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Wed, 20 May 2026 12:56:01 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1779281768; bh=IHcIhsCGtp+zFimfYJhlVXiifBlHPpWiDOsOVHQEJSk=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=odw+ysF2fSGs0CYgREQtSvLkA7C1vnjLq+yTGH8uUyZQL5gIwy8l0sgOhMQFrvR/U uhiwWUqoaRwvkgB0dpBHlc/BUJ1q5+UUOAqod11j8iXpTYoTcMdKWbeJzuR6xMAqQ+ OvTmgxH6iYYQP1hB8J7APtE6URdQBmktp1g/AVzlcWRxqXXJ8L/XVRnaqQo7zC7Xng ETzTvLZhkt1SvKxjfJTjvTYcMABci75fb1o2/YPfIpAPbJkiRERe++WMYYQiw7kmc/ 0b+T91xT73D5pkQltTZRR+EMWtVC4246AhUUcvutCmfPfiXeX8053NtPz89QYDfQc2 xRaHqWoL1+79g== Date: Wed, 20 May 2026 13:55:58 +0100 From: Lorenzo Stoakes To: "David Hildenbrand (Arm)" Cc: Suren Baghdasaryan , 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 Subject: Re: [PATCH v2 0/5] mm: reduce mmap_lock contention and improve page fault performance Message-ID: References: X-Mailing-List: linuxppc-dev@lists.ozlabs.org List-Id: List-Help: List-Owner: List-Post: List-Archive: , List-Subscribe: , , List-Unsubscribe: Precedence: list MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: On Wed, May 20, 2026 at 12:33:56PM +0200, David Hildenbrand (Arm) wrote: > 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] Thanks for the background will read through! :) But yeah I think the transition from !vma->anon_vma -> vma->anon_vma being a bit slow is kinda ok most page faults will of course have anon_vma populated. Be interesting with CoW context, because we won't need to mmap read lock there at all :) > > [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 Cheers, Lorenzo