Re: [PATCH v19 2/5] fs/proc/task_mmu: Implement IOCTL to get and optionally clear info about PTEs

[Muhammad Usama Anjum <usama.anjum@collabora.com>]

On 6/21/23 11:42 AM, Andrei Vagin wrote:
> On Mon, Jun 19, 2023 at 11:06:36AM +0500, Muhammad Usama Anjum wrote:
>> On 6/17/23 11:39 AM, Andrei Vagin wrote:
>>> On Thu, Jun 15, 2023 at 07:11:41PM +0500, Muhammad Usama Anjum wrote:
>>>> +static int pagemap_scan_pmd_entry(pmd_t *pmd, unsigned long start,
>>>> +				  unsigned long end, struct mm_walk *walk)
>>>> +{
>>>> +	bool is_written, flush = false, is_interesting = true;
>>>> +	struct pagemap_scan_private *p = walk->private;
>>>> +	struct vm_area_struct *vma = walk->vma;
>>>> +	unsigned long bitmap, addr = end;
>>>> +	pte_t *pte, *orig_pte, ptent;
>>>> +	spinlock_t *ptl;
>>>> +	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;
>>>> +
>>>> +		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;
>>>> +		}
>>>> +
>>>> +		bitmap = PM_SCAN_FLAGS(is_written, (bool)vma->vm_file,
>>>> +				       pmd_present(*pmd), is_swap_pmd(*pmd));
>>>> +
>>>> +		if (IS_PM_SCAN_GET(p->flags)) {
>>>> +			is_interesting = pagemap_scan_is_interesting_page(bitmap, p);
>>>> +			if (is_interesting)
>>>> +				ret = pagemap_scan_output(bitmap, p, start, n_pages);
>>>> +		}
>>>> +
>>>> +		if (IS_PM_SCAN_WP(p->flags) && is_written && is_interesting &&
>>>> +		    ret >= 0) {
>>>> +			make_uffd_wp_pmd(vma, start, pmd);
>>>> +			flush_tlb_range(vma, start, end);
>>>> +		}
>>>> +
>>>> +		spin_unlock(ptl);
>>>> +
>>>> +		arch_leave_lazy_mmu_mode();
>>>> +		return ret;
>>>> +	}
>>>> +
>>>> +process_smaller_pages:
>>>> +#endif
>>>> +
>>>> +	orig_pte = pte = pte_offset_map_lock(vma->vm_mm, pmd, start, &ptl);
>>>> +	if (!pte) {
>>>
>>> Do we need to unlock ptl here?
>>>
>>> 		spin_unlock(ptl);
>> No, please look at these recently merged patches:
>> https://lore.kernel.org/all/c1c9a74a-bc5b-15ea-e5d2-8ec34bc921d@google.com
>>
>>>
>>>> +		walk->action = ACTION_AGAIN;
>>>> +		return 0;
>>>> +	}
>>>> +
>>>> +	for (addr = start; addr < end && !ret; pte++, addr += PAGE_SIZE) {
>>>> +		ptent = ptep_get(pte);
>>>> +		is_written = !is_pte_uffd_wp(ptent);
>>>> +
>>>> +		bitmap = PM_SCAN_FLAGS(is_written, (bool)vma->vm_file,
>>>> +				       pte_present(ptent), is_swap_pte(ptent));
>>>
>>> The vma->vm_file check isn't correct in this case. You can look when
>>> pte_to_pagemap_entry sets PM_FILE. This flag is used to detect what
>>> pages have a file backing store and what pages are anonymous.
>> I'll update.
>>
>>>
>>> I was trying to integrate this new interace into CRIU and I found
>>> one more thing that is required. We need to detect zero pages.
>> Should we name it ZERO_PFN_PRESENT_PAGE to be exact or what?
> 
> IMHO, ZERO_PFN_PRESENT_PAGE looks a bit monstrous.
> It looks like zero page is a proper noun in the kernel, so PAGE_IS_ZERO
> might be a good choice here, but it is up to you.
> 
>>
>>>
>>> It should look something like this:
>>>
>>> #define PM_SCAN_FLAGS(wt, file, present, swap, zero)   \
>>>        ((wt) | ((file) << 1) | ((present) << 2) | ((swap) << 3) | ((zero) << 4))
>>>
>>>
>>> bitmap = PM_SCAN_FLAGS(is_written, page && !PageAnon(page),
>>> 		      pte_present(ptent), is_swap_pte(ptent),
>>> 		      pte_present(ptent) && is_zero_pfn(pte_pfn(ptent)));
>> Okay. Can you please confirm my assumptions:
>> - A THP cannot be file backed. (PM_FILE isn't being set for THP case)
> 
> ```
> Currently THP only works for anonymous memory mappings and tmpfs/shmem.
> But in the future it can expand to other filesystems. 
> ```
> https://www.kernel.org/doc/html/next/admin-guide/mm/transhuge.html
> 
> so THP can be "file backed".
> 
>> - A hole is also not file backed.
>>
>> A hole isn't present in memory. So its pfn would be zero. But as it isn't
>> present, it shouldn't report zero page. Right? For hole::
>>
>> PM_SCAN_FLAGS(false, false, false, false, false)
> 
> This looks correct to me.
Thanks. I'll sent next version in a few minutes.


> 

-- 
BR,
Muhammad Usama Anjum