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b=CxN08qtpOBJ8IAiYT1NGbATSMveN2VuvCz+XRydDhhXI+x5QqjrTyhe5UIS691Kb2 bS9w1ztocYeYGUfpqcashfOVc/Pb/0rjFRC4zdEfjCwigY5cmMMgpzI1gKRdtA3pI4 v/VV+NqUVSeovPsja64sp9wWP72hVhslEn03pxycaef5UGhc7dYBhy2FZ2/Av0j3h0 1dh5UBOVeGESCRqbxSUbKDq9gTnfNXjfMTy6NPbKLtEkxHtIHeWDfR+iPX1ZvW7L/h LBrO+E3xuhU/cNw3kbOQoavYiADtcJMriOizeGdr3j5sT3HlIxKigegez3GS/kKTHd Rpyd30xJMvmFw== Date: Tue, 19 May 2026 14:42:11 +0100 From: Lorenzo Stoakes To: "David Hildenbrand (Arm)" Cc: Barry Song , Matthew Wilcox , surenb@google.com, 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: <20260430040427.4672-1-baohua@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: X-Rspamd-Server: rspam02 X-Rspamd-Queue-Id: 154524000D X-Rspam-User: X-Stat-Signature: 1u8e9df9h3fohy78h8bxepn3nkkf9ukt X-HE-Tag: 1779198142-330140 X-HE-Meta: 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 NNNjBP1j vaTrZSL8MO91sEXRfPs+55Jfbgs4yae9WxuIZsl6kxqX8TCge8LU1OUPg3wcoua3VnOCYRKij/yGFpizsyxq8L3zgQvc/88VFhSjE8Y5x93HeTzz7/0nE3QShdjS+nB2OjYUlemW3cyaPP6Qva5wvnLNaxzhDGZqvxvu3UI3ma63uLOPAvrv6SI6He80/QfaMberbSqvM6JFVIiMae6epWjHM1iT8u/ouPk86X/2yIZopwqI4O9o2OgvUaQG1uo5XvS5u94rAh62P+MYPcLPxIKo+4jWH88xtF84JpnYWm0/QNuYsGotuJy9LiupH+89W5ZByQpvJLWkkV3HJVfb6xHnfpYAPYbsMcLI5QdG9ZLrxWhIV3RoyBludxe/UQPia4aeEvXsB4rJv7yfGaiieNALqj3dy0BoXg4rZZtZ3aLYH4xw+YK8ohUkUcLA/X8HG+nLAEICVEKQqwVs= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Mon, May 18, 2026 at 11:53:37AM +0200, David Hildenbrand (Arm) wrote: > 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. Yes :) I made pretty much the same comment though I forgot the correct helper :P > > -- > Cheers, > > David Cheers, Lorenzo