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baolin.wang@linux.alibaba.com, baohua@kernel.org, ryan.roberts@arm.com, dev.jain@arm.com, npache@redhat.com, riel@surriel.com, Liam.Howlett@oracle.com, vbabka@suse.cz, harry.yoo@oracle.com, jannh@google.com, matthew.brost@intel.com, joshua.hahnjy@gmail.com, rakie.kim@sk.com, byungchul@sk.com, gourry@gourry.net, ying.huang@linux.alibaba.com, apopple@nvidia.com, usamaarif642@gmail.com, yuzhao@google.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org, ioworker0@gmail.com, stable@vger.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 Date: Tue, 30 Sep 2025 11:00:03 -0400 X-Mailer: MailMate (2.0r6283) Message-ID: <98A8D177-A476-4D97-B6C4-DC3F34E91126@nvidia.com> In-Reply-To: <20250930081040.80926-1-lance.yang@linux.dev> References: <20250930081040.80926-1-lance.yang@linux.dev> Content-Type: text/plain Content-Transfer-Encoding: quoted-printable X-ClientProxiedBy: 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 R8Lqybte 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 30 Sep 2025, at 4:10, Lance Yang wrote: > From: Lance Yang > > When splitting an mTHP and replacing a zero-filled subpage with the sha= red > zeropage, try_to_map_unused_to_zeropage() currently drops several impor= tant > PTE bits. > > For userspace tools like CRIU, which rely on the soft-dirty mechanism f= or > incremental snapshots, losing the soft-dirty bit means modified pages a= re > 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 dat= a > 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 > --- > v4 -> v5: > - Move ptep_get() call after the !pvmw.pte check, which handles PMD-ma= pped > THP migration entries. > - https://lore.kernel.org/linux-mm/20250930071053.36158-1-lance.yang@l= inux.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@l= inux.dev/ > > v2 -> v3: > - ptep_get() gets called only once per iteration (per Dev) > - https://lore.kernel.org/linux-mm/20250930043351.34927-1-lance.yang@l= inux.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@l= inux.dev/ > > mm/migrate.c | 15 ++++++++++----- > 1 file changed, 10 insertions(+), 5 deletions(-) > Acked-by: Zi Yan Best Regards, Yan, Zi