Commit 9658d698 authored by Lance Yang's avatar Lance Yang Committed by Andrew Morton
Browse files

mm/rmap: fix soft-dirty and uffd-wp bit loss when remapping zero-filled mTHP... 

mm/rmap: fix soft-dirty and uffd-wp bit loss when remapping zero-filled mTHP subpage to shared zeropage

When splitting an mTHP and replacing a zero-filled subpage with the shared
zeropage, try_to_map_unused_to_zeropage() currently drops several
important PTE bits.

For userspace tools like CRIU, which rely on the soft-dirty mechanism for
incremental snapshots, losing the soft-dirty bit means modified pages are
missed, leading to inconsistent memory state after restore.

As pointed out by David, the more critical uffd-wp bit is also dropped. 
This breaks the userfaultfd write-protection mechanism, causing writes to
be silently missed by monitoring applications, which can lead to data
corruption.

Preserve both the soft-dirty and uffd-wp bits from the old PTE when
creating the new zeropage mapping to ensure they are correctly tracked.

Link: https://lkml.kernel.org/r/20250930081040.80926-1-lance.yang@linux.dev


Fixes: b1f20206 ("mm: remap unused subpages to shared zeropage when splitting isolated thp")
Signed-off-by: default avatarLance Yang <lance.yang@linux.dev>
Suggested-by: default avatarDavid Hildenbrand <david@redhat.com>
Suggested-by: default avatarDev Jain <dev.jain@arm.com>
Acked-by: default avatarDavid Hildenbrand <david@redhat.com>
Reviewed-by: default avatarDev Jain <dev.jain@arm.com>
Acked-by: default avatarZi Yan <ziy@nvidia.com>
Reviewed-by: default avatarLiam R. Howlett <Liam.Howlett@oracle.com>
Reviewed-by: default avatarHarry Yoo <harry.yoo@oracle.com>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Barry Song <baohua@kernel.org>
Cc: Byungchul Park <byungchul@sk.com>
Cc: Gregory Price <gourry@gourry.net>
Cc: "Huang, Ying" <ying.huang@linux.alibaba.com>
Cc: Jann Horn <jannh@google.com>
Cc: Joshua Hahn <joshua.hahnjy@gmail.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Mariano Pache <npache@redhat.com>
Cc: Mathew Brost <matthew.brost@intel.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Rakie Kim <rakie.kim@sk.com>
Cc: Rik van Riel <riel@surriel.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Usama Arif <usamaarif642@gmail.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Yu Zhao <yuzhao@google.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
parent 1ce6473d

mm/migrate.c

+10 −5
Original line number Diff line number Diff line
@@ -296,8 +296,7 @@ bool isolate_folio_to_list(struct folio *folio, struct list_head *list) 
}



static bool try_to_map_unused_to_zeropage(struct page_vma_mapped_walk *pvmw,

					  struct folio *folio,

					  unsigned long idx)

		struct folio *folio, pte_t old_pte, unsigned long idx)

{

	struct page *page = folio_page(folio, idx);

	pte_t newpte;
@@ -306,7 +305,7 @@ static bool try_to_map_unused_to_zeropage(struct page_vma_mapped_walk *pvmw, 
		return false;

	VM_BUG_ON_PAGE(!PageAnon(page), page);

	VM_BUG_ON_PAGE(!PageLocked(page), page);

	VM_BUG_ON_PAGE(pte_present(ptep_get(pvmw->pte)), page);

	VM_BUG_ON_PAGE(pte_present(old_pte), page);



	if (folio_test_mlocked(folio) || (pvmw->vma->vm_flags & VM_LOCKED) ||

	    mm_forbids_zeropage(pvmw->vma->vm_mm))
@@ -322,6 +321,12 @@ static bool try_to_map_unused_to_zeropage(struct page_vma_mapped_walk *pvmw, 


	newpte = pte_mkspecial(pfn_pte(my_zero_pfn(pvmw->address),

					pvmw->vma->vm_page_prot));



	if (pte_swp_soft_dirty(old_pte))

		newpte = pte_mksoft_dirty(newpte);

	if (pte_swp_uffd_wp(old_pte))

		newpte = pte_mkuffd_wp(newpte);



	set_pte_at(pvmw->vma->vm_mm, pvmw->address, pvmw->pte, newpte);



	dec_mm_counter(pvmw->vma->vm_mm, mm_counter(folio));
@@ -364,13 +369,13 @@ static bool remove_migration_pte(struct folio *folio, 
			continue;

		}

#endif

		old_pte = ptep_get(pvmw.pte);

		if (rmap_walk_arg->map_unused_to_zeropage &&

		    try_to_map_unused_to_zeropage(&pvmw, folio, idx))

		    try_to_map_unused_to_zeropage(&pvmw, folio, old_pte, idx))

			continue;



		folio_get(folio);

		pte = mk_pte(new, READ_ONCE(vma->vm_page_prot));

		old_pte = ptep_get(pvmw.pte);



		entry = pte_to_swp_entry(old_pte);

		if (!is_migration_entry_young(entry))