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 vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 4A99DC636D7 for ; Fri, 17 Feb 2023 10:21:39 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229773AbjBQKVi (ORCPT ); Fri, 17 Feb 2023 05:21:38 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:40596 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229558AbjBQKVg (ORCPT ); Fri, 17 Feb 2023 05:21:36 -0500 Received: from mail-yb1-xb30.google.com (mail-yb1-xb30.google.com [IPv6:2607:f8b0:4864:20::b30]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 6226860A57 for ; Fri, 17 Feb 2023 02:21:35 -0800 (PST) Received: by mail-yb1-xb30.google.com with SMTP id 188so490717ybi.8 for ; Fri, 17 Feb 2023 02:21:35 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20210112; h=cc:to:subject:message-id:date:from:in-reply-to:references :mime-version:from:to:cc:subject:date:message-id:reply-to; bh=H9BBhPTLGGaUBxW3AOHp6ZsKFR75sYiq19lXrasIXls=; b=myOxV9gamhIWyliGmdmN2/mk/L1tbECLokR9V0fsZYL+Aj/t5D9uNxFdPdzQY8b5Eb thUND1gGBMTB39ipgJZf2Ql2LwujdfXsd2EzrT0IKVckNP1pDgIt9Lw3ICJtr2va/jkA WiVF19oUY+8ELCnfLAH8YG5HyU9uEcJ7o4MYoepq66YAimbAXisPnHBVOsYIERKI9ViL ty30kR8wtoIG+qnXXowYn0X8x22Hf3AhdUqHDULpynsgtTJmn69CgLLXR2KaH0cSsKFu 3jTW/4sCmj6lx99hEANBNFSMI5FxNAw7CWRBuN7NnhQp+zZF7xQg+Fa//77roL19p0Kt LVlg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=cc:to:subject:message-id:date:from:in-reply-to:references :mime-version:x-gm-message-state:from:to:cc:subject:date:message-id :reply-to; bh=H9BBhPTLGGaUBxW3AOHp6ZsKFR75sYiq19lXrasIXls=; b=T753ek2j0JJzD0yyiDT/RzfZ7PY6ZSFBbZhiV4dTud2ENCvsmfjUiSpOerlWMlKwAR dkln3OV2rIgWLghVVFKmF2rojkk5i1XsVShMyMb85YBdLsTQ21wkIvfbPBDJCani9OB1 ZA64Hume+gg5bCTAXlDzHecpvfUH1GVN7SvO2EkpQ3uD35DO/jvVsH/eUt9uANodOX1n C+uVqUoRm7lYTPbeeieoJHP5vxT95fgC9r4w+gpmpAzToS3zMcyoXfVFKesUDBxqz4SS dX2bTyiD3jHxkK+XUwjXTzXIzMjhh6MFVWhWvQ6iP92Ip8Xo0B+1WgGmpUUifR7a7+V1 R2Hw== X-Gm-Message-State: AO0yUKUg9IJqdxlqLdBunfsdvYl+1N+ktkTDG6Iyl347SBf0xbd6gzaS /yz7awRnC25ORCFAvwkldMP3RpybjMSBPSyg5bo= X-Google-Smtp-Source: AK7set9XZzct7X7WLcYp9C1YQZ54GdA3ZML/EFmNgnNLzMxsPTlkXoYn0KU+aTYkj1KX4pt6o21ZLWxWDcs5n3vh8n0= X-Received: by 2002:a05:6902:308:b0:969:a7c9:d7b8 with SMTP id b8-20020a056902030800b00969a7c9d7b8mr386954ybs.624.1676629294516; Fri, 17 Feb 2023 02:21:34 -0800 (PST) MIME-Version: 1.0 References: <20230216051750.3125598-1-surenb@google.com> <20230216051750.3125598-27-surenb@google.com> In-Reply-To: From: Hyeonggon Yoo <42.hyeyoo@gmail.com> Date: Fri, 17 Feb 2023 19:21:21 +0900 Message-ID: Subject: Re: [PATCH v3 26/35] mm: fall back to mmap_lock if vma->anon_vma is not yet set To: Suren Baghdasaryan 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 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. > > 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.