linux-kernel - Re: [PATCH v5 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: <98A8D177-A476-4D97-B6C4-DC3F34E91126@nvidia.com>
Date: Tue, 30 Sep 2025 11:00:03 -0400
From: Zi Yan <ziy@...dia.com>
To: Lance Yang <lance.yang@...ux.dev>
Cc: akpm@...ux-foundation.org, david@...hat.com, lorenzo.stoakes@...cle.com,
 peterx@...hat.com, baolin.wang@...ux.alibaba.com, baohua@...nel.org,
 ryan.roberts@....com, dev.jain@....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 v5 1/1] mm/rmap: fix soft-dirty and uffd-wp bit loss when
 remapping zero-filled mTHP subpage to shared zeropage

On 30 Sep 2025, at 4:10, 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>
> ---
> v4 -> v5:
>  - Move ptep_get() call after the !pvmw.pte check, which handles PMD-mapped
>    THP migration entries.
>  - https://lore.kernel.org/linux-mm/20250930071053.36158-1-lance.yang@linux.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 | 15 ++++++++++-----
>  1 file changed, 10 insertions(+), 5 deletions(-)
>

Acked-by: Zi Yan <ziy@...dia.com>


Best Regards,
Yan, Zi