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b=k0S3S/keSBLaGmyqupwK3Z1mvr66dN2JMGWF3ieVRlx99hH1pXaQueAW+z7vgQZ43 FLddevi4zJjipFoHV8RO/xzDu2eW9Ph1tnMl9S5JrNxSfKr4IyUkNNUz/TuOjmvQ9P TfQSYdlPaCepQYkAi1wuPiRy349QjRGU+zQCiB3bmhODCk48llGycLn/gBzTK7mkYK eFYbb7rydTwhcEVglxFxRS1oGV5dyfP6l0CptCFYZUnF9mX1EEI1icTLZvmR+2Ap2G yD8FRNrZRvT5G47Y74X4p4e0ovLOLp6kYC3eA6xjU58ieE3ACOpqv7VnYaRg3b780f K35saKMpZRS4Q== Message-ID: Date: Wed, 20 May 2026 12:33:56 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 0/5] mm: reduce mmap_lock contention and improve page fault performance To: Lorenzo Stoakes , Suren Baghdasaryan Cc: Barry Song , Matthew Wilcox , akpm@linux-foundation.org, linux-mm@kvack.org, liam@infradead.org, vbabka@kernel.org, rppt@kernel.org, mhocko@suse.com, jack@suse.cz, pfalcato@suse.de, wanglian@kylinos.cn, chentao@kylinos.cn, lianux.mm@gmail.com, kunwu.chan@gmail.com, liyangouwen1@oppo.com, chrisl@kernel.org, kasong@tencent.com, shikemeng@huaweicloud.com, nphamcs@gmail.com, bhe@redhat.com, youngjun.park@lge.com, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, loongarch@lists.linux.dev, linuxppc-dev@lists.ozlabs.org, linux-riscv@lists.infradead.org, linux-s390@vger.kernel.org, Nanzhe Zhao References: <20260430040427.4672-1-baohua@kernel.org> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspam-User: X-Rspamd-Queue-Id: AB8FE40008 X-Rspamd-Server: rspam04 X-Stat-Signature: 7hfwxmfqpyfm6hqgsry8hdwpofrg1dcz X-HE-Tag: 1779273250-399481 X-HE-Meta: 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 S/RN2+lY 9H2POC2E4lXvuS0JWEPZ3yD2wfqFfBTCni9vWYCnu/E2ZNQ8cxAECNMkLVA/L9yEpjIpM06q6LGhjG2fkKqjBHtMdf2aP0puvv57XRXhByO+MXph9JqFJMOEjJX7bskVP3zPrvGs17QGlxp7AWWfsRigCb5XEZ/KOIqUoylREJWd+4/MqKtrLU3c7g3DZekYT3Dvq3RCF+RDBb/u9fvsbFCqe31mAQSGqOWtcAokyr0G+UN6RdBph6KbcoauKAXWuNPaIBlUOKBfVEZ6WjVSkHO6u+EYRuedxP6+OYn89d7qhWhzi4bqQl3jNQYutXboiQdG7Vwab7y7alE7tyA2ez05OheG7vxwGqnkNxAUamrAEaKbZkYnNHbgitX9OfjNRamVi4tIYUl5QrFruA3eQy4EidPaZmb+Hozxm82wJye8K2Ak= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 5/19/26 14:53, Lorenzo Stoakes wrote: > On Mon, May 18, 2026 at 12:56:59PM -0700, Suren Baghdasaryan wrote: > >>> >>> I think we either need to fix `fork()`, or keep the current >>> behavior of dropping the VMA lock before performing I/O. >> >> I see. So, this problem arises from the fact that we are changing the >> pagefaults requiring I/O operation to hold VMA lock... >> And you want to lock VMA on fork only if vma_is_anonymous(vma) || >> is_cow_mapping(vma->vm_flags). So, we will be blocking page faults for >> anonymous and COW VMAs only while holding mmap_write_lock, preventing >> any VMA modification. On the surface, that looks ok to me but I might >> be missing some corner cases. If nobody sees any obvious issues, I >> think it's worth a try. > > Not sure if you noticed but I did raise concerns ;) > > I wonder if you've confused the fault path and fork here, as I think Barry has > been a little unclear on that. > > What's being suggested in this thread is to fundamentally change fork behaviour > so it's different from the entire history of the kernel (or - presumably - at > least recent history :) I don't want fork() to become different in that regard. There is already a slight difference with vs. without per-VMA locks, because there is a window in-between us taking the write mmap_lock and all the per-VMA locks. I raised that previously [1] and assumed that it is probably fine. I also raised in the past why I think we must not allow concurrent page faults, at least as soon as anonymous memory is involved [2]. ... and I raised that this is pretty much slower by design right now: "Well, the design decision that CONFIG_PER_VMA_LOCK made for now to make page faults fast and to make blocking any page faults from happening to be slower ..." [3] [1] https://lore.kernel.org/all/970295ab-e85d-7af3-76e6-df53a5c52f8b@redhat.com/ [2] https://lore.kernel.org/all/7e3f35cc-59b9-bf12-b8b1-4ed78223844a@redhat.com/ [3] https://lore.kernel.org/all/2efa2c89-3765-721d-2c3c-00590054aa5b@redhat.com/ -- Cheers, David