Re: [PATCH v7 3/4] fs/proc/task_mmu: Implement IOCTL to get and/or the clear info about PTEs

[Peter Xu <peterx@redhat.com>]

On Mon, Jan 09, 2023 at 11:45:18AM +0500, Muhammad Usama Anjum 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_Wt),
>   file mapped (PAGE_IS_FILE), present (PAGE_IS_PRESENT) or swapped
>   (PAGE_IS_SWAPPED).
> - Write-protect the pages (PAGEMAP_WP_ENGAGE) to start finding which
>   pages have been written-to.
> - Find pages which have been written-to and write protect the pages
>   (atomic PAGE_IS_NOT_WP + PAGEMAP_WP_ENGAGE)
> 
> The uffd should have been registered on the memory range before performing
> any WP/WT (Write Protect/Writtern-To) related operations with the IOCTL.
> 
> struct pagemap_scan_args is used as the argument of the IOCTL. In this
> struct:
> - The range is specified through start and len.
> - The output buffer and size is specified as vec and vec_len.
> - The optional maximum requested pages are specified in the max_pages.
> - The flags can be specified in the flags field. The PAGEMAP_WP_ENGAGE
>   is the only added flag at this time.
> - The masks are specified in required_mask, anyof_mask, excluded_ mask
>   and return_mask.
> 
> This IOCTL can be extended to get information about more PTE bits. This
> patch has evolved from a basic patch from Gabriel Krisman Bertazi.
> 
> Signed-off-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
> ---
> Changes in v7:
> - Rebase on top of latest next
> - Fix some corner cases
> - Base soft-dirty on the uffd wp async
> - Update the terminologies
> - Optimize the memory usage inside the ioctl
> 
> Changes in v6:
> - Rename variables and update comments
> - Make IOCTL independent of soft_dirty config
> - Change masks and bitmap type to _u64
> - Improve code quality
> 
> Changes in v5:
> - Remove tlb flushing even for clear operation
> 
> Changes in v4:
> - Update the interface and implementation
> 
> Changes in v3:
> - Tighten the user-kernel interface by using explicit types and add more
>   error checking
> 
> Changes in v2:
> - Convert the interface from syscall to ioctl
> - Remove pidfd support as it doesn't make sense in ioctl
> ---
>  fs/proc/task_mmu.c            | 300 ++++++++++++++++++++++++++++++++++
>  include/uapi/linux/fs.h       |  50 ++++++
>  tools/include/uapi/linux/fs.h |  50 ++++++
>  3 files changed, 400 insertions(+)
> 
> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> index e35a0398db63..ba70faadf403 100644
> --- a/fs/proc/task_mmu.c
> +++ b/fs/proc/task_mmu.c
> @@ -19,6 +19,7 @@
>  #include <linux/shmem_fs.h>
>  #include <linux/uaccess.h>
>  #include <linux/pkeys.h>
> +#include <linux/minmax.h>
>  
>  #include <asm/elf.h>
>  #include <asm/tlb.h>
> @@ -1135,6 +1136,22 @@ static inline void clear_soft_dirty(struct vm_area_struct *vma,
>  }
>  #endif
>  
> +static inline bool is_pte_uffd_wp(pte_t pte)
> +{
> +	if ((pte_present(pte) && pte_uffd_wp(pte)) ||
> +	    (is_swap_pte(pte) && pte_swp_uffd_wp(pte)))
> +		return true;

This is an interesting way to detect whether the page is written..  I would
have thought you would reuse soft-dirty bit.

For swap case, you missed the pte markers (which can exist for !anon
memory).  You can have a look at pte_swp_uffd_wp_any().

> +	return false;
> +}
> +
> +static inline bool is_pmd_uffd_wp(pmd_t pmd)
> +{
> +	if ((pmd_present(pmd) && pmd_uffd_wp(pmd)) ||
> +	    (is_swap_pmd(pmd) && pmd_swp_uffd_wp(pmd)))
> +		return true;
> +	return false;
> +}
> +
>  #if defined(CONFIG_MEM_SOFT_DIRTY) && defined(CONFIG_TRANSPARENT_HUGEPAGE)
>  static inline void clear_soft_dirty_pmd(struct vm_area_struct *vma,
>  		unsigned long addr, pmd_t *pmdp)
> @@ -1763,11 +1780,294 @@ static int pagemap_release(struct inode *inode, struct file *file)
>  	return 0;
>  }
>  
> +#define PAGEMAP_OP_MASK		(PAGE_IS_WT | PAGE_IS_FILE |	\
> +				 PAGE_IS_PRESENT | PAGE_IS_SWAPPED)
> +#define PAGEMAP_NONWT_OP_MASK	(PAGE_IS_FILE |	PAGE_IS_PRESENT | PAGE_IS_SWAPPED)
> +#define IS_WP_ENGAGE_OP(a)	(a->flags & PAGEMAP_WP_ENGAGE)
> +#define IS_GET_OP(a)		(a->vec)
> +#define PAGEMAP_SCAN_BITMAP(wt, file, present, swap)	\
> +	(wt | file << 1 | present << 2 | swap << 3)
> +#define IS_WT_REQUIRED(a)				\
> +	((a->required_mask & PAGE_IS_WT) ||	\
> +	 (a->anyof_mask & PAGE_IS_WT))
> +
> +struct pagemap_scan_private {
> +	struct page_region *vec;
> +	struct page_region prev;
> +	unsigned long vec_len, vec_index;
> +	unsigned int max_pages, found_pages, flags;
> +	unsigned long required_mask, anyof_mask, excluded_mask, return_mask;
> +};
> +
> +static int pagemap_scan_test_walk(unsigned long start, unsigned long end, struct mm_walk *walk)
> +{
> +	struct pagemap_scan_private *p = walk->private;
> +	struct vm_area_struct *vma = walk->vma;
> +
> +	if (IS_WT_REQUIRED(p) && !vma_can_userfault(vma, vma->vm_flags))
> +		return -EPERM;

vma_can_userfault() is too coarse.  Maybe what you wanted to check is
userfaultfd_wp(vma)?

> +	if (IS_GET_OP(p) && p->max_pages && (p->found_pages == p->max_pages))
> +		return -ENOSPC;

This is the function to test "whether the walker should walk the vma
specified".  This check should IIUC be meaningless because found_pages
doesn't boost during vma switching, while OTOH your pmd walker fn should do
proper check when increasing found_pages and return -ENOSPC properly when
the same condition met.  That should be enough, IMHO.

I saw it already a few times in this patch, I think maybe you want a helper
for this one taking *p as argument.

> +	if (vma->vm_flags & VM_PFNMAP)
> +		return 1;
> +	return 0;
> +}
> +
> +static inline int add_to_out(bool wt, bool file, bool pres, bool swap,
> +			     struct pagemap_scan_private *p, unsigned long addr, unsigned int len)
> +{
> +	unsigned long bitmap, cur = PAGEMAP_SCAN_BITMAP(wt, file, pres, swap);
> +	bool cpy = true;
> +	struct page_region *prev = &p->prev;
> +
> +	if (p->required_mask)
> +		cpy = ((p->required_mask & cur) == p->required_mask);
> +	if (cpy && p->anyof_mask)
> +		cpy = (p->anyof_mask & cur);
> +	if (cpy && p->excluded_mask)
> +		cpy = !(p->excluded_mask & cur);
> +	bitmap = cur & p->return_mask;
> +	if (cpy && bitmap) {
> +		if ((prev->len) && (prev->bitmap == bitmap) &&
> +		    (prev->start + prev->len * PAGE_SIZE == addr)) {
> +			prev->len += len;
> +			p->found_pages += len;
> +		} else if (p->vec_index < p->vec_len) {
> +			if (prev->len) {
> +				memcpy(&p->vec[p->vec_index], prev, sizeof(struct page_region));
> +				p->vec_index++;
> +			}
> +			prev->start = addr;
> +			prev->len = len;
> +			prev->bitmap = bitmap;
> +			p->found_pages += len;
> +		} else {
> +			return -ENOSPC;
> +		}

[1]

> +	}
> +	return 0;
> +}
> +
> +static inline int export_prev_to_out(struct pagemap_scan_private *p, struct page_region __user *vec,
> +				     unsigned long *vec_index)
> +{
> +	struct page_region *prev = &p->prev;
> +
> +	if (prev->len) {
> +		if (copy_to_user(&vec[*vec_index], prev, sizeof(struct page_region)))
> +			return -EFAULT;
> +		p->vec_index++;
> +		(*vec_index)++;
> +		prev->len = 0;
> +	}
> +	return 0;
> +}

I'd rather not have this helper; it doesn't really help a lot.

> +
> +static inline int pagemap_scan_pmd_entry(pmd_t *pmd, unsigned long addr,
> +					 unsigned long end, struct mm_walk *walk)
> +{
> +	struct pagemap_scan_private *p = walk->private;
> +	struct vm_area_struct *vma = walk->vma;
> +	unsigned long start = addr;
> +	unsigned int len;
> +	spinlock_t *ptl;
> +	int ret = 0;
> +	pte_t *pte;
> +	bool pmd_wt;
> +
> +	if ((walk->vma->vm_end < addr) || (p->max_pages && p->found_pages == p->max_pages))
> +		return 0;
> +	end = min(end, walk->vma->vm_end);

None of the check above is needed, I think..

vma ranges are checked before pmd_entry() called.  found_pages should be
checked when increased (I think it's [1] above).

> +
> +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> +	ptl = pmd_trans_huge_lock(pmd, vma);
> +	if (ptl) {
> +		pmd_wt = !is_pmd_uffd_wp(*pmd);
> +		/*
> +		 * Break huge page into small pages if operation needs to be performed is
> +		 * on a portion of the huge page or the return buffer cannot store complete
> +		 * data.
> +		 */
> +		if (pmd_wt && (IS_WP_ENGAGE_OP(p) && (end - addr < HPAGE_SIZE))) {
> +			spin_unlock(ptl);
> +			split_huge_pmd(vma, pmd, addr);
> +			goto process_smaller_pages;
> +		}
> +		if (IS_GET_OP(p)) {
> +			len = (end - addr)/PAGE_SIZE;
> +			if (p->max_pages && p->found_pages + len > p->max_pages)
> +				len = p->max_pages - p->found_pages;
> +
> +			ret = add_to_out(pmd_wt, vma->vm_file, pmd_present(*pmd),
> +					 is_swap_pmd(*pmd), p, addr, len);
> +			if (ret) {
> +				spin_unlock(ptl);
> +				return ret;
> +			}
> +		}
> +		spin_unlock(ptl);
> +		if (IS_WP_ENGAGE_OP(p) && pmd_wt) {
> +			BUG_ON(start & ~HPAGE_MASK);
> +
> +			ret = wp_range_async(vma, addr, HPAGE_SIZE);
> +		}
> +		return ret;
> +	}
> +process_smaller_pages:
> +	if (pmd_trans_unstable(pmd))
> +		return 0;
> +#endif /* CONFIG_TRANSPARENT_HUGEPAGE */
> +
> +	pte = pte_offset_map_lock(vma->vm_mm, pmd, addr, &ptl);
> +	for (; addr < end && !ret && (!p->max_pages || (p->found_pages < p->max_pages))
> +	     ; pte++, addr += PAGE_SIZE) {
> +		if (IS_GET_OP(p)) {

This 'if' should be out - skip loop if not needed.

> +			ret = add_to_out(!is_pte_uffd_wp(*pte), vma->vm_file, pte_present(*pte),
> +					 is_swap_pte(*pte), p, addr, 1);
> +			if (ret)
> +				break;
> +		}
> +	}
> +	pte_unmap_unlock(pte - 1, ptl);
> +	if (IS_WP_ENGAGE_OP(p)) {
> +		BUG_ON(start & ~PAGE_MASK);
> +		ret = wp_range_async(vma, start, addr - start);
> +	}
> +
> +	cond_resched();
> +	return ret;
> +}
> +
> +static int pagemap_scan_pte_hole(unsigned long addr, unsigned long end, int depth,
> +				 struct mm_walk *walk)
> +{
> +	struct pagemap_scan_private *p = walk->private;
> +	struct vm_area_struct *vma = walk->vma;
> +	unsigned int len;
> +	int ret = 0;
> +
> +	if (vma) {
> +		len = (end - addr)/PAGE_SIZE;
> +		if (p->max_pages && p->found_pages + len > p->max_pages)
> +			len = p->max_pages - p->found_pages;
> +		ret = add_to_out(false, vma->vm_file, false, false, p, addr, len);
> +	}
> +
> +	return ret;
> +}
> +
> +static const struct mm_walk_ops pagemap_scan_ops = {
> +	.test_walk = pagemap_scan_test_walk,
> +	.pmd_entry = pagemap_scan_pmd_entry,

Do we care about hugetlb at all?  So far it seems you don't.  It's fine,
then if you decided to go the uffd-wp way you can explicit declare no
support on hugetlb, as uffd-wp sync supports it so it should be by default
supported otherwise.

> +	.pte_hole = pagemap_scan_pte_hole,
> +};

-- 
Peter Xu