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Wed, 20 May 2026 10:33:58 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1779273249; bh=1LDWtyaODcbYP5iCV92+/8GVnwUqEJ5fHGR9cz0/w+U=; h=Date:Subject:To:Cc:References:From:In-Reply-To; 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 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 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 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 From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 069D8CD4F54 for ; Wed, 20 May 2026 10:34:25 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender: Content-Transfer-Encoding:Content-Type:List-Subscribe:List-Help:List-Post: List-Archive:List-Unsubscribe:List-Id:In-Reply-To:From:References:Cc:To: Subject:MIME-Version:Date:Message-ID:Reply-To:Content-ID:Content-Description: Resent-Date:Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID: List-Owner; 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Wed, 20 May 2026 10:33:58 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1779273249; bh=1LDWtyaODcbYP5iCV92+/8GVnwUqEJ5fHGR9cz0/w+U=; h=Date:Subject:To:Cc:References:From:In-Reply-To; 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="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-riscv" Errors-To: linux-riscv-bounces+linux-riscv=archiver.kernel.org@lists.infradead.org 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 _______________________________________________ linux-riscv mailing list linux-riscv@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-riscv