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Fri, 17 Feb 2023 08:13:12 -0800 (PST) MIME-Version: 1.0 References: <20230216051750.3125598-1-surenb@google.com> <20230216051750.3125598-27-surenb@google.com> In-Reply-To: From: Suren Baghdasaryan Date: Fri, 17 Feb 2023 08:13:01 -0800 Message-ID: Subject: Re: [PATCH v3 26/35] mm: fall back to mmap_lock if vma->anon_vma is not yet set To: Hyeonggon Yoo <42.hyeyoo@gmail.com> Cc: Matthew Wilcox , akpm@linux-foundation.org, michel@lespinasse.org, jglisse@google.com, mhocko@suse.com, vbabka@suse.cz, hannes@cmpxchg.org, mgorman@techsingularity.net, dave@stgolabs.net, liam.howlett@oracle.com, peterz@infradead.org, ldufour@linux.ibm.com, paulmck@kernel.org, mingo@redhat.com, will@kernel.org, luto@kernel.org, songliubraving@fb.com, peterx@redhat.com, david@redhat.com, dhowells@redhat.com, hughd@google.com, bigeasy@linutronix.de, kent.overstreet@linux.dev, punit.agrawal@bytedance.com, lstoakes@gmail.com, peterjung1337@gmail.com, rientjes@google.com, chriscli@google.com, axelrasmussen@google.com, joelaf@google.com, minchan@google.com, rppt@kernel.org, jannh@google.com, shakeelb@google.com, tatashin@google.com, edumazet@google.com, gthelen@google.com, gurua@google.com, arjunroy@google.com, soheil@google.com, leewalsh@google.com, posk@google.com, michalechner92@googlemail.com, linux-mm@kvack.org, linux-arm-kernel@lists.infradead.org, linuxppc-dev@lists.ozlabs.org, x86@kernel.org, linux-kernel@vger.kernel.org, kernel-team@android.com Content-Type: text/plain; charset="UTF-8" Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Feb 17, 2023 at 2:21 AM Hyeonggon Yoo <42.hyeyoo@gmail.com> wrote: > > On Fri, Feb 17, 2023 at 11:15 AM Suren Baghdasaryan wrote: > > > > On Thu, Feb 16, 2023 at 11:43 AM Suren Baghdasaryan wrote: > > > > > > On Thu, Feb 16, 2023 at 7:44 AM Matthew Wilcox wrote: > > > > > > > > On Wed, Feb 15, 2023 at 09:17:41PM -0800, Suren Baghdasaryan wrote: > > > > > When vma->anon_vma is not set, page fault handler will set it by either > > > > > reusing anon_vma of an adjacent VMA if VMAs are compatible or by > > > > > allocating a new one. find_mergeable_anon_vma() walks VMA tree to find > > > > > a compatible adjacent VMA and that requires not only the faulting VMA > > > > > to be stable but also the tree structure and other VMAs inside that tree. > > > > > Therefore locking just the faulting VMA is not enough for this search. > > > > > Fall back to taking mmap_lock when vma->anon_vma is not set. This > > > > > situation happens only on the first page fault and should not affect > > > > > overall performance. > > > > > > > > I think I asked this before, but don't remember getting an aswer. > > > > Why do we defer setting anon_vma to the first fault? Why don't we > > > > set it up at mmap time? > > > > > > Yeah, I remember that conversation Matthew and I could not find the > > > definitive answer at the time. I'll look into that again or maybe > > > someone can answer it here. > > > > After looking into it again I'm still under the impression that > > vma->anon_vma is populated lazily (during the first page fault rather > > than at mmap time) to avoid doing extra work for areas which are never > > faulted. Though I might be missing some important detail here. > > I think this is because the kernel cannot merge VMAs that have > different anon_vmas? > > Enabling lazy population of anon_vma could potentially increase the > chances of merging VMAs. Hmm. Do you have a clear explanation why merging chances increase this way? A couple of possibilities I can think of would be: 1. If after mmap'ing a VMA and before faulting the first page into it we often change something that affects anon_vma_compatible() decision, like vm_policy; 2. When mmap'ing VMAs we do not map them consecutively but the final arrangement is actually contiguous. Don't think either of those cases would be very representative of a usual case but maybe I'm wrong or there is another reason? > > > > In the end rather than changing that logic I decided to skip > > > vma->anon_vma==NULL cases because I measured them being less than > > > 0.01% of all page faults, so ROI from changing that would be quite > > > low. But I agree that the logic is weird and maybe we can improve > > > that. I will have to review that again when I'm working on eliminating > > > all these special cases we skip, like swap/userfaults/etc. > > -- > To unsubscribe from this group and stop receiving emails from it, send an email to kernel-team+unsubscribe@android.com. >