Commit a2880202 authored by Kiryl Shutsemau's avatar Kiryl Shutsemau Committed by Andrew Morton
Browse files

mm/rmap: fix a mlock race condition in folio_referenced_one() 

The mlock_vma_folio() function requires the page table lock to be held in
order to safely mlock the folio.  However, folio_referenced_one() mlocks a
large folios outside of the page_vma_mapped_walk() loop where the page
table lock has already been dropped.

Rework the mlock logic to use the same code path inside the loop for both
large and small folios.

Use PVMW_PGTABLE_CROSSED to detect when the folio is mapped across a page
table boundary.

[akpm@linux-foundation.org: s/CROSSSED/CROSSED/]
Link: https://lkml.kernel.org/r/20250923110711.690639-3-kirill@shutemov.name


Signed-off-by: default avatarKiryl Shutsemau <kas@kernel.org>
Reviewed-by: default avatarShakeel Butt <shakeel.butt@linux.dev>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
parent 2db57983

mm/rmap.c

+20 −37
Original line number Diff line number Diff line
@@ -850,35 +850,35 @@ static bool folio_referenced_one(struct folio *folio, 
{

	struct folio_referenced_arg *pra = arg;

	DEFINE_FOLIO_VMA_WALK(pvmw, folio, vma, address, 0);

	int referenced = 0;

	unsigned long start = address, ptes = 0;

	int ptes = 0, referenced = 0;



	while (page_vma_mapped_walk(&pvmw)) {

		address = pvmw.address;



		if (vma->vm_flags & VM_LOCKED) {

			if (!folio_test_large(folio) || !pvmw.pte) {

			ptes++;

			pra->mapcount--;



			/* Only mlock fully mapped pages */

			if (pvmw.pte && ptes != pvmw.nr_pages)

				continue;



			/*

			 * All PTEs must be protected by page table lock in

			 * order to mlock the page.

			 *

			 * If page table boundary has been cross, current ptl

			 * only protect part of ptes.

			 */

			if (pvmw.flags & PVMW_PGTABLE_CROSSED)

				continue;



			/* Restore the mlock which got missed */

			mlock_vma_folio(folio, vma);

			page_vma_mapped_walk_done(&pvmw);

			pra->vm_flags |= VM_LOCKED;

			return false; /* To break the loop */

		}

			/*

			 * For large folio fully mapped to VMA, will

			 * be handled after the pvmw loop.

			 *

			 * For large folio cross VMA boundaries, it's

			 * expected to be picked  by page reclaim. But

			 * should skip reference of pages which are in

			 * the range of VM_LOCKED vma. As page reclaim

			 * should just count the reference of pages out

			 * the range of VM_LOCKED vma.

			 */

			ptes++;

			pra->mapcount--;

			continue;

		}



		/*

		 * Skip the non-shared swapbacked folio mapped solely by
@@ -913,23 +913,6 @@ static bool folio_referenced_one(struct folio *folio, 
		pra->mapcount--;

	}



	if ((vma->vm_flags & VM_LOCKED) &&

			folio_test_large(folio) &&

			folio_within_vma(folio, vma)) {

		unsigned long s_align, e_align;



		s_align = ALIGN_DOWN(start, PMD_SIZE);

		e_align = ALIGN_DOWN(start + folio_size(folio) - 1, PMD_SIZE);



		/* folio doesn't cross page table boundary and fully mapped */

		if ((s_align == e_align) && (ptes == folio_nr_pages(folio))) {

			/* Restore the mlock which got missed */

			mlock_vma_folio(folio, vma);

			pra->vm_flags |= VM_LOCKED;

			return false; /* To break the loop */

		}

	}



	if (referenced)

		folio_clear_idle(folio);

	if (folio_test_clear_young(folio))