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X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Tue, Sep 30, 2025 at 04:10:40PM +0800, Lance Yang wrote: > From: Lance Yang > > 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: > Fixes: b1f202060afe ("mm: remap unused subpages to shared zeropage when splitting isolated thp") > Suggested-by: David Hildenbrand > Suggested-by: Dev Jain > Acked-by: David Hildenbrand > Reviewed-by: Dev Jain > Signed-off-by: Lance Yang > --- Looks good to me, Reviewed-by: Harry Yoo -- Cheers, Harry / Hyeonggon