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Message-ID: <2d21c9bc-e299-4ca6-85ba-b01a1f346d9d@linux.dev>
Date: Tue, 30 Sep 2025 15:29:52 +0800
From: Lance Yang <lance.yang@...ux.dev>
To: david@...hat.com, dev.jain@....com
Cc: peterx@...hat.com, ziy@...dia.com, baolin.wang@...ux.alibaba.com,
baohua@...nel.org, ryan.roberts@....com, npache@...hat.com,
riel@...riel.com, Liam.Howlett@...cle.com, vbabka@...e.cz,
harry.yoo@...cle.com, jannh@...gle.com, matthew.brost@...el.com,
joshua.hahnjy@...il.com, rakie.kim@...com, byungchul@...com,
gourry@...rry.net, ying.huang@...ux.alibaba.com, apopple@...dia.com,
usamaarif642@...il.com, yuzhao@...gle.com, linux-kernel@...r.kernel.org,
linux-mm@...ck.org, ioworker0@...il.com, stable@...r.kernel.org,
lorenzo.stoakes@...cle.com, akpm@...ux-foundation.org
Subject: Re: [PATCH v4 1/1] mm/rmap: fix soft-dirty and uffd-wp bit loss when
remapping zero-filled mTHP subpage to shared zeropage
On 2025/9/30 15:10, Lance Yang wrote:
> From: Lance Yang <lance.yang@...ux.dev>
>
> 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.
>
> Cc: <stable@...r.kernel.org>
> Fixes: b1f202060afe ("mm: remap unused subpages to shared zeropage when splitting isolated thp")
> Suggested-by: David Hildenbrand <david@...hat.com>
> Suggested-by: Dev Jain <dev.jain@....com>
> Acked-by: David Hildenbrand <david@...hat.com>
> Reviewed-by: Dev Jain <dev.jain@....com>
> Signed-off-by: Lance Yang <lance.yang@...ux.dev>
> ---
> v3 -> v4:
> - Minor formatting tweak in try_to_map_unused_to_zeropage() function
> signature (per David and Dev)
> - Collect Reviewed-by from Dev - thanks!
> - https://lore.kernel.org/linux-mm/20250930060557.85133-1-lance.yang@linux.dev/
>
> v2 -> v3:
> - ptep_get() gets called only once per iteration (per Dev)
> - https://lore.kernel.org/linux-mm/20250930043351.34927-1-lance.yang@linux.dev/
>
> v1 -> v2:
> - Avoid calling ptep_get() multiple times (per Dev)
> - Double-check the uffd-wp bit (per David)
> - Collect Acked-by from David - thanks!
> - https://lore.kernel.org/linux-mm/20250928044855.76359-1-lance.yang@linux.dev/
>
> mm/migrate.c | 16 ++++++++++------
> 1 file changed, 10 insertions(+), 6 deletions(-)
>
> diff --git a/mm/migrate.c b/mm/migrate.c
> index ce83c2c3c287..21a2a1bf89f7 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -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));
> @@ -344,7 +349,7 @@ static bool remove_migration_pte(struct folio *folio,
>
> while (page_vma_mapped_walk(&pvmw)) {
> rmap_t rmap_flags = RMAP_NONE;
> - pte_t old_pte;
> + pte_t old_pte = ptep_get(pvmw.pte);
Oops, I just found a NULL pointer dereference bug in my changes to
remove_migration_pte() when we encounter a PMD-mapped THP migration
entry.
#ifdef CONFIG_ARCH_ENABLE_THP_MIGRATION
/* PMD-mapped THP migration entry */
if (!pvmw.pte) {
VM_BUG_ON_FOLIO(folio_test_hugetlb(folio) ||
!folio_test_pmd_mappable(folio), folio);
remove_migration_pmd(&pvmw, new);
continue;
}
#endif
ptep_get() is called too early... before the !pvmw.pte check for
PMD-mapped entries.
The initialization of old_pte must be moved to after that if block.
Sorry for the churn :(
Lance
> pte_t pte;
> swp_entry_t entry;
> struct page *new;
> @@ -365,12 +370,11 @@ static bool remove_migration_pte(struct folio *folio,
> }
> #endif
> 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))
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