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 A56B9C636CC for ; Thu, 16 Feb 2023 19:43:34 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229738AbjBPTnd (ORCPT ); Thu, 16 Feb 2023 14:43:33 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:49962 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229512AbjBPTnb (ORCPT ); Thu, 16 Feb 2023 14:43:31 -0500 Received: from mail-yb1-xb36.google.com (mail-yb1-xb36.google.com [IPv6:2607:f8b0:4864:20::b36]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id C0FCB4E5C7 for ; Thu, 16 Feb 2023 11:43:30 -0800 (PST) Received: by mail-yb1-xb36.google.com with SMTP id 139so3484066ybe.3 for ; Thu, 16 Feb 2023 11:43:30 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.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=ecCMrPuH47M6x+7l+8oSjk53rrpfDEPsyzkbMwvMYGE=; b=nQjhqTpWmrsidiAuvoCcaH+1Nn+MdzCD0u1wOKp3+6nMie0EEOCShOkZxWAStmC0Z7 0DAiGxaTX/84wos6YcnH4ZcBy4oFJENSMkSF4PKWKvGrLfFlv9miv6D1euEV3WJw2Bja U/Tc4VyRAp8gojN5lr+rESfLvTfTK5vezctBWGmB51gEhz9cXu1jwSFmGRSfVyIe7KJW 9985FtKADC1PSU2QKatY5svbWvXHeK/lG4CpKBFBZzA8TlGvpIV5VpHjgkpocjYvo2Sn 2t+SpqQDf8D5EE+w43rAN8WY2iOuZ6m2DhxKzx3UvSVmlCnkWCxNbjgkfk9I9fPsoDou rcSQ== 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=ecCMrPuH47M6x+7l+8oSjk53rrpfDEPsyzkbMwvMYGE=; b=uCgMQ+QaX2kmLMSOnQLeziA4E0/2ALJM/cfJNcPLNTdui6L3r7BwXwSHiQxt/HU+tF dPleWwN9LEyOLTUKZ/mLvH1tB+XulJPE31vny+onx9gNvFHYxjJOUqSrVUH/qQ6PY8wV 7JGnoOEF5ECJT4Bh/MpANrnoPRIuMN2LkmH6TFgRU7bKd1bSUx8OsdC7j+SotThT/gd6 1Nyixhb92yNrEkgsG/RwZ7/PBbyvFqcL5scZJsLtInC7L/855VeUa05TnB+zVm5u1sau M3UmLikVauY3tWZwrUKH1LUCW0OZ1Wueve0BFF8oeiV3YNzY+dAAU0v+mLO+EKNNFewT BYPQ== X-Gm-Message-State: AO0yUKVs4UoWIrbGvQ/Ugtlnb992OZJL/h2rx6pJbtzWAaSoG/p5tWZm 7GAaycIT0S4jsfZ2nTn9vcgc+LNO5ofX9AxYqL79cw== X-Google-Smtp-Source: AK7set+GOKNhzuykRCoIMckW7I82pXv7V9YjFwgSjZ09J90OxeIVOanNo7rnoi+gq6vhCwETbxbWJ7fJFGy2XpbKeOY= X-Received: by 2002:a25:9941:0:b0:90c:de27:7f15 with SMTP id n1-20020a259941000000b0090cde277f15mr876421ybo.428.1676576609777; Thu, 16 Feb 2023 11:43:29 -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: Thu, 16 Feb 2023 11:43:18 -0800 Message-ID: Subject: Re: [PATCH v3 26/35] mm: fall back to mmap_lock if vma->anon_vma is not yet set To: Matthew Wilcox 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 Content-Type: text/plain; charset="UTF-8" Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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. 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.