From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from lists.ozlabs.org (lists.ozlabs.org [112.213.38.117]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 5D8AAC636D6 for ; Fri, 17 Feb 2023 16:07:41 +0000 (UTC) Received: from boromir.ozlabs.org (localhost [IPv6:::1]) by lists.ozlabs.org (Postfix) with ESMTP id 4PJGtM3HGrz3fTN for ; Sat, 18 Feb 2023 03:07:39 +1100 (AEDT) Authentication-Results: lists.ozlabs.org; dkim=fail reason="signature verification failed" (2048-bit key; secure) header.d=infradead.org header.i=@infradead.org header.a=rsa-sha256 header.s=casper.20170209 header.b=RECpzUKP; dkim-atps=neutral Authentication-Results: lists.ozlabs.org; spf=none (no SPF record) smtp.mailfrom=infradead.org (client-ip=2001:8b0:10b:1236::1; helo=casper.infradead.org; envelope-from=willy@infradead.org; receiver=) Authentication-Results: lists.ozlabs.org; dkim=pass (2048-bit key; secure) header.d=infradead.org header.i=@infradead.org header.a=rsa-sha256 header.s=casper.20170209 header.b=RECpzUKP; dkim-atps=neutral Received: from casper.infradead.org (casper.infradead.org [IPv6:2001:8b0:10b:1236::1]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by lists.ozlabs.org (Postfix) with ESMTPS id 4PJGsF1zcgz3bTs for ; Sat, 18 Feb 2023 03:06:40 +1100 (AEDT) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.20170209; h=In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=COgV16QpVFvzSyezj8sQCRnnPRGtqN7Jdmva/Sfnf6s=; b=RECpzUKPshI8Gr0TMEm4IPdtUF sAEs6G2dshqrzDveO36GIvZT8g51OMfCJndtvNTg55RA8n/7m/0UG6ZAK1GFsSN8y1dk9KOTtXpl0 BZws7o+RrILvSQjUBE5aWHre5Tp4+zXQrNBLI68HEk8qdrB9vD5ZaX+zYj80x5aJTzc8Np5/J9hJy rVHUrkhkUi0rDmWmvZWftItyP0vBb+2H4RtAzs0gycKAd9E4zBeMqNk6J+FV9oWWCrsMXg/BcRQ9i lXif2mYzZUBzqknhqsTIA4YtXMWVUAZkHCXQYWJxdB0JQFT4PLIViYkXx49ykdTfXHscRIcVpAC7t u2/OAI2g==; 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 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-BeenThere: linuxppc-dev@lists.ozlabs.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: michel@lespinasse.org, joelaf@google.com, songliubraving@fb.com, mhocko@suse.com, leewalsh@google.com, david@redhat.com, peterz@infradead.org, bigeasy@linutronix.de, peterx@redhat.com, dhowells@redhat.com, linux-mm@kvack.org, edumazet@google.com, jglisse@google.com, punit.agrawal@bytedance.com, will@kernel.org, arjunroy@google.com, chriscli@google.com, dave@stgolabs.net, minchan@google.com, x86@kernel.org, hughd@google.com, gurua@google.com, mingo@redhat.com, linux-arm-kernel@lists.infradead.org, rientjes@google.com, axelrasmussen@google.com, kernel-team@android.com, michalechner92@googlemail.com, soheil@google.com, paulmck@kernel.org, jannh@google.com, liam.howlett@oracle.com, shakeelb@google.com, luto@kernel.org, gthelen@google.com, ldufour@linux.ibm.com, vbabka@suse.cz, posk@google.com, lstoakes@gmail.com, peterjung1337@gmail.com, linuxppc-dev@lists.ozlabs.org, kent.overstreet@linux.dev, linux-kernel@vger.kernel.org, hannes@cmpxchg.org, akpm@linux-foundation.org, tatashin@goog le.com, mgorman@techsingularity.net, rppt@kernel.org Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Sender: "Linuxppc-dev" 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.