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Howlett" , Mike Rapoport , Andrew Morton , David Carlier , Heechan Kang , Michael Bommarito , Peter Xu , linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH RESEND] userfaultfd: snapshot VMA state across UFFDIO_COPY retry Message-ID: References: <20260519052516.3315196-1-rppt@kernel.org> <855a00a7-c1f4-4c6d-bd4a-f3ccb0eb1eab@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <855a00a7-c1f4-4c6d-bd4a-f3ccb0eb1eab@kernel.org> X-Rspam-User: X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: 83F662000C X-Stat-Signature: etceju9iner3857u5b4iay4iobyc6qxz X-HE-Tag: 1779809141-494734 X-HE-Meta: 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 qI7hdBuC UOLoGKTFUNJLSBZLDe/5wAcAlCE/n8sDqPdea6sLz2qxTCfPojfJXa625pP+pe5QOXy0asYKMCZmpFtgN7glwv5vXv4UECS4RDwS+o6GuhjvqXccbY7lnjopHt5JAhdk0biuMcyQPQeeIsp+KFcYalK7WntiaOgihv9PZmNzvuJczX7IAG7fwjxlGll64FrWXH4/Nfi6IeidT/XnDvqkyzI5RiKl8DfKsKdQ6yG6VVxcgFFzd9aNnE4b19Ikwb12WhRxlh5CY65tfGh+dLKVxcWd8br1m8bBLTH1QwoVNoUcu3XqiD1h/m7QZjw6Ri3yCVuo6wMbfyZrdbUKBeYpnV2u8mYqcKG8f3Aks68IzRO/02efvk1qsayBH+g== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Tue, May 26, 2026 at 02:47:45PM +0200, David Hildenbrand (Arm) wrote: > On 5/25/26 19:12, Liam R. Howlett wrote: > > On 26/05/20 04:38PM, David Hildenbrand (Arm) wrote: > >> On 5/20/26 16:12, Mike Rapoport wrote: > >>> > >>> Let me reiterate: > >>> > >>> A thread doing UFFDIO_COPY releases the VMA in mfill_copy_folio_retry(), > >>> re-gets the VMA and checks if the per-MM counter stayed the same. > >>> > >>> If another thread makes any change to VMA while mfill_copy_folio_retry() > >>> waits to re-get the VMA, the counter would be incremented by another > >>> thread. mfill_copy_folio_retry() will see the change after mfill_get_vma() > >>> and will bail out with -EAGAIN. > >>> > >> > >> Yeah. > > > > This isn't bulletproof anyways. The sequence count can wrap. So, if > > someone can replace the vma then cause the sequence counter wrap, then > > you can be fooled into thinking it's okay (we had this problem years ago > > with the vmacache using a 32 bit counter, iirc). > > If you can get it to wrap for such short durations, then how would sequence > counters possibly work in any reasonable context? Surely even for a 32-bit value, we can be pretty confident we're not going to see a wrap that matters (the seqnum will != the prev seqnum unless 4 billion VMA write locks were obtained)? I may be missing something though! > > -- > Cheers, > > David Thanks, Lorenzo