linux-kernel - Re: [PATCH v3 1/1] mm/rmap: fix soft-dirty and uffd-wp bit loss when remapping zero-filled mTHP subpage to shared zeropage

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Message-ID: <838505c8-053e-49af-b37b-0475520daf68@arm.com>
Date: Tue, 30 Sep 2025 12:03:36 +0530
From: Dev Jain <dev.jain@....com>
To: Lance Yang <lance.yang@...ux.dev>, akpm@...ux-foundation.org,
 david@...hat.com, lorenzo.stoakes@...cle.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
Subject: Re: [PATCH v3 1/1] mm/rmap: fix soft-dirty and uffd-wp bit loss when
 remapping zero-filled mTHP subpage to shared zeropage


On 30/09/25 11:35 am, 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>
> Signed-off-by: Lance Yang <lance.yang@...ux.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 | 14 ++++++++++----
>   1 file changed, 10 insertions(+), 4 deletions(-)
>
> diff --git a/mm/migrate.c b/mm/migrate.c
> index ce83c2c3c287..bafd8cb3bebe 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -297,6 +297,7 @@ 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,
> +					  pte_t old_pte,
>   					  unsigned long idx)

Could have just added this in the same line as folio?

>   {
>   	struct page *page = folio_page(folio, idx);
> @@ -306,7 +307,7 @@ 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 +323,12 @@ 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 +351,7 @@ 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);
>   		pte_t pte;
>   		swp_entry_t entry;
>   		struct page *new;
> @@ -365,12 +372,11 @@ 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))

Looks good, the special bit does not overlay on any arch with the soft-dirty bit.
It shouldn't overlay with uffd-wp as well since split_huge_zero_page_pmd does the
same bit preservation.

Reviewed-by: Dev Jain <dev.jain@....com>