Re: [PATCH v17 2/5] fs/proc/task_mmu: Implement IOCTL to get and optionally clear info about PTEs

Re: [PATCH v17 2/5] fs/proc/task_mmu: Implement IOCTL to get and optionally clear info about PTEs

[kernel test robot]

Hi Muhammad,

kernel test robot noticed the following build errors:

[auto build test ERROR on akpm-mm/mm-everything]
[also build test ERROR on next-20230606]
[cannot apply to linus/master v6.4-rc5]
[If your patch is applied to the wrong git tree, kindly drop us a note.
And when submitting patch, we suggest to use '--base' as documented in
https://git-scm.com/docs/git-format-patch#_base_tree_information]

url:    https://github.com/intel-lab-lkp/linux/commits/Muhammad-Usama-Anjum/userfaultfd-UFFD_FEATURE_WP_ASYNC/20230606-141114
base:   https://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm.git mm-everything
patch link:    https://lore.kernel.org/r/20230606060822.1065182-3-usama.anjum%40collabora.com
patch subject: [PATCH v17 2/5] fs/proc/task_mmu: Implement IOCTL to get and optionally clear info about PTEs
config: arc-allyesconfig (https://download.01.org/0day-ci/archive/20230607/202306070414.XDn2ITuw-lkp@intel.com/config)
compiler: arceb-elf-gcc (GCC) 12.3.0
reproduce (this is a W=1 build):
        mkdir -p ~/bin
        wget https://raw.githubusercontent.com/intel/lkp-tests/master/sbin/make.cross -O ~/bin/make.cross
        chmod +x ~/bin/make.cross
        git remote add akpm-mm https://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm.git
        git fetch akpm-mm mm-everything
        git checkout akpm-mm/mm-everything
        b4 shazam https://lore.kernel.org/r/20230606060822.1065182-3-usama.anjum@collabora.com
        # save the config file
        mkdir build_dir && cp config build_dir/.config
        COMPILER_INSTALL_PATH=$HOME/0day COMPILER=gcc-12.3.0 ~/bin/make.cross W=1 O=build_dir ARCH=arc olddefconfig
        COMPILER_INSTALL_PATH=$HOME/0day COMPILER=gcc-12.3.0 ~/bin/make.cross W=1 O=build_dir ARCH=arc SHELL=/bin/bash

If you fix the issue, kindly add following tag where applicable
| Reported-by: kernel test robot <lkp@intel.com>
| Closes: https://lore.kernel.org/oe-kbuild-all/202306070414.XDn2ITuw-lkp@intel.com/

All errors (new ones prefixed by >>):

   fs/proc/task_mmu.c: In function 'pagemap_scan_pmd_entry':
>> fs/proc/task_mmu.c:1960:31: error: 'HPAGE_SIZE' undeclared (first use in this function); did you mean 'PAGE_SIZE'?
    1960 |                     n_pages < HPAGE_SIZE/PAGE_SIZE) {
         |                               ^~~~~~~~~~
         |                               PAGE_SIZE
   fs/proc/task_mmu.c:1960:31: note: each undeclared identifier is reported only once for each function it appears in


vim +1960 fs/proc/task_mmu.c

  1931	
  1932	static int pagemap_scan_pmd_entry(pmd_t *pmd, unsigned long start,
  1933					  unsigned long end, struct mm_walk *walk)
  1934	{
  1935		struct pagemap_scan_private *p = walk->private;
  1936		struct vm_area_struct *vma = walk->vma;
  1937		unsigned long addr = end;
  1938		pte_t *pte, *orig_pte;
  1939		spinlock_t *ptl;
  1940		bool is_written;
  1941		int ret = 0;
  1942	
  1943		arch_enter_lazy_mmu_mode();
  1944	
  1945	#ifdef CONFIG_TRANSPARENT_HUGEPAGE
  1946		ptl = pmd_trans_huge_lock(pmd, vma);
  1947		if (ptl) {
  1948			unsigned long n_pages = (end - start)/PAGE_SIZE;
  1949	
  1950			if (p->max_pages && n_pages > p->max_pages - p->found_pages)
  1951				n_pages = p->max_pages - p->found_pages;
  1952	
  1953			is_written = !is_pmd_uffd_wp(*pmd);
  1954	
  1955			/*
  1956			 * Break huge page into small pages if the WP operation need to
  1957			 * be performed is on a portion of the huge page.
  1958			 */
  1959			if (is_written && IS_PM_SCAN_WP(p->flags) &&
> 1960			    n_pages < HPAGE_SIZE/PAGE_SIZE) {
  1961				spin_unlock(ptl);
  1962	
  1963				split_huge_pmd(vma, pmd, start);
  1964				goto process_smaller_pages;
  1965			}
  1966	
  1967			if (IS_PM_SCAN_GET(p->flags))
  1968				ret = pagemap_scan_output(is_written, vma->vm_file,
  1969							  pmd_present(*pmd),
  1970							  is_swap_pmd(*pmd),
  1971							  p, start, n_pages);
  1972	
  1973			if (ret >= 0 && is_written && IS_PM_SCAN_WP(p->flags))
  1974				make_uffd_wp_pmd(vma, addr, pmd);
  1975	
  1976			if (IS_PM_SCAN_WP(p->flags))
  1977				flush_tlb_range(vma, start, end);
  1978	
  1979			spin_unlock(ptl);
  1980	
  1981			arch_leave_lazy_mmu_mode();
  1982			return ret;
  1983		}
  1984	
  1985	process_smaller_pages:
  1986		if (pmd_trans_unstable(pmd)) {
  1987			arch_leave_lazy_mmu_mode();
  1988			walk->action = ACTION_AGAIN;
  1989			return 0;
  1990		}
  1991	#endif
  1992	
  1993		orig_pte = pte = pte_offset_map_lock(vma->vm_mm, pmd, start, &ptl);
  1994		for (addr = start; addr < end && !ret; pte++, addr += PAGE_SIZE) {
  1995			is_written = !is_pte_uffd_wp(*pte);
  1996	
  1997			if (IS_PM_SCAN_GET(p->flags))
  1998				ret = pagemap_scan_output(is_written, vma->vm_file,
  1999							  pte_present(*pte),
  2000							  is_swap_pte(*pte),
  2001							  p, addr, 1);
  2002	
  2003			if (ret >= 0 && is_written && IS_PM_SCAN_WP(p->flags))
  2004				make_uffd_wp_pte(vma, addr, pte);
  2005		}
  2006	
  2007		if (IS_PM_SCAN_WP(p->flags))
  2008			flush_tlb_range(vma, start, addr);
  2009	
  2010		pte_unmap_unlock(orig_pte, ptl);
  2011		arch_leave_lazy_mmu_mode();
  2012	
  2013		cond_resched();
  2014		return ret;
  2015	}
  2016	

-- 
0-DAY CI Kernel Test Service
https://github.com/intel/lkp-tests/wiki

Re: [PATCH v17 2/5] fs/proc/task_mmu: Implement IOCTL to get and optionally clear info about PTEs

[Michał Mirosław]

On Tue, 6 Jun 2023 at 08:08, Muhammad Usama Anjum
<usama.anjum@collabora.com> wrote:
> This IOCTL, PAGEMAP_SCAN on pagemap file can be used to get and/or clear
> the info about page table entries. The following operations are supported
> in this ioctl:
> - Get the information if the pages have been written-to (PAGE_IS_WRITTEN),
>   file mapped (PAGE_IS_FILE), present (PAGE_IS_PRESENT) or swapped
>   (PAGE_IS_SWAPPED).
> - Find pages which have been written-to and/or write protect the pages
>   (atomic PM_SCAN_OP_GET + PM_SCAN_OP_WP)
>
> This IOCTL can be extended to get information about more PTE bits.
[...]
> --- a/fs/proc/task_mmu.c
> +++ b/fs/proc/task_mmu.c
[...]
> +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> +static inline bool is_pmd_uffd_wp(pmd_t pmd)
> +{
> +       return (pmd_present(pmd) && pmd_uffd_wp(pmd)) ||
> +              (is_swap_pmd(pmd) && pmd_swp_uffd_wp(pmd));
> +}
[...]
> +#ifdef CONFIG_HUGETLB_PAGE
> +static inline bool is_huge_pte_uffd_wp(pte_t pte)
> +{
> +       return ((pte_present(pte) && huge_pte_uffd_wp(pte)) ||
> +              pte_swp_uffd_wp_any(pte));

Nit: please remove the outer parentheses (it is already done for
similar finctuons above).

> +}

> +static inline bool pagemap_scan_check_page_written(struct pagemap_scan_private *p)
> +{
> +       return (p->required_mask | p->anyof_mask | p->excluded_mask) &
> +              PAGE_IS_WRITTEN;
> +}

This could be precalculated and put as a flag into
pagemap_scan_private - it is kernel-private structure and there are a
few spare bits in `flags` if you'd prefer not to add an explicit
boolean.

[...]
> +static int pagemap_scan_output(bool wt, bool file, bool pres, bool swap,
> +                              struct pagemap_scan_private *p,
> +                              unsigned long addr, unsigned int n_pages)
> +{
> +       unsigned long bitmap = PM_SCAN_BITMAP(wt, file, pres, swap);
> +       struct page_region *cur = &p->cur;
> +
> +       if (!n_pages)
> +               return -EINVAL;
> +
> +       if ((p->required_mask & bitmap) != p->required_mask)
> +               return 0;
> +       if (p->anyof_mask && !(p->anyof_mask & bitmap))
> +               return 0;
> +       if (p->excluded_mask & bitmap)
> +               return 0;
> +
> +       bitmap &= p->return_mask;
> +       if (!bitmap)
> +               return 0;
> +
> +       if (cur->bitmap == bitmap &&
> +           cur->start + cur->len * PAGE_SIZE == addr) {
> +               cur->len += n_pages;
> +               p->found_pages += n_pages;
> +       } else {
> +               /*
> +                * All data is copied to cur first. When more data is found, we
> +                * push cur to vec and copy new data to cur. The vec_index
> +                * represents the current index of vec array. We add 1 to the
> +                * vec_index while performing checks to account for data in cur.
> +                */
> +               if (cur->len && (p->vec_index + 1) >= p->vec_len)
> +                       return -ENOSPC;
> +
> +               if (cur->len) {
> +                       memcpy(&p->vec[p->vec_index], cur, sizeof(*p->vec));
> +                       p->vec_index++;
> +               }
> +
> +               cur->start = addr;
> +               cur->len = n_pages;
> +               cur->bitmap = bitmap;
> +               p->found_pages += n_pages;
> +       }
> +
> +       if (p->max_pages && (p->found_pages == p->max_pages))
> +               return PM_SCAN_FOUND_MAX_PAGES;
> +
> +       return 0;
> +}
> +
> +static int pagemap_scan_pmd_entry(pmd_t *pmd, unsigned long start,
> +                                 unsigned long end, struct mm_walk *walk)
> +{
> +       struct pagemap_scan_private *p = walk->private;
> +       struct vm_area_struct *vma = walk->vma;
> +       unsigned long addr = end;
> +       pte_t *pte, *orig_pte;
> +       spinlock_t *ptl;
> +       bool is_written;
> +       int ret = 0;
> +
> +       arch_enter_lazy_mmu_mode();
> +
> +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> +       ptl = pmd_trans_huge_lock(pmd, vma);
> +       if (ptl) {
> +               unsigned long n_pages = (end - start)/PAGE_SIZE;
> +
> +               if (p->max_pages && n_pages > p->max_pages - p->found_pages)
> +                       n_pages = p->max_pages - p->found_pages;

Since p->found_pages is only ever increased in `pagemap_scan_output()`
and that function is only called for GET or GET+WP operations, maybe
the logic could be folded to pagemap_scan_output() to avoid
duplication?
In this function the calculation is used only when WP op is done to
split the HP if n_pages limit would be hit, but if using plain WP
(without GET) it doesn't make sense to use the limit.
(pagemap_scan_output() is trivial enough so I think it could be pulled
inside the spinlocked region.)

> +
> +               is_written = !is_pmd_uffd_wp(*pmd);
> +
> +               /*
> +                * Break huge page into small pages if the WP operation need to
> +                * be performed is on a portion of the huge page.
> +                */
> +               if (is_written && IS_PM_SCAN_WP(p->flags) &&
> +                   n_pages < HPAGE_SIZE/PAGE_SIZE) {
> +                       spin_unlock(ptl);
> +
> +                       split_huge_pmd(vma, pmd, start);
> +                       goto process_smaller_pages;
> +               }
> +
> +               if (IS_PM_SCAN_GET(p->flags))
> +                       ret = pagemap_scan_output(is_written, vma->vm_file,
> +                                                 pmd_present(*pmd),
> +                                                 is_swap_pmd(*pmd),
> +                                                 p, start, n_pages);
> +
> +               if (ret >= 0 && is_written && IS_PM_SCAN_WP(p->flags))
> +                       make_uffd_wp_pmd(vma, addr, pmd);
> +
> +               if (IS_PM_SCAN_WP(p->flags))

Why `is_written` is not checked? If is_written is false, then the WP
op should be a no-op and so won't need TLB flushing, will it? [Same
for the PTE case below.]

> +                       flush_tlb_range(vma, start, end);
> +
[...]
> +       if (IS_PM_SCAN_WP(p->flags))
> +               flush_tlb_range(vma, start, addr);
> +
> +       pte_unmap_unlock(orig_pte, ptl);
> +       arch_leave_lazy_mmu_mode();
> +
> +       cond_resched();
> +       return ret;
> +}
> +
> +#ifdef CONFIG_HUGETLB_PAGE
> +static int pagemap_scan_hugetlb_entry(pte_t *ptep, unsigned long hmask,
> +                                     unsigned long start, unsigned long end,
> +                                     struct mm_walk *walk)
> +{
> +       unsigned long n_pages = (end - start)/PAGE_SIZE;
> +       struct pagemap_scan_private *p = walk->private;
> +       struct vm_area_struct *vma = walk->vma;
> +       struct hstate *h = hstate_vma(vma);
> +       spinlock_t *ptl;
> +       bool is_written;
> +       int ret = 0;
> +       pte_t pte;
> +
> +       if (p->max_pages && n_pages > p->max_pages - p->found_pages)
> +               n_pages = p->max_pages - p->found_pages;
> +
> +       if (IS_PM_SCAN_WP(p->flags)) {
> +               i_mmap_lock_write(vma->vm_file->f_mapping);
> +               ptl = huge_pte_lock(h, vma->vm_mm, ptep);
> +       }
> +
> +       pte = huge_ptep_get(ptep);
> +       is_written = !is_huge_pte_uffd_wp(pte);
> +
> +       /*
> +        * Partial hugetlb page clear isn't supported
> +        */
> +       if (is_written && IS_PM_SCAN_WP(p->flags) &&
> +           n_pages < HPAGE_SIZE/PAGE_SIZE) {
> +               ret = -EPERM;

Shouldn't this be ENOSPC, conveying that the operation would overflow
the n_pages limit?

> +               goto unlock_and_return;
> +       }
> +
> +       if (IS_PM_SCAN_GET(p->flags)) {
> +               ret = pagemap_scan_output(is_written, vma->vm_file,
> +                                         pte_present(pte), is_swap_pte(pte),
> +                                         p, start, n_pages);
> +               if (ret < 0)
> +                       goto unlock_and_return;
> +       }
> +
> +       if (is_written && IS_PM_SCAN_WP(p->flags)) {

Oh, this case does check `is_written` before flushing TLB, contrary to
what the cases above do.

> +               make_uffd_wp_huge_pte(vma, start, ptep, pte);
> +               flush_hugetlb_tlb_range(vma, start, end);
> +       }
> +
> +unlock_and_return:
> +       if (IS_PM_SCAN_WP(p->flags)) {
> +               spin_unlock(ptl);
> +               i_mmap_unlock_write(vma->vm_file->f_mapping);
> +       }
> +
> +       return ret;
> +}
> +#else
> +#define pagemap_scan_hugetlb_entry NULL
> +#endif
> +
> +static int pagemap_scan_pte_hole(unsigned long addr, unsigned long end,
> +                                int depth, struct mm_walk *walk)
> +{
> +       unsigned long n_pages = (end - addr)/PAGE_SIZE;
> +       struct pagemap_scan_private *p = walk->private;
> +       struct vm_area_struct *vma = walk->vma;
> +       int ret = 0;
> +
> +       if (!vma || !IS_PM_SCAN_GET(p->flags))
> +               return 0;
> +
> +       if (p->max_pages && n_pages > p->max_pages - p->found_pages)
> +               n_pages = p->max_pages - p->found_pages;

Nit: If the page flags don't match (wouldn't be output), the limit
would not be hit and the calculation is unnecessary. But if it was
done in pagemap_scan_output() instead after all the flags checks...

> +       ret = pagemap_scan_output(false, vma->vm_file, false, false, p, addr,
> +                                 n_pages);
> +
> +       return ret;
> +}
[...]
> +static long do_pagemap_scan(struct mm_struct *mm,
> +                           struct pm_scan_arg __user *uarg)
> +{
> +       unsigned long start, end, walk_start, walk_end;
> +       unsigned long empty_slots, vec_index = 0;
> +       struct mmu_notifier_range range;
> +       struct page_region __user *vec;
> +       struct pagemap_scan_private p;
> +       struct pm_scan_arg arg;
> +       int ret = 0;
> +
> +       if (copy_from_user(&arg, uarg, sizeof(arg)))
> +               return -EFAULT;
> +
> +       start = untagged_addr((unsigned long)arg.start);
> +       vec = (struct page_region *)untagged_addr((unsigned long)arg.vec);
> +
> +       ret = pagemap_scan_args_valid(&arg, start, vec);
> +       if (ret)
> +               return ret;
> +
> +       end = start + arg.len;
> +       p.max_pages = arg.max_pages;
> +       p.found_pages = 0;
> +       p.flags = arg.flags;
> +       p.required_mask = arg.required_mask;
> +       p.anyof_mask = arg.anyof_mask;
> +       p.excluded_mask = arg.excluded_mask;
> +       p.return_mask = arg.return_mask;
> +       p.cur.start = p.cur.len = p.cur.bitmap = 0;
> +       p.vec = NULL;
> +       p.vec_len = PAGEMAP_WALK_SIZE >> PAGE_SHIFT;

If p.vec_len would not count the entry held in `cur` (IOW: vec_len =
WALK_SIZE - 1), then pagemap_scan_output() wouldn't need the big
comment about adding or subtracting 1 when checking for overflow. The
output vector needs to have space for at least one entrry to make GET
useful. Maybe `cur` could be renamed or annotated to express that it
always holds the last entry?

> +
> +       /*
> +        * Allocate smaller buffer to get output from inside the page walk
> +        * functions and walk page range in PAGEMAP_WALK_SIZE size chunks. As
> +        * we want to return output to user in compact form where no two
> +        * consecutive regions should be continuous and have the same flags.
> +        * So store the latest element in p.cur between different walks and
> +        * store the p.cur at the end of the walk to the user buffer.
> +        */
> +       if (IS_PM_SCAN_GET(p.flags)) {
> +               p.vec = kmalloc_array(p.vec_len, sizeof(*p.vec), GFP_KERNEL);
> +               if (!p.vec)
> +                       return -ENOMEM;
> +       }
> +
> +       if (IS_PM_SCAN_WP(p.flags)) {
> +               mmu_notifier_range_init(&range, MMU_NOTIFY_PROTECTION_VMA, 0,
> +                                       mm, start, end);
> +               mmu_notifier_invalidate_range_start(&range);
> +       }
> +
> +       walk_start = walk_end = start;
> +       while (walk_end < end && !ret) {
> +               if (IS_PM_SCAN_GET(p.flags)) {
> +                       p.vec_index = 0;
> +
> +                       empty_slots = arg.vec_len - vec_index;

Can `empty_slots` be zero here? I don't see anything prohibiting this case.

> +                       p.vec_len = min(p.vec_len, empty_slots);

( If not counting `cur`, it would be min(p.vec_len, empty_slots - 1); )

> +               }
> +
> +               walk_end = (walk_start + PAGEMAP_WALK_SIZE) & PAGEMAP_WALK_MASK;
> +               if (walk_end > end)
> +                       walk_end = end;
> +
> +               ret = mmap_read_lock_killable(mm);
> +               if (ret)
> +                       goto free_data;
> +               ret = walk_page_range(mm, walk_start, walk_end,
> +                                     &pagemap_scan_ops, &p);
> +               mmap_read_unlock(mm);
> +
> +               if (ret && ret != -ENOSPC && ret != PM_SCAN_FOUND_MAX_PAGES)
> +                       goto free_data;
> +
> +               walk_start = walk_end;
> +               if (IS_PM_SCAN_GET(p.flags) && p.vec_index) {
> +                       if (copy_to_user(&vec[vec_index], p.vec,
> +                                        p.vec_index * sizeof(*p.vec))) {
> +                               /*
> +                                * Return error even though the OP succeeded
> +                                */
> +                               ret = -EFAULT;
> +                               goto free_data;
> +                       }
> +                       vec_index += p.vec_index;
> +               }
> +       }
> +
> +       if (IS_PM_SCAN_GET(p.flags) && p.cur.len) {

Nit: p.cur.len can be non-zero only if we do a GET (or GET+WP) operation.

> +               if (copy_to_user(&vec[vec_index], &p.cur, sizeof(*p.vec))) {

Nit: sizeof(*p.cur); (even though this is the same type)

> +                       ret = -EFAULT;
> +                       goto free_data;
> +               }
> +               vec_index++;
> +       }
> +
> +       ret = vec_index;
> +
> +free_data:
> +       if (IS_PM_SCAN_WP(p.flags))
> +               mmu_notifier_invalidate_range_end(&range);
> +
> +       kfree(p.vec);
> +       return ret;
> +}
> +
> +static long do_pagemap_cmd(struct file *file, unsigned int cmd,
> +                          unsigned long arg)
> +{
> +       struct pm_scan_arg __user *uarg = (struct pm_scan_arg __user *)arg;

The cast should be in do_pagemap_scan() as if there comes another
`cmd`, then it might use a different argument type.

> +       struct mm_struct *mm = file->private_data;
> +
> +       switch (cmd) {
> +       case PAGEMAP_SCAN:
> +               return do_pagemap_scan(mm, uarg);
> +
> +       default:
> +               return -EINVAL;
> +       }
> +}

Best Regards
Michał Mirosław

Re: [PATCH v17 2/5] fs/proc/task_mmu: Implement IOCTL to get and optionally clear info about PTEs

[Muhammad Usama Anjum]

Hi Michał,

Thank you for taking time to review!

On 6/7/23 7:52 PM, Michał Mirosław wrote:
> On Tue, 6 Jun 2023 at 08:08, Muhammad Usama Anjum
> <usama.anjum@collabora.com> wrote:
>> This IOCTL, PAGEMAP_SCAN on pagemap file can be used to get and/or clear
>> the info about page table entries. The following operations are supported
>> in this ioctl:
>> - Get the information if the pages have been written-to (PAGE_IS_WRITTEN),
>>   file mapped (PAGE_IS_FILE), present (PAGE_IS_PRESENT) or swapped
>>   (PAGE_IS_SWAPPED).
>> - Find pages which have been written-to and/or write protect the pages
>>   (atomic PM_SCAN_OP_GET + PM_SCAN_OP_WP)
>>
>> This IOCTL can be extended to get information about more PTE bits.
> [...]
>> --- a/fs/proc/task_mmu.c
>> +++ b/fs/proc/task_mmu.c
> [...]
>> +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
>> +static inline bool is_pmd_uffd_wp(pmd_t pmd)
>> +{
>> +       return (pmd_present(pmd) && pmd_uffd_wp(pmd)) ||
>> +              (is_swap_pmd(pmd) && pmd_swp_uffd_wp(pmd));
>> +}
> [...]
>> +#ifdef CONFIG_HUGETLB_PAGE
>> +static inline bool is_huge_pte_uffd_wp(pte_t pte)
>> +{
>> +       return ((pte_present(pte) && huge_pte_uffd_wp(pte)) ||
>> +              pte_swp_uffd_wp_any(pte));
> 
> Nit: please remove the outer parentheses (it is already done for
> similar finctuons above).
Will remove.

> 
>> +}
> 
>> +static inline bool pagemap_scan_check_page_written(struct pagemap_scan_private *p)
>> +{
>> +       return (p->required_mask | p->anyof_mask | p->excluded_mask) &
>> +              PAGE_IS_WRITTEN;
>> +}
> 
> This could be precalculated and put as a flag into
> pagemap_scan_private - it is kernel-private structure and there are a
> few spare bits in `flags` if you'd prefer not to add an explicit
> boolean.
This inline function is only being used at one spot. I can remove the
function altogether. I don't like putting it in flags. It'll bring some
complexity.

> 
> [...]
>> +static int pagemap_scan_output(bool wt, bool file, bool pres, bool swap,
>> +                              struct pagemap_scan_private *p,
>> +                              unsigned long addr, unsigned int n_pages)
>> +{
>> +       unsigned long bitmap = PM_SCAN_BITMAP(wt, file, pres, swap);
>> +       struct page_region *cur = &p->cur;
>> +
>> +       if (!n_pages)
>> +               return -EINVAL;
>> +
>> +       if ((p->required_mask & bitmap) != p->required_mask)
>> +               return 0;
>> +       if (p->anyof_mask && !(p->anyof_mask & bitmap))
>> +               return 0;
>> +       if (p->excluded_mask & bitmap)
>> +               return 0;
>> +
>> +       bitmap &= p->return_mask;
>> +       if (!bitmap)
>> +               return 0;
>> +
>> +       if (cur->bitmap == bitmap &&
>> +           cur->start + cur->len * PAGE_SIZE == addr) {
>> +               cur->len += n_pages;
>> +               p->found_pages += n_pages;
>> +       } else {
>> +               /*
>> +                * All data is copied to cur first. When more data is found, we
>> +                * push cur to vec and copy new data to cur. The vec_index
>> +                * represents the current index of vec array. We add 1 to the
>> +                * vec_index while performing checks to account for data in cur.
>> +                */
>> +               if (cur->len && (p->vec_index + 1) >= p->vec_len)
>> +                       return -ENOSPC;
>> +
>> +               if (cur->len) {
>> +                       memcpy(&p->vec[p->vec_index], cur, sizeof(*p->vec));
>> +                       p->vec_index++;
>> +               }
>> +
>> +               cur->start = addr;
>> +               cur->len = n_pages;
>> +               cur->bitmap = bitmap;
>> +               p->found_pages += n_pages;
>> +       }
>> +
>> +       if (p->max_pages && (p->found_pages == p->max_pages))
>> +               return PM_SCAN_FOUND_MAX_PAGES;
>> +
>> +       return 0;
>> +}
>> +
>> +static int pagemap_scan_pmd_entry(pmd_t *pmd, unsigned long start,
>> +                                 unsigned long end, struct mm_walk *walk)
>> +{
>> +       struct pagemap_scan_private *p = walk->private;
>> +       struct vm_area_struct *vma = walk->vma;
>> +       unsigned long addr = end;
>> +       pte_t *pte, *orig_pte;
>> +       spinlock_t *ptl;
>> +       bool is_written;
>> +       int ret = 0;
>> +
>> +       arch_enter_lazy_mmu_mode();
>> +
>> +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
>> +       ptl = pmd_trans_huge_lock(pmd, vma);
>> +       if (ptl) {
>> +               unsigned long n_pages = (end - start)/PAGE_SIZE;
>> +
>> +               if (p->max_pages && n_pages > p->max_pages - p->found_pages)
>> +                       n_pages = p->max_pages - p->found_pages;
> 
> Since p->found_pages is only ever increased in `pagemap_scan_output()`
> and that function is only called for GET or GET+WP operations, maybe
> the logic could be folded to pagemap_scan_output() to avoid
> duplication?
> In this function the calculation is used only when WP op is done to
> split the HP if n_pages limit would be hit, but if using plain WP
> (without GET) it doesn't make sense to use the limit.
The n_pages is needed to decide if THP need to be broken down and it is
used in pagemap_scan_output(). I've brought this condition out of
pagemap_scan_output() to cater this former condition. If I move it to
pagemap_scan_output(), I'll have to write same condition to find out if I
need to breakt he THP. This seems like repetition, but we have same use
case for tlbhuge page.

> (pagemap_scan_output() is trivial enough so I think it could be pulled
> inside the spinlocked region.)
It is already in spinlocked region. Spin lock is being released after tlb
flush.

> 
>> +
>> +               is_written = !is_pmd_uffd_wp(*pmd);
>> +
>> +               /*
>> +                * Break huge page into small pages if the WP operation need to
>> +                * be performed is on a portion of the huge page.
>> +                */
>> +               if (is_written && IS_PM_SCAN_WP(p->flags) &&
>> +                   n_pages < HPAGE_SIZE/PAGE_SIZE) {
>> +                       spin_unlock(ptl);
>> +
>> +                       split_huge_pmd(vma, pmd, start);
>> +                       goto process_smaller_pages;
>> +               }
>> +
>> +               if (IS_PM_SCAN_GET(p->flags))
>> +                       ret = pagemap_scan_output(is_written, vma->vm_file,
>> +                                                 pmd_present(*pmd),
>> +                                                 is_swap_pmd(*pmd),
>> +                                                 p, start, n_pages);
>> +
>> +               if (ret >= 0 && is_written && IS_PM_SCAN_WP(p->flags))
>> +                       make_uffd_wp_pmd(vma, addr, pmd);
>> +
>> +               if (IS_PM_SCAN_WP(p->flags))
> 
> Why `is_written` is not checked? If is_written is false, then the WP
> op should be a no-op and so won't need TLB flushing, will it? [Same
> for the PTE case below.]
It can be done for THP. But for ptes we cannot trust is_written as
is_written only represent last pte state.

> 
>> +                       flush_tlb_range(vma, start, end);
>> +
> [...]
>> +       if (IS_PM_SCAN_WP(p->flags))
>> +               flush_tlb_range(vma, start, addr);
>> +
>> +       pte_unmap_unlock(orig_pte, ptl);
>> +       arch_leave_lazy_mmu_mode();
>> +
>> +       cond_resched();
>> +       return ret;
>> +}
>> +
>> +#ifdef CONFIG_HUGETLB_PAGE
>> +static int pagemap_scan_hugetlb_entry(pte_t *ptep, unsigned long hmask,
>> +                                     unsigned long start, unsigned long end,
>> +                                     struct mm_walk *walk)
>> +{
>> +       unsigned long n_pages = (end - start)/PAGE_SIZE;
>> +       struct pagemap_scan_private *p = walk->private;
>> +       struct vm_area_struct *vma = walk->vma;
>> +       struct hstate *h = hstate_vma(vma);
>> +       spinlock_t *ptl;
>> +       bool is_written;
>> +       int ret = 0;
>> +       pte_t pte;
>> +
>> +       if (p->max_pages && n_pages > p->max_pages - p->found_pages)
>> +               n_pages = p->max_pages - p->found_pages;
>> +
>> +       if (IS_PM_SCAN_WP(p->flags)) {
>> +               i_mmap_lock_write(vma->vm_file->f_mapping);
>> +               ptl = huge_pte_lock(h, vma->vm_mm, ptep);
>> +       }
>> +
>> +       pte = huge_ptep_get(ptep);
>> +       is_written = !is_huge_pte_uffd_wp(pte);
>> +
>> +       /*
>> +        * Partial hugetlb page clear isn't supported
>> +        */
>> +       if (is_written && IS_PM_SCAN_WP(p->flags) &&
>> +           n_pages < HPAGE_SIZE/PAGE_SIZE) {
>> +               ret = -EPERM;
> 
> Shouldn't this be ENOSPC, conveying that the operation would overflow
> the n_pages limit?
We are testing here is user has asked us to engage WP on a part of the
hugetlb or we can only perform WP on a part of the engage as user buffer is
full. We cannot judge this has happened because of the former or later
condition. So I'm assuming that user's parameters aren't solid enough and
returning -EPERM. It seemed more suitable to me. But I can return -ENOSPC
as well, if you say?

> 
>> +               goto unlock_and_return;
>> +       }
>> +
>> +       if (IS_PM_SCAN_GET(p->flags)) {
>> +               ret = pagemap_scan_output(is_written, vma->vm_file,
>> +                                         pte_present(pte), is_swap_pte(pte),
>> +                                         p, start, n_pages);
>> +               if (ret < 0)
>> +                       goto unlock_and_return;
>> +       }
>> +
>> +       if (is_written && IS_PM_SCAN_WP(p->flags)) {
> 
> Oh, this case does check `is_written` before flushing TLB, contrary to
> what the cases above do.
> 
>> +               make_uffd_wp_huge_pte(vma, start, ptep, pte);
>> +               flush_hugetlb_tlb_range(vma, start, end);
>> +       }
>> +
>> +unlock_and_return:
>> +       if (IS_PM_SCAN_WP(p->flags)) {
>> +               spin_unlock(ptl);
>> +               i_mmap_unlock_write(vma->vm_file->f_mapping);
>> +       }
>> +
>> +       return ret;
>> +}
>> +#else
>> +#define pagemap_scan_hugetlb_entry NULL
>> +#endif
>> +
>> +static int pagemap_scan_pte_hole(unsigned long addr, unsigned long end,
>> +                                int depth, struct mm_walk *walk)
>> +{
>> +       unsigned long n_pages = (end - addr)/PAGE_SIZE;
>> +       struct pagemap_scan_private *p = walk->private;
>> +       struct vm_area_struct *vma = walk->vma;
>> +       int ret = 0;
>> +
>> +       if (!vma || !IS_PM_SCAN_GET(p->flags))
>> +               return 0;
>> +
>> +       if (p->max_pages && n_pages > p->max_pages - p->found_pages)
>> +               n_pages = p->max_pages - p->found_pages;
> 
> Nit: If the page flags don't match (wouldn't be output), the limit
> would not be hit and the calculation is unnecessary. But if it was
> done in pagemap_scan_output() instead after all the flags checks...
Correct for this use case. But moving to pagemap_scan_output() would make
me do duplicate calculation for other 2 cases as explained above.

> 
>> +       ret = pagemap_scan_output(false, vma->vm_file, false, false, p, addr,
>> +                                 n_pages);
>> +
>> +       return ret;
>> +}
> [...]
>> +static long do_pagemap_scan(struct mm_struct *mm,
>> +                           struct pm_scan_arg __user *uarg)
>> +{
>> +       unsigned long start, end, walk_start, walk_end;
>> +       unsigned long empty_slots, vec_index = 0;
>> +       struct mmu_notifier_range range;
>> +       struct page_region __user *vec;
>> +       struct pagemap_scan_private p;
>> +       struct pm_scan_arg arg;
>> +       int ret = 0;
>> +
>> +       if (copy_from_user(&arg, uarg, sizeof(arg)))
>> +               return -EFAULT;
>> +
>> +       start = untagged_addr((unsigned long)arg.start);
>> +       vec = (struct page_region *)untagged_addr((unsigned long)arg.vec);
>> +
>> +       ret = pagemap_scan_args_valid(&arg, start, vec);
>> +       if (ret)
>> +               return ret;
>> +
>> +       end = start + arg.len;
>> +       p.max_pages = arg.max_pages;
>> +       p.found_pages = 0;
>> +       p.flags = arg.flags;
>> +       p.required_mask = arg.required_mask;
>> +       p.anyof_mask = arg.anyof_mask;
>> +       p.excluded_mask = arg.excluded_mask;
>> +       p.return_mask = arg.return_mask;
>> +       p.cur.start = p.cur.len = p.cur.bitmap = 0;
>> +       p.vec = NULL;
>> +       p.vec_len = PAGEMAP_WALK_SIZE >> PAGE_SHIFT;
> 
> If p.vec_len would not count the entry held in `cur` (IOW: vec_len =
> WALK_SIZE - 1), then pagemap_scan_output() wouldn't need the big
> comment about adding or subtracting 1 when checking for overflow. The
> output vector needs to have space for at least one entrry to make GET
> useful. Maybe `cur` could be renamed or annotated to express that it
> always holds the last entry?
Ohhh.. This can be done by doing subtracting 1 from empty_slots. But I've
explored the idea. I don't see any benefit. If we do this, then I'll have
to put a comment why subtracting 1 is needed. Seems like same problem:

--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -1909,7 +1909,7 @@ static int pagemap_scan_output(bool wt, bool file,
bool pres, bool swap,
                 * represents the current index of vec array. We add 1 to the
                 * vec_index while performing checks to account for data in
cur.
                 */
-               if (cur->len && (p->vec_index + 1) >= p->vec_len)
+               if (cur->len && p->vec_index >= p->vec_len)
                        return -ENOSPC;

                if (cur->len) {
@@ -2202,7 +2202,7 @@ static long do_pagemap_scan(struct mm_struct *mm,
                if (IS_PM_SCAN_GET(p.flags)) {
                        p.vec_index = 0;

-                       empty_slots = arg.vec_len - vec_index;
+                       empty_slots = arg.vec_len - 1 - vec_index;
                        p.vec_len = min(p.vec_len, empty_slots);
                }

Lets leave it as it is. I can change `cur` to `last` or any other name.
Please suggest.

> 
>> +
>> +       /*
>> +        * Allocate smaller buffer to get output from inside the page walk
>> +        * functions and walk page range in PAGEMAP_WALK_SIZE size chunks. As
>> +        * we want to return output to user in compact form where no two
>> +        * consecutive regions should be continuous and have the same flags.
>> +        * So store the latest element in p.cur between different walks and
>> +        * store the p.cur at the end of the walk to the user buffer.
>> +        */
>> +       if (IS_PM_SCAN_GET(p.flags)) {
>> +               p.vec = kmalloc_array(p.vec_len, sizeof(*p.vec), GFP_KERNEL);
>> +               if (!p.vec)
>> +                       return -ENOMEM;
>> +       }
>> +
>> +       if (IS_PM_SCAN_WP(p.flags)) {
>> +               mmu_notifier_range_init(&range, MMU_NOTIFY_PROTECTION_VMA, 0,
>> +                                       mm, start, end);
>> +               mmu_notifier_invalidate_range_start(&range);
>> +       }
>> +
>> +       walk_start = walk_end = start;
>> +       while (walk_end < end && !ret) {
>> +               if (IS_PM_SCAN_GET(p.flags)) {
>> +                       p.vec_index = 0;
>> +
>> +                       empty_slots = arg.vec_len - vec_index;
> 
> Can `empty_slots` be zero here? I don't see anything prohibiting this case.
I'll add a check here and abort the loop here instead of continuing the
operation.

> 
>> +                       p.vec_len = min(p.vec_len, empty_slots);
> 
> ( If not counting `cur`, it would be min(p.vec_len, empty_slots - 1); )
> 
>> +               }
>> +
>> +               walk_end = (walk_start + PAGEMAP_WALK_SIZE) & PAGEMAP_WALK_MASK;
>> +               if (walk_end > end)
>> +                       walk_end = end;
>> +
>> +               ret = mmap_read_lock_killable(mm);
>> +               if (ret)
>> +                       goto free_data;
>> +               ret = walk_page_range(mm, walk_start, walk_end,
>> +                                     &pagemap_scan_ops, &p);
>> +               mmap_read_unlock(mm);
>> +
>> +               if (ret && ret != -ENOSPC && ret != PM_SCAN_FOUND_MAX_PAGES)
>> +                       goto free_data;
>> +
>> +               walk_start = walk_end;
>> +               if (IS_PM_SCAN_GET(p.flags) && p.vec_index) {
>> +                       if (copy_to_user(&vec[vec_index], p.vec,
>> +                                        p.vec_index * sizeof(*p.vec))) {
>> +                               /*
>> +                                * Return error even though the OP succeeded
>> +                                */
>> +                               ret = -EFAULT;
>> +                               goto free_data;
>> +                       }
>> +                       vec_index += p.vec_index;
>> +               }
>> +       }
>> +
>> +       if (IS_PM_SCAN_GET(p.flags) && p.cur.len) {
> 
> Nit: p.cur.len can be non-zero only if we do a GET (or GET+WP) operation.
I'll remove the first half of condition.

> 
>> +               if (copy_to_user(&vec[vec_index], &p.cur, sizeof(*p.vec))) {
> 
> Nit: sizeof(*p.cur); (even though this is the same type)
Good point.

> 
>> +                       ret = -EFAULT;
>> +                       goto free_data;
>> +               }
>> +               vec_index++;
>> +       }
>> +
>> +       ret = vec_index;
>> +
>> +free_data:
>> +       if (IS_PM_SCAN_WP(p.flags))
>> +               mmu_notifier_invalidate_range_end(&range);
>> +
>> +       kfree(p.vec);
>> +       return ret;
>> +}
>> +
>> +static long do_pagemap_cmd(struct file *file, unsigned int cmd,
>> +                          unsigned long arg)
>> +{
>> +       struct pm_scan_arg __user *uarg = (struct pm_scan_arg __user *)arg;
> 
> The cast should be in do_pagemap_scan() as if there comes another
> `cmd`, then it might use a different argument type.
I'll update.

> 
>> +       struct mm_struct *mm = file->private_data;
>> +
>> +       switch (cmd) {
>> +       case PAGEMAP_SCAN:
>> +               return do_pagemap_scan(mm, uarg);
>> +
>> +       default:
>> +               return -EINVAL;
>> +       }
>> +}
> 
> Best Regards
> Michał Mirosław

-- 
BR,
Muhammad Usama Anjum

Re: [PATCH v17 2/5] fs/proc/task_mmu: Implement IOCTL to get and optionally clear info about PTEs

[Michał Mirosław]

On Wed, 7 Jun 2023 at 18:13, Muhammad Usama Anjum
<usama.anjum@collabora.com> wrote:
>
> Hi Michał,
>
> Thank you for taking time to review!
>
> On 6/7/23 7:52 PM, Michał Mirosław wrote:
> > On Tue, 6 Jun 2023 at 08:08, Muhammad Usama Anjum
> > <usama.anjum@collabora.com> wrote:
> >> This IOCTL, PAGEMAP_SCAN on pagemap file can be used to get and/or clear
> >> the info about page table entries. The following operations are supported
> >> in this ioctl:
> >> - Get the information if the pages have been written-to (PAGE_IS_WRITTEN),
> >>   file mapped (PAGE_IS_FILE), present (PAGE_IS_PRESENT) or swapped
> >>   (PAGE_IS_SWAPPED).
> >> - Find pages which have been written-to and/or write protect the pages
> >>   (atomic PM_SCAN_OP_GET + PM_SCAN_OP_WP)
> >>
> >> This IOCTL can be extended to get information about more PTE bits.
> > [...]
> >> --- a/fs/proc/task_mmu.c
> >> +++ b/fs/proc/task_mmu.c
[...]
> >> +static inline bool pagemap_scan_check_page_written(struct pagemap_scan_private *p)
> >> +{
> >> +       return (p->required_mask | p->anyof_mask | p->excluded_mask) &
> >> +              PAGE_IS_WRITTEN;
> >> +}
> >
> > This could be precalculated and put as a flag into
> > pagemap_scan_private - it is kernel-private structure and there are a
> > few spare bits in `flags` if you'd prefer not to add an explicit
> > boolean.
> This inline function is only being used at one spot. I can remove the
> function altogether. I don't like putting it in flags. It'll bring some
> complexity.

The difference at the call site will be function call vs field access.
Do you mean that moving the function to where the struct is
initialized would add complexity? Why is that?

> > [...]
> >> +static int pagemap_scan_output(bool wt, bool file, bool pres, bool swap,
> >> +                              struct pagemap_scan_private *p,
> >> +                              unsigned long addr, unsigned int n_pages)
> >> +{
> >> +       unsigned long bitmap = PM_SCAN_BITMAP(wt, file, pres, swap);
> >> +       struct page_region *cur = &p->cur;
> >> +
> >> +       if (!n_pages)
> >> +               return -EINVAL;
> >> +
> >> +       if ((p->required_mask & bitmap) != p->required_mask)
> >> +               return 0;
> >> +       if (p->anyof_mask && !(p->anyof_mask & bitmap))
> >> +               return 0;
> >> +       if (p->excluded_mask & bitmap)
> >> +               return 0;
> >> +
> >> +       bitmap &= p->return_mask;
> >> +       if (!bitmap)
> >> +               return 0;
> >> +
> >> +       if (cur->bitmap == bitmap &&
> >> +           cur->start + cur->len * PAGE_SIZE == addr) {
> >> +               cur->len += n_pages;
> >> +               p->found_pages += n_pages;
> >> +       } else {
> >> +               /*
> >> +                * All data is copied to cur first. When more data is found, we
> >> +                * push cur to vec and copy new data to cur. The vec_index
> >> +                * represents the current index of vec array. We add 1 to the
> >> +                * vec_index while performing checks to account for data in cur.
> >> +                */
> >> +               if (cur->len && (p->vec_index + 1) >= p->vec_len)
> >> +                       return -ENOSPC;
> >> +
> >> +               if (cur->len) {
> >> +                       memcpy(&p->vec[p->vec_index], cur, sizeof(*p->vec));
> >> +                       p->vec_index++;
> >> +               }
> >> +
> >> +               cur->start = addr;
> >> +               cur->len = n_pages;
> >> +               cur->bitmap = bitmap;
> >> +               p->found_pages += n_pages;
> >> +       }
> >> +
> >> +       if (p->max_pages && (p->found_pages == p->max_pages))
> >> +               return PM_SCAN_FOUND_MAX_PAGES;
> >> +
> >> +       return 0;
> >> +}
> >> +
> >> +static int pagemap_scan_pmd_entry(pmd_t *pmd, unsigned long start,
> >> +                                 unsigned long end, struct mm_walk *walk)
> >> +{
> >> +       struct pagemap_scan_private *p = walk->private;
> >> +       struct vm_area_struct *vma = walk->vma;
> >> +       unsigned long addr = end;
> >> +       pte_t *pte, *orig_pte;
> >> +       spinlock_t *ptl;
> >> +       bool is_written;
> >> +       int ret = 0;
> >> +
> >> +       arch_enter_lazy_mmu_mode();
> >> +
> >> +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> >> +       ptl = pmd_trans_huge_lock(pmd, vma);
> >> +       if (ptl) {
> >> +               unsigned long n_pages = (end - start)/PAGE_SIZE;
> >> +
> >> +               if (p->max_pages && n_pages > p->max_pages - p->found_pages)
> >> +                       n_pages = p->max_pages - p->found_pages;
> >
> > Since p->found_pages is only ever increased in `pagemap_scan_output()`
> > and that function is only called for GET or GET+WP operations, maybe
> > the logic could be folded to pagemap_scan_output() to avoid
> > duplication?
> > In this function the calculation is used only when WP op is done to
> > split the HP if n_pages limit would be hit, but if using plain WP
> > (without GET) it doesn't make sense to use the limit.
> The n_pages is needed to decide if THP need to be broken down and it is
> used in pagemap_scan_output(). I've brought this condition out of
> pagemap_scan_output() to cater this former condition. If I move it to
> pagemap_scan_output(), I'll have to write same condition to find out if I
> need to breakt he THP. This seems like repetition, but we have same use
> case for tlbhuge page.

My point is that you need to split the THP only if doing a GET+WP
operation. If you only do GET, then the worst case would be for the
process to report a spurious WRITTEN bit if an earlier-visited part of
THP was modified and the scan restarted in the middle of a THP.

>> > (pagemap_scan_output() is trivial enough so I think it could be pulled
> > inside the spinlocked region.)
> It is already in spinlocked region. Spin lock is being released after tlb
> flush.

Ah, I was thinking about calling pagemap_scan_output() before checking
split_huge_pmd() case - and at that use the pagemap_scan_output()'s
return value to do the check.

> >> +               is_written = !is_pmd_uffd_wp(*pmd);
> >> +
> >> +               /*
> >> +                * Break huge page into small pages if the WP operation need to
> >> +                * be performed is on a portion of the huge page.
> >> +                */
> >> +               if (is_written && IS_PM_SCAN_WP(p->flags) &&
> >> +                   n_pages < HPAGE_SIZE/PAGE_SIZE) {
> >> +                       spin_unlock(ptl);
> >> +
> >> +                       split_huge_pmd(vma, pmd, start);
> >> +                       goto process_smaller_pages;
> >> +               }
> >> +
> >> +               if (IS_PM_SCAN_GET(p->flags))
> >> +                       ret = pagemap_scan_output(is_written, vma->vm_file,
> >> +                                                 pmd_present(*pmd),
> >> +                                                 is_swap_pmd(*pmd),
> >> +                                                 p, start, n_pages);
> >> +
> >> +               if (ret >= 0 && is_written && IS_PM_SCAN_WP(p->flags))
> >> +                       make_uffd_wp_pmd(vma, addr, pmd);
> >> +
> >> +               if (IS_PM_SCAN_WP(p->flags))
> >
> > Why `is_written` is not checked? If is_written is false, then the WP
> > op should be a no-op and so won't need TLB flushing, will it? [Same
> > for the PTE case below.]
> It can be done for THP. But for ptes we cannot trust is_written as
> is_written only represent last pte state.

Ok, so the PTE case could use a flag recording whether any PTE had WP
applied instead of `is_written`.

> >> +                       flush_tlb_range(vma, start, end);
> >> +
> > [...]
> >> +       if (IS_PM_SCAN_WP(p->flags))
> >> +               flush_tlb_range(vma, start, addr);
> >> +
> >> +       pte_unmap_unlock(orig_pte, ptl);
> >> +       arch_leave_lazy_mmu_mode();
> >> +
> >> +       cond_resched();
> >> +       return ret;
> >> +}
> >> +
> >> +#ifdef CONFIG_HUGETLB_PAGE
> >> +static int pagemap_scan_hugetlb_entry(pte_t *ptep, unsigned long hmask,
> >> +                                     unsigned long start, unsigned long end,
> >> +                                     struct mm_walk *walk)
> >> +{
> >> +       unsigned long n_pages = (end - start)/PAGE_SIZE;
> >> +       struct pagemap_scan_private *p = walk->private;
> >> +       struct vm_area_struct *vma = walk->vma;
> >> +       struct hstate *h = hstate_vma(vma);
> >> +       spinlock_t *ptl;
> >> +       bool is_written;
> >> +       int ret = 0;
> >> +       pte_t pte;
> >> +
> >> +       if (p->max_pages && n_pages > p->max_pages - p->found_pages)
> >> +               n_pages = p->max_pages - p->found_pages;
> >> +
> >> +       if (IS_PM_SCAN_WP(p->flags)) {
> >> +               i_mmap_lock_write(vma->vm_file->f_mapping);
> >> +               ptl = huge_pte_lock(h, vma->vm_mm, ptep);
> >> +       }
> >> +
> >> +       pte = huge_ptep_get(ptep);
> >> +       is_written = !is_huge_pte_uffd_wp(pte);
> >> +
> >> +       /*
> >> +        * Partial hugetlb page clear isn't supported
> >> +        */
> >> +       if (is_written && IS_PM_SCAN_WP(p->flags) &&
> >> +           n_pages < HPAGE_SIZE/PAGE_SIZE) {
> >> +               ret = -EPERM;
> >
> > Shouldn't this be ENOSPC, conveying that the operation would overflow
> > the n_pages limit?
> We are testing here is user has asked us to engage WP on a part of the
> hugetlb or we can only perform WP on a part of the engage as user buffer is
> full. We cannot judge this has happened because of the former or later
> condition. So I'm assuming that user's parameters aren't solid enough and
> returning -EPERM. It seemed more suitable to me. But I can return -ENOSPC
> as well, if you say?

Those two cases can be differentiated when checked before truncating
n_pages. If a user requests partial WP for a hugetlb page wouldn't
EINVAL (or other error - as this can't ever work) be more appropriate
(this check could happen only at the start of scan)? If the request is
due to max_pages limit (with found_pages > 0), then I'd return ENOSPC
and expect the user to restart the scan with a new buffer.

Our discussion here makes me wonder: what is the expected return value
for the ioctl WP (without GET) operation? If it would return e.g. the
number of 4K-pages successfully scanned, then the caller would be able
to detect the partial tail hugepage case and act accordingly.

> >> +static int pagemap_scan_pte_hole(unsigned long addr, unsigned long end,
> >> +                                int depth, struct mm_walk *walk)
> >> +{
> >> +       unsigned long n_pages = (end - addr)/PAGE_SIZE;
> >> +       struct pagemap_scan_private *p = walk->private;
> >> +       struct vm_area_struct *vma = walk->vma;
> >> +       int ret = 0;
> >> +
> >> +       if (!vma || !IS_PM_SCAN_GET(p->flags))
> >> +               return 0;
> >> +
> >> +       if (p->max_pages && n_pages > p->max_pages - p->found_pages)
> >> +               n_pages = p->max_pages - p->found_pages;
> >
> > Nit: If the page flags don't match (wouldn't be output), the limit
> > would not be hit and the calculation is unnecessary. But if it was
> > done in pagemap_scan_output() instead after all the flags checks...
> Correct for this use case. But moving to pagemap_scan_output() would make
> me do duplicate calculation for other 2 cases as explained above.

(responded below the cases above)

> >> +       ret = pagemap_scan_output(false, vma->vm_file, false, false, p, addr,
> >> +                                 n_pages);
> >> +
> >> +       return ret;
> >> +}
> > [...]
> >> +static long do_pagemap_scan(struct mm_struct *mm,
> >> +                           struct pm_scan_arg __user *uarg)
> >> +{
> >> +       unsigned long start, end, walk_start, walk_end;
> >> +       unsigned long empty_slots, vec_index = 0;
> >> +       struct mmu_notifier_range range;
> >> +       struct page_region __user *vec;
> >> +       struct pagemap_scan_private p;
> >> +       struct pm_scan_arg arg;
> >> +       int ret = 0;
> >> +
> >> +       if (copy_from_user(&arg, uarg, sizeof(arg)))
> >> +               return -EFAULT;
> >> +
> >> +       start = untagged_addr((unsigned long)arg.start);
> >> +       vec = (struct page_region *)untagged_addr((unsigned long)arg.vec);
> >> +
> >> +       ret = pagemap_scan_args_valid(&arg, start, vec);
> >> +       if (ret)
> >> +               return ret;
> >> +
> >> +       end = start + arg.len;
> >> +       p.max_pages = arg.max_pages;
> >> +       p.found_pages = 0;
> >> +       p.flags = arg.flags;
> >> +       p.required_mask = arg.required_mask;
> >> +       p.anyof_mask = arg.anyof_mask;
> >> +       p.excluded_mask = arg.excluded_mask;
> >> +       p.return_mask = arg.return_mask;
> >> +       p.cur.start = p.cur.len = p.cur.bitmap = 0;
> >> +       p.vec = NULL;
> >> +       p.vec_len = PAGEMAP_WALK_SIZE >> PAGE_SHIFT;
> >
> > If p.vec_len would not count the entry held in `cur` (IOW: vec_len =
> > WALK_SIZE - 1), then pagemap_scan_output() wouldn't need the big
> > comment about adding or subtracting 1 when checking for overflow. The
> > output vector needs to have space for at least one entrry to make GET
> > useful. Maybe `cur` could be renamed or annotated to express that it
> > always holds the last entry?
> Ohhh.. This can be done by doing subtracting 1 from empty_slots. But I've
> explored the idea. I don't see any benefit. If we do this, then I'll have
> to put a comment why subtracting 1 is needed. Seems like same problem:
>
> --- a/fs/proc/task_mmu.c
> +++ b/fs/proc/task_mmu.c
> @@ -1909,7 +1909,7 @@ static int pagemap_scan_output(bool wt, bool file,
> bool pres, bool swap,
>                  * represents the current index of vec array. We add 1 to the
>                  * vec_index while performing checks to account for data in
> cur.
>                  */
> -               if (cur->len && (p->vec_index + 1) >= p->vec_len)
> +               if (cur->len && p->vec_index >= p->vec_len)
>                         return -ENOSPC;
>
>                 if (cur->len) {
> @@ -2202,7 +2202,7 @@ static long do_pagemap_scan(struct mm_struct *mm,
>                 if (IS_PM_SCAN_GET(p.flags)) {
>                         p.vec_index = 0;
>
> -                       empty_slots = arg.vec_len - vec_index;
> +                       empty_slots = arg.vec_len - 1 - vec_index;
>                         p.vec_len = min(p.vec_len, empty_slots);
>                 }
>
> Lets leave it as it is. I can change `cur` to `last` or any other name.
> Please suggest.

The difference is that you have the subtraction only once per the
outer page_walk loop iteration, but in the current version the
addition has to happen every pagemap_scan_output() call after a hole.

From the readability perspective, "if (next_index >= vec_len)" is
short and self-documenting. Also I'd use "p.vec_len = min(p.vec_len,
empty_slots - 1)" as it also conveys the intent better (in that `vec`
is holding all but the last entry, but arg.vec_len holds the final
output buffer length).

If we're picking colors, then maybe make `arg.vec_len` have a
different name than `p.vec_len` (same for `vec_index`) so that there
is less confusion possible as to what they refer to. Maybe keep
`arg.vec_len`, but have `p.vec_buf`, `p.vec_buf_len`, and
`p.next_buf_index`?

(Note: you'd also need to decrement p.vec_len where it is first assigned.)

[...]
Best Regards
Michał Mirosław

Re: [PATCH v17 2/5] fs/proc/task_mmu: Implement IOCTL to get and optionally clear info about PTEs

[Muhammad Usama Anjum]

On 6/7/23 9:52 PM, Michał Mirosław wrote:
> On Wed, 7 Jun 2023 at 18:13, Muhammad Usama Anjum
> <usama.anjum@collabora.com> wrote:
>>
>> Hi Michał,
>>
>> Thank you for taking time to review!
>>
>> On 6/7/23 7:52 PM, Michał Mirosław wrote:
>>> On Tue, 6 Jun 2023 at 08:08, Muhammad Usama Anjum
>>> <usama.anjum@collabora.com> wrote:
>>>> This IOCTL, PAGEMAP_SCAN on pagemap file can be used to get and/or clear
>>>> the info about page table entries. The following operations are supported
>>>> in this ioctl:
>>>> - Get the information if the pages have been written-to (PAGE_IS_WRITTEN),
>>>>   file mapped (PAGE_IS_FILE), present (PAGE_IS_PRESENT) or swapped
>>>>   (PAGE_IS_SWAPPED).
>>>> - Find pages which have been written-to and/or write protect the pages
>>>>   (atomic PM_SCAN_OP_GET + PM_SCAN_OP_WP)
>>>>
>>>> This IOCTL can be extended to get information about more PTE bits.
>>> [...]
>>>> --- a/fs/proc/task_mmu.c
>>>> +++ b/fs/proc/task_mmu.c
> [...]
>>>> +static inline bool pagemap_scan_check_page_written(struct pagemap_scan_private *p)
>>>> +{
>>>> +       return (p->required_mask | p->anyof_mask | p->excluded_mask) &
>>>> +              PAGE_IS_WRITTEN;
>>>> +}
>>>
>>> This could be precalculated and put as a flag into
>>> pagemap_scan_private - it is kernel-private structure and there are a
>>> few spare bits in `flags` if you'd prefer not to add an explicit
>>> boolean.
>> This inline function is only being used at one spot. I can remove the
>> function altogether. I don't like putting it in flags. It'll bring some
>> complexity.
> 
> The difference at the call site will be function call vs field access.
> Do you mean that moving the function to where the struct is
> initialized would add complexity? Why is that?
In my view, adding this calculation to `flag` would make the `flag double
meaning. 1) user flags 2) wp flag is turn on or off
Okay, I've added the flag and removed this function call from
frpagemap_scan_test_walk().

> 
>>> [...]
>>>> +static int pagemap_scan_output(bool wt, bool file, bool pres, bool swap,
>>>> +                              struct pagemap_scan_private *p,
>>>> +                              unsigned long addr, unsigned int n_pages)
>>>> +{
>>>> +       unsigned long bitmap = PM_SCAN_BITMAP(wt, file, pres, swap);
>>>> +       struct page_region *cur = &p->cur;
>>>> +
>>>> +       if (!n_pages)
>>>> +               return -EINVAL;
>>>> +
>>>> +       if ((p->required_mask & bitmap) != p->required_mask)
>>>> +               return 0;
>>>> +       if (p->anyof_mask && !(p->anyof_mask & bitmap))
>>>> +               return 0;
>>>> +       if (p->excluded_mask & bitmap)
>>>> +               return 0;
>>>> +
>>>> +       bitmap &= p->return_mask;
>>>> +       if (!bitmap)
>>>> +               return 0;
>>>> +
>>>> +       if (cur->bitmap == bitmap &&
>>>> +           cur->start + cur->len * PAGE_SIZE == addr) {
>>>> +               cur->len += n_pages;
>>>> +               p->found_pages += n_pages;
>>>> +       } else {
>>>> +               /*
>>>> +                * All data is copied to cur first. When more data is found, we
>>>> +                * push cur to vec and copy new data to cur. The vec_index
>>>> +                * represents the current index of vec array. We add 1 to the
>>>> +                * vec_index while performing checks to account for data in cur.
>>>> +                */
>>>> +               if (cur->len && (p->vec_index + 1) >= p->vec_len)
>>>> +                       return -ENOSPC;
>>>> +
>>>> +               if (cur->len) {
>>>> +                       memcpy(&p->vec[p->vec_index], cur, sizeof(*p->vec));
>>>> +                       p->vec_index++;
>>>> +               }
>>>> +
>>>> +               cur->start = addr;
>>>> +               cur->len = n_pages;
>>>> +               cur->bitmap = bitmap;
>>>> +               p->found_pages += n_pages;
>>>> +       }
>>>> +
>>>> +       if (p->max_pages && (p->found_pages == p->max_pages))
>>>> +               return PM_SCAN_FOUND_MAX_PAGES;
>>>> +
>>>> +       return 0;
>>>> +}
>>>> +
>>>> +static int pagemap_scan_pmd_entry(pmd_t *pmd, unsigned long start,
>>>> +                                 unsigned long end, struct mm_walk *walk)
>>>> +{
>>>> +       struct pagemap_scan_private *p = walk->private;
>>>> +       struct vm_area_struct *vma = walk->vma;
>>>> +       unsigned long addr = end;
>>>> +       pte_t *pte, *orig_pte;
>>>> +       spinlock_t *ptl;
>>>> +       bool is_written;
>>>> +       int ret = 0;
>>>> +
>>>> +       arch_enter_lazy_mmu_mode();
>>>> +
>>>> +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
>>>> +       ptl = pmd_trans_huge_lock(pmd, vma);
>>>> +       if (ptl) {
>>>> +               unsigned long n_pages = (end - start)/PAGE_SIZE;
>>>> +
>>>> +               if (p->max_pages && n_pages > p->max_pages - p->found_pages)
>>>> +                       n_pages = p->max_pages - p->found_pages;
>>>
>>> Since p->found_pages is only ever increased in `pagemap_scan_output()`
>>> and that function is only called for GET or GET+WP operations, maybe
>>> the logic could be folded to pagemap_scan_output() to avoid
>>> duplication?
>>> In this function the calculation is used only when WP op is done to
>>> split the HP if n_pages limit would be hit, but if using plain WP
>>> (without GET) it doesn't make sense to use the limit.
>> The n_pages is needed to decide if THP need to be broken down and it is
>> used in pagemap_scan_output(). I've brought this condition out of
>> pagemap_scan_output() to cater this former condition. If I move it to
>> pagemap_scan_output(), I'll have to write same condition to find out if I
>> need to breakt he THP. This seems like repetition, but we have same use
>> case for tlbhuge page.
> 
> My point is that you need to split the THP only if doing a GET+WP
> operation. If you only do GET, then the worst case would be for the
> process to report a spurious WRITTEN bit if an earlier-visited part of
> THP was modified and the scan restarted in the middle of a THP.
We only need to split if doing GET+WP or WP only. In case of WP op, I need
to check if we have less than HPAGE_SIZE/PAGE_SIZE pages and only then
split. So moving this  if condition would not be beneficial.

> 
>>>> (pagemap_scan_output() is trivial enough so I think it could be pulled
>>> inside the spinlocked region.)
>> It is already in spinlocked region. Spin lock is being released after tlb
>> flush.
> 
> Ah, I was thinking about calling pagemap_scan_output() before checking
> split_huge_pmd() case - and at that use the pagemap_scan_output()'s
> return value to do the check.
> 
>>>> +               is_written = !is_pmd_uffd_wp(*pmd);
>>>> +
>>>> +               /*
>>>> +                * Break huge page into small pages if the WP operation need to
>>>> +                * be performed is on a portion of the huge page.
>>>> +                */
>>>> +               if (is_written && IS_PM_SCAN_WP(p->flags) &&
>>>> +                   n_pages < HPAGE_SIZE/PAGE_SIZE) {
>>>> +                       spin_unlock(ptl);
>>>> +
>>>> +                       split_huge_pmd(vma, pmd, start);
>>>> +                       goto process_smaller_pages;
>>>> +               }
>>>> +
>>>> +               if (IS_PM_SCAN_GET(p->flags))
>>>> +                       ret = pagemap_scan_output(is_written, vma->vm_file,
>>>> +                                                 pmd_present(*pmd),
>>>> +                                                 is_swap_pmd(*pmd),
>>>> +                                                 p, start, n_pages);
>>>> +
>>>> +               if (ret >= 0 && is_written && IS_PM_SCAN_WP(p->flags))
>>>> +                       make_uffd_wp_pmd(vma, addr, pmd);
>>>> +
>>>> +               if (IS_PM_SCAN_WP(p->flags))
>>>
>>> Why `is_written` is not checked? If is_written is false, then the WP
>>> op should be a no-op and so won't need TLB flushing, will it? [Same
>>> for the PTE case below.]
>> It can be done for THP. But for ptes we cannot trust is_written as
>> is_written only represent last pte state.
> 
> Ok, so the PTE case could use a flag recording whether any PTE had WP
> applied instead of `is_written`.
Definately, but I'll have to add a extra variable. I'll add it as you have
asked.

> 
>>>> +                       flush_tlb_range(vma, start, end);
>>>> +
>>> [...]
>>>> +       if (IS_PM_SCAN_WP(p->flags))
>>>> +               flush_tlb_range(vma, start, addr);
>>>> +
>>>> +       pte_unmap_unlock(orig_pte, ptl);
>>>> +       arch_leave_lazy_mmu_mode();
>>>> +
>>>> +       cond_resched();
>>>> +       return ret;
>>>> +}
>>>> +
>>>> +#ifdef CONFIG_HUGETLB_PAGE
>>>> +static int pagemap_scan_hugetlb_entry(pte_t *ptep, unsigned long hmask,
>>>> +                                     unsigned long start, unsigned long end,
>>>> +                                     struct mm_walk *walk)
>>>> +{
>>>> +       unsigned long n_pages = (end - start)/PAGE_SIZE;
>>>> +       struct pagemap_scan_private *p = walk->private;
>>>> +       struct vm_area_struct *vma = walk->vma;
>>>> +       struct hstate *h = hstate_vma(vma);
>>>> +       spinlock_t *ptl;
>>>> +       bool is_written;
>>>> +       int ret = 0;
>>>> +       pte_t pte;
>>>> +
>>>> +       if (p->max_pages && n_pages > p->max_pages - p->found_pages)
>>>> +               n_pages = p->max_pages - p->found_pages;
>>>> +
>>>> +       if (IS_PM_SCAN_WP(p->flags)) {
>>>> +               i_mmap_lock_write(vma->vm_file->f_mapping);
>>>> +               ptl = huge_pte_lock(h, vma->vm_mm, ptep);
>>>> +       }
>>>> +
>>>> +       pte = huge_ptep_get(ptep);
>>>> +       is_written = !is_huge_pte_uffd_wp(pte);
>>>> +
>>>> +       /*
>>>> +        * Partial hugetlb page clear isn't supported
>>>> +        */
>>>> +       if (is_written && IS_PM_SCAN_WP(p->flags) &&
>>>> +           n_pages < HPAGE_SIZE/PAGE_SIZE) {
>>>> +               ret = -EPERM;
>>>
>>> Shouldn't this be ENOSPC, conveying that the operation would overflow
>>> the n_pages limit?
>> We are testing here is user has asked us to engage WP on a part of the
>> hugetlb or we can only perform WP on a part of the engage as user buffer is
>> full. We cannot judge this has happened because of the former or later
>> condition. So I'm assuming that user's parameters aren't solid enough and
>> returning -EPERM. It seemed more suitable to me. But I can return -ENOSPC
>> as well, if you say?
> 
> Those two cases can be differentiated when checked before truncating
> n_pages. If a user requests partial WP for a hugetlb page wouldn't
> EINVAL (or other error - as this can't ever work) be more appropriate
> (this check could happen only at the start of scan)? If the request is
> due to max_pages limit (with found_pages > 0), then I'd return ENOSPC
> and expect the user to restart the scan with a new buffer.
Okay. I'll update.

> 
> Our discussion here makes me wonder: what is the expected return value
> for the ioctl WP (without GET) operation? If it would return e.g. the
> number of 4K-pages successfully scanned, then the caller would be able
> to detect the partial tail hugepage case and act accordingly.
Returning error in case of only WP means partial hugetlb is hit. User
should expect the partial wp on the memory area already.

If we really want to return number of pages cleared, possible option of
return value:
* Return 0 means whole region is wp-ed
* Return number of pages wp-ed if some error occurred (we need to handle
this only for hugetlb) OR return -1 * number of pages wp-ed?

An another uniform way can be to sum all wp-ed pages every time and return
them * -. -1 is needed to let the know user that wp operation got halted
mid way.

> 
>>>> +static int pagemap_scan_pte_hole(unsigned long addr, unsigned long end,
>>>> +                                int depth, struct mm_walk *walk)
>>>> +{
>>>> +       unsigned long n_pages = (end - addr)/PAGE_SIZE;
>>>> +       struct pagemap_scan_private *p = walk->private;
>>>> +       struct vm_area_struct *vma = walk->vma;
>>>> +       int ret = 0;
>>>> +
>>>> +       if (!vma || !IS_PM_SCAN_GET(p->flags))
>>>> +               return 0;
>>>> +
>>>> +       if (p->max_pages && n_pages > p->max_pages - p->found_pages)
>>>> +               n_pages = p->max_pages - p->found_pages;
>>>
>>> Nit: If the page flags don't match (wouldn't be output), the limit
>>> would not be hit and the calculation is unnecessary. But if it was
>>> done in pagemap_scan_output() instead after all the flags checks...
>> Correct for this use case. But moving to pagemap_scan_output() would make
>> me do duplicate calculation for other 2 cases as explained above.
> 
> (responded below the cases above)
> 
>>>> +       ret = pagemap_scan_output(false, vma->vm_file, false, false, p, addr,
>>>> +                                 n_pages);
>>>> +
>>>> +       return ret;
>>>> +}
>>> [...]
>>>> +static long do_pagemap_scan(struct mm_struct *mm,
>>>> +                           struct pm_scan_arg __user *uarg)
>>>> +{
>>>> +       unsigned long start, end, walk_start, walk_end;
>>>> +       unsigned long empty_slots, vec_index = 0;
>>>> +       struct mmu_notifier_range range;
>>>> +       struct page_region __user *vec;
>>>> +       struct pagemap_scan_private p;
>>>> +       struct pm_scan_arg arg;
>>>> +       int ret = 0;
>>>> +
>>>> +       if (copy_from_user(&arg, uarg, sizeof(arg)))
>>>> +               return -EFAULT;
>>>> +
>>>> +       start = untagged_addr((unsigned long)arg.start);
>>>> +       vec = (struct page_region *)untagged_addr((unsigned long)arg.vec);
>>>> +
>>>> +       ret = pagemap_scan_args_valid(&arg, start, vec);
>>>> +       if (ret)
>>>> +               return ret;
>>>> +
>>>> +       end = start + arg.len;
>>>> +       p.max_pages = arg.max_pages;
>>>> +       p.found_pages = 0;
>>>> +       p.flags = arg.flags;
>>>> +       p.required_mask = arg.required_mask;
>>>> +       p.anyof_mask = arg.anyof_mask;
>>>> +       p.excluded_mask = arg.excluded_mask;
>>>> +       p.return_mask = arg.return_mask;
>>>> +       p.cur.start = p.cur.len = p.cur.bitmap = 0;
>>>> +       p.vec = NULL;
>>>> +       p.vec_len = PAGEMAP_WALK_SIZE >> PAGE_SHIFT;
>>>
>>> If p.vec_len would not count the entry held in `cur` (IOW: vec_len =
>>> WALK_SIZE - 1), then pagemap_scan_output() wouldn't need the big
>>> comment about adding or subtracting 1 when checking for overflow. The
>>> output vector needs to have space for at least one entrry to make GET
>>> useful. Maybe `cur` could be renamed or annotated to express that it
>>> always holds the last entry?
>> Ohhh.. This can be done by doing subtracting 1 from empty_slots. But I've
>> explored the idea. I don't see any benefit. If we do this, then I'll have
>> to put a comment why subtracting 1 is needed. Seems like same problem:
>>
>> --- a/fs/proc/task_mmu.c
>> +++ b/fs/proc/task_mmu.c
>> @@ -1909,7 +1909,7 @@ static int pagemap_scan_output(bool wt, bool file,
>> bool pres, bool swap,
>>                  * represents the current index of vec array. We add 1 to the
>>                  * vec_index while performing checks to account for data in
>> cur.
>>                  */
>> -               if (cur->len && (p->vec_index + 1) >= p->vec_len)
>> +               if (cur->len && p->vec_index >= p->vec_len)
>>                         return -ENOSPC;
>>
>>                 if (cur->len) {
>> @@ -2202,7 +2202,7 @@ static long do_pagemap_scan(struct mm_struct *mm,
>>                 if (IS_PM_SCAN_GET(p.flags)) {
>>                         p.vec_index = 0;
>>
>> -                       empty_slots = arg.vec_len - vec_index;
>> +                       empty_slots = arg.vec_len - 1 - vec_index;
>>                         p.vec_len = min(p.vec_len, empty_slots);
>>                 }
>>
>> Lets leave it as it is. I can change `cur` to `last` or any other name.
>> Please suggest.
> 
> The difference is that you have the subtraction only once per the
> outer page_walk loop iteration, but in the current version the
> addition has to happen every pagemap_scan_output() call after a hole.
I'll update as you are saying. After updating, we cannot abort this outer
loop if length is 0 as the last element is present in cur.

> 
> From the readability perspective, "if (next_index >= vec_len)" is
> short and self-documenting. Also I'd use "p.vec_len = min(p.vec_len,
> empty_slots - 1)" as it also conveys the intent better (in that `vec`
> is holding all but the last entry, but arg.vec_len holds the final
> output buffer length).
> 
> If we're picking colors, then maybe make `arg.vec_len` have a
> different name than `p.vec_len` (same for `vec_index`) so that there
> is less confusion possible as to what they refer to. Maybe keep
> `arg.vec_len`, but have `p.vec_buf`, `p.vec_buf_len`, and
> `p.next_buf_index`?
So we do have colors in variable names. :) I'll update.

> 
> (Note: you'd also need to decrement p.vec_len where it is first assigned.)
> 
> [...]
> Best Regards
> Michał Mirosław

-- 
BR,
Muhammad Usama Anjum