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Received: from willy by casper.infradead.org with local (Exim 4.94.2 #2 (Red Hat Linux)) id 1pT3F1-009RNk-2v; Fri, 17 Feb 2023 16:05:43 +0000 Date: Fri, 17 Feb 2023 16:05:43 +0000 From: Matthew Wilcox To: Suren Baghdasaryan Cc: 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 Subject: Re: [PATCH v3 26/35] mm: fall back to mmap_lock if vma->anon_vma is not yet set Message-ID: References: <20230216051750.3125598-1-surenb@google.com> <20230216051750.3125598-27-surenb@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspam-User: X-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: 7EE9710000B X-Stat-Signature: 71pxiapczozj69nfkazuea46r7dsasdh X-HE-Tag: 1676649990-771754 X-HE-Meta: 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 1mgiT3L8 7ZmKMrmy508fGZSoMZaME6dv+Di3CqahFm/ab9XqhrPtb++yCZTpbVtL/8pm9m5P6ytSySXcM82ReXp/q4Xp9HdauDSUHoafRUKssBhPQ/UP9I/dIl7Mhqbe2W/ZhcpycNyU1fMXBnUE7tc/LA0Zc2m/y9YNTIYTYsfs67L7d6LAIb5xsThzSS/WOsJuZkN4mPSnv5ZivvF/cmZ7wqWfU2hJ91DEGTh/m/QA+G2lXrT2byblPyT2BJ5p8Pn1RBPg90nSC7i10rdRMm0AGVrKRWLf2miIk1QV+9Lmc X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Thu, Feb 16, 2023 at 06:14:59PM -0800, 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. How often does userspace call mmap() and then _never_ fault on it? I appreciate that userspace might mmap() gigabytes of address space and then only end up using a small amount of it, so populating it lazily makes sense. But creating a region and never faulting on it? The only use-case I can think of is loading shared libraries: openat(AT_FDCWD, "/lib/x86_64-linux-gnu/libc.so.6", O_RDONLY|O_CLOEXEC) = 3 (...) mmap(NULL, 1970000, PROT_READ, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x7f0ce612e000 mmap(0x7f0ce6154000, 1396736, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x26000) = 0x7f0ce6154000 mmap(0x7f0ce62a9000, 339968, PROT_READ, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x17b000) = 0x7f0ce62a9000 mmap(0x7f0ce62fc000, 24576, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x1ce000) = 0x7f0ce62fc000 mmap(0x7f0ce6302000, 53072, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x7f0ce6302000 but that's a file-backed VMA, not an anon VMA.