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([2a01:599:901:4a65:f2e2:845:f3d2:404d]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-40fb9e1b665sm21132349f8f.27.2025.09.29.23.31.51 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 29 Sep 2025 23:31:53 -0700 (PDT) Message-ID: <026a2673-8195-4927-8cde-f7517b601125@redhat.com> Date: Tue, 30 Sep 2025 08:31:50 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird 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 To: Lance Yang , akpm@linux-foundation.org, lorenzo.stoakes@oracle.com Cc: peterx@redhat.com, ziy@nvidia.com, 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 References: <20250930060557.85133-1-lance.yang@linux.dev> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Stat-Signature: yjakicixdwm7rxgqhnyi7y4kgmmrz85g X-Rspam-User: X-Rspamd-Queue-Id: F31AA100009 X-Rspamd-Server: rspam04 X-HE-Tag: 1759213929-484751 X-HE-Meta: U2FsdGVkX1++W+daJ9UhPVukstnIP+XEhHpBB4BHdUcWQkXqD8LOVoRNZnY4yzKONzlJnIBjC2XWK8+y0r1W2LZCsn4ti1rGCmshpyUz/wyovvSjfSAs9WVVhAtJpCIMp124HL6k2GNI/DvcFXQNVjOe47a58keJ7s8J7MZ/7QG57X52EAneWf5jTEtVkWuRwC17xT5NOKpvecqF3jSs2auwz9rWuJudanxoQGgflZqOe8YZtM28mEX3St2P4QpdNgT3Rc0FEpTs15N2bf4a2t/Daysx3AobJL9ahC1EP9JVs1/TEr1PqvhBdUD55OeCcvrrGS4fqWbq1jEmXGNqCiwOuCWW+3YhMk6I00hVtpAgdQr2KdDVTtqWwtswLbivB5DWpMoPztwOXWAPsW+AFC2nNIDy/y9vTNw0jZt3a7RBabIY0lO62GE/XOnR0m8y4A4YLWqRIOQM9Fwi3lyfNJAUMyqHU59UqRhYRXUEeBOeiSIhfGR5f4dVTpEN6zDIQdE+QUjsBadH4ksjETz7zOBB10XUwGfudACJ4HsvqM7Xe66pWKy9S1wV8PFLawlmS4t/re+tNrulb/rMGjdE0tNNs7GO2cTg3rFAN708P3wGfhZNJGqPbZsjNBUstHP4UrqL2UltEh25t3bMcJ36cT2IijsFKoKm12wHsZpwQeLtu//E4ntxG5JLFz4bDN3tM7HvwJP/ZlBBNimRdnpjHQm0XfR+0Agrq3yDB9bTtOuxj23fO9kKSAz7n1eUINHWhDWNurjJ4b9jVtPDaaQOKBikouk+itJL6JaTHLY/0B72Tf2piDHRjzbrdfUD1gioxbm+Clr5kyI7iijhf14J9o2HolsFCx9WwWxVZOlgpmIEWbcvEP3VXlfDn0xVsMBRFab2U9jDIi9qA0htm200gGVH7RRT1xQjU2nzRUP6TjXAVrMZGiZkVOK0EKlJvH25QWBcd2vVYobTq4W1oE9 fHdSbo4A 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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 30.09.25 08:05, 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 > Signed-off-by: Lance Yang > --- > 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) Nit: static bool try_to_map_unused_to_zeropage(struct page_vma_mapped_walk *pvmw, struct folio *folio, pte_t old_pte, unsigned long idx) LGTM, Thanks! -- Cheers David / dhildenb