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Mon, 18 May 2026 09:53:40 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1779098029; bh=W8Y9KhF7UdtIAt1c+NPri553EDOVp1AAPUaE7TKCmjc=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=pd65kd8GwKjdONJbfTVC0Cbl/3yXe5p7s3Odr3JoeXeW5LNYe95k6Po6QAxURYpS7 B/fp4Vt5Z3BIPBT6bfhGi63GZlBdT1O4IjpTSFgqkdrl1GS76ZEqwVPiCihl8mgw25 ewkaG1poBVEHGPVLrhZkCfUmmYcBYaMy8YNHSrh0SLnR/GsnPO8dcDE7DsfBCg+0WR zKuF/vEy1e8RoDYGByJjQS7ArI72vcW8qFhPO+TFbT+RYPfD0MuVuXglet+3AD+81z MO7vGLYPvMjA8AvRMC7wYWoPCUZs2JpKZdhJml4rhXSJWvwHBF9InDkR9UY50YRVCV Cp57Dwam949Zg== Message-ID: Date: Mon, 18 May 2026 11:53:37 +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: Barry Song , Matthew Wilcox , surenb@google.com Cc: akpm@linux-foundation.org, linux-mm@kvack.org, ljs@kernel.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: 8bit On 5/17/26 10:45, Barry Song wrote: > On Sat, May 2, 2026 at 1:58 AM Matthew Wilcox wrote: >> >> On Sat, May 02, 2026 at 01:44:34AM +0800, Barry Song wrote: >>> >>> It doesn’t have to involve unmapping or applying mprotect to >>> the entire VMA—just a portion of it is sufficient. >> >> Yes, but that still fails to answer "does this actually happen". How much >> performance is all this complexity in the page fault handler buying us? >> If you don't answer this question, I'm just going to go in and rip it >> all out. >> > > Hi Matthew (and Lorenzo, Jan, and anyone else who may be > waiting for answers), > > As promised during LSF/MM/BPF, we conducted thorough > testing on Android phones to determine whether performing > I/O in `filemap_fault()` can block `vma_start_write()`. > I wanted to give a quick update on this question. > > Nanzhe at Xiaomi created tracing scripts and ran various > applications on Android devices with I/O performed under > the VMA lock in `filemap_fault()`. We found that: > > 1. There are very few cases where unmap() is blocked by > page faults. I assume this is due to buggy user code > or poor synchronization between reads and unmap(). > So I assume it is not a problem. > > 2. We observed many cases where `vma_start_write()` > is blocked by page-fault I/O in some applications. > The blocking occurs in the `dup_mmap()` path during > fork(). > > With Suren's commit fb49c455323ff ("fork: lock VMAs of > the parent process when forking"), we now always hold > `vma_write_lock()` for each VMA. Note that the > `mmap_lock` write lock is also held, which could lead to > chained waiting if page-fault I/O is performed without > releasing the VMA lock. > > My gut feeling is that Suren's commit may be overshooting, > so my rough idea is that we might want to do something like > the following (we haven't tested it yet and it might be > wrong): > > diff --git a/mm/mmap.c b/mm/mmap.c > index 2311ae7c2ff4..5ddaf297f31a 100644 > --- a/mm/mmap.c > +++ b/mm/mmap.c > @@ -1762,7 +1762,13 @@ __latent_entropy int dup_mmap(struct mm_struct > *mm, struct mm_struct *oldmm) > for_each_vma(vmi, mpnt) { > struct file *file; > > - retval = vma_start_write_killable(mpnt); > + /* > + * For anonymous or writable private VMAs, prevent > + * concurrent CoW faults. > + */ > + if (!mpnt->vm_file || (!(mpnt->vm_flags & VM_SHARED) && > + (mpnt->vm_flags & VM_WRITE))) > + retval = vma_start_write_killable(mpnt); Likely is_cow_mapping() is what you would want to check to handle VMAs that could have anonymous pages in them. -- Cheers, David