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

[Muhammad Usama Anjum]

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@xxxxxxxxxx

> 
>> +		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?

> 
> 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)
- 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)


> 
> Thanks,
> Andrei

-- 
BR,
Muhammad Usama Anjum