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

[Andrei Vagin <avagin@gmail.com>]

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);

> +		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 was trying to integrate this new interace into CRIU and I found
one more thing that is required. We need to detect zero pages.

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)));

Thanks,
Andrei