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Date: Thu, 16 Feb 2023 18:14:59 -0800 From: Suren Baghdasaryan <surenb@...gle.com> To: Matthew Wilcox <willy@...radead.org> Cc: akpm@...ux-foundation.org, michel@...pinasse.org, jglisse@...gle.com, mhocko@...e.com, vbabka@...e.cz, hannes@...xchg.org, mgorman@...hsingularity.net, dave@...olabs.net, liam.howlett@...cle.com, peterz@...radead.org, ldufour@...ux.ibm.com, paulmck@...nel.org, mingo@...hat.com, will@...nel.org, luto@...nel.org, songliubraving@...com, peterx@...hat.com, david@...hat.com, dhowells@...hat.com, hughd@...gle.com, bigeasy@...utronix.de, kent.overstreet@...ux.dev, punit.agrawal@...edance.com, lstoakes@...il.com, peterjung1337@...il.com, rientjes@...gle.com, chriscli@...gle.com, axelrasmussen@...gle.com, joelaf@...gle.com, minchan@...gle.com, rppt@...nel.org, jannh@...gle.com, shakeelb@...gle.com, tatashin@...gle.com, edumazet@...gle.com, gthelen@...gle.com, gurua@...gle.com, arjunroy@...gle.com, soheil@...gle.com, leewalsh@...gle.com, posk@...gle.com, michalechner92@...glemail.com, linux-mm@...ck.org, linux-arm-kernel@...ts.infradead.org, linuxppc-dev@...ts.ozlabs.org, x86@...nel.org, linux-kernel@...r.kernel.org, kernel-team@...roid.com Subject: Re: [PATCH v3 26/35] mm: fall back to mmap_lock if vma->anon_vma is not yet set On Thu, Feb 16, 2023 at 11:43 AM Suren Baghdasaryan <surenb@...gle.com> wrote: > > On Thu, Feb 16, 2023 at 7:44 AM Matthew Wilcox <willy@...radead.org> 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. > > 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.
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