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 B9F21C25B50 for ; Mon, 23 Jan 2023 20:08:40 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232957AbjAWUIj (ORCPT ); Mon, 23 Jan 2023 15:08:39 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:42876 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232824AbjAWUIh (ORCPT ); Mon, 23 Jan 2023 15:08:37 -0500 Received: from mail-wm1-x32b.google.com (mail-wm1-x32b.google.com [IPv6:2a00:1450:4864:20::32b]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 78B177A9C for ; Mon, 23 Jan 2023 12:08:36 -0800 (PST) Received: by mail-wm1-x32b.google.com with SMTP id k16so9941490wms.2 for ; Mon, 23 Jan 2023 12:08:36 -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=Xx0+AS/R5cTFxTer1KPJhFwelUzEZqlaudVJ4C5UqlM=; b=RyYyVfXdy0EOGXtjgeoYEX0CSfXeDuawJy3LtwRKGEXH7xPRUd7FzH5vM0ZD/5/uNP fWBSe/Vuek0fbxHy7ulBo2tzxgjytQXIhDoKjaRk3SO4pUlWanXPzUwdk9sCfPJMgGQV p18xwdky3Y0ZFWjj2oRsWB1r8iCofsyuM+DRRi3eBUWKYZ0nN2FXeurkdZEHXIwEXyHo 0hKaiu62QCi9O+hzr1ZvTjmsOHLWnqe2N/6m1fgfhTobQv9HYlX2hUiV/odvHm54lxez NCc0aIPRkybQcRORZZdU5UX7FU9qwPmbIdE06KG8aUUiUZFG2ePpUffvAsR6tWHeElFT rYKg== 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=Xx0+AS/R5cTFxTer1KPJhFwelUzEZqlaudVJ4C5UqlM=; b=CNTyybfjS8U+APbUmqCYi0KdEOs/q8dz/nLGfCMhVi787SF1+Et9hfUdE51/3jrCFK kQwWf4IsxPTAHytBw9lYucgXk6ZD2X6eXGz6zz3ZPcMbml9m0s4MQhECwi4/N0tPelch TgAI+rAqu9DN4BoDh8Y0Mh2OJ6jcwxaUw91P1e5czdlWybBErrORBlfFiFw63ZwPBLzr 8XMXyUTu2dvxodU0SvG/0K0ckv4wG8rdOe6/cTg05jqG1dg+EMo8pkX88gWY3jBcLCzz DQ0waeIjRbUduZNNesDsJmolZME9X2N4UFEOwobQAQyb+r0k/7aWAP9kOVLPD0LLzWfy qWNw== X-Gm-Message-State: AFqh2krVOPgYayMCLCrW76kFgTzyWrcHOIm9gcf3fY0rxEMxYTH4uAyS VPkW7xtMNXz5E50hAcqDE7MnB/ZPb+d9o/D+gBgVOQ== X-Google-Smtp-Source: AMrXdXsAYItHDpk4yH6h+tdfdbjpmDtWUNcVfzcOaPv8ocdm14en5egjG3x9mr/LGbfmiQJ1437Ba8Qp+R4Oyn+yujs= X-Received: by 2002:a05:600c:3412:b0:3d0:a619:c445 with SMTP id y18-20020a05600c341200b003d0a619c445mr1027494wmp.17.1674504514768; Mon, 23 Jan 2023 12:08:34 -0800 (PST) MIME-Version: 1.0 References: In-Reply-To: From: Suren Baghdasaryan Date: Mon, 23 Jan 2023 12:08:21 -0800 Message-ID: Subject: Re: [PATCH 39/41] kernel/fork: throttle call_rcu() calls in vm_area_free To: Michal Hocko Cc: Matthew Wilcox , "Liam R. Howlett" , akpm@linux-foundation.org, michel@lespinasse.org, jglisse@google.com, vbabka@suse.cz, hannes@cmpxchg.org, mgorman@techsingularity.net, dave@stgolabs.net, peterz@infradead.org, ldufour@linux.ibm.com, laurent.dufour@fr.ibm.com, paulmck@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, axelrasmussen@google.com, joelaf@google.com, minchan@google.com, jannh@google.com, shakeelb@google.com, tatashin@google.com, edumazet@google.com, gthelen@google.com, gurua@google.com, arjunroy@google.com, soheil@google.com, hughlynch@google.com, leewalsh@google.com, posk@google.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 Mon, Jan 23, 2023 at 12:00 PM Michal Hocko wrote: > > On Mon 23-01-23 19:30:43, Matthew Wilcox wrote: > > On Mon, Jan 23, 2023 at 08:18:37PM +0100, Michal Hocko wrote: > > > On Mon 23-01-23 18:23:08, Matthew Wilcox wrote: > > > > On Mon, Jan 23, 2023 at 09:46:20AM -0800, Suren Baghdasaryan wrote: > > > [...] > > > > > Yes, batching the vmas into a list and draining it in remove_mt() and > > > > > exit_mmap() as you suggested makes sense to me and is quite simple. > > > > > Let's do that if nobody has objections. > > > > > > > > I object. We *know* nobody has a reference to any of the VMAs because > > > > you have to have a refcount on the mm before you can get a reference > > > > to a VMA. If Michal is saying that somebody could do: > > > > > > > > mmget(mm); > > > > vma = find_vma(mm); > > > > lock_vma(vma); > > > > mmput(mm); > > > > vma->a = b; > > > > unlock_vma(mm, vma); > > > > > > > > then that's something we'd catch in review -- you obviously can't use > > > > the mm after you've dropped your reference to it. > > > > > > I am not claiming this is possible now. I do not think we want to have > > > something like that in the future either but that is really hard to > > > envision. I am claiming that it is subtle and potentially error prone to > > > have two different ways of mass vma freeing wrt. locking. Also, don't we > > > have a very similar situation during last munmaps? > > > > We shouldn't have two ways of mass VMA freeing. Nobody's suggesting that. > > There are two cases; there's munmap(), which typically frees a single > > VMA (yes, theoretically, you can free hundreds of VMAs with a single > > call which spans multiple VMAs, but in practice that doesn't happen), > > and there's exit_mmap() which happens on exec() and exit(). > > This requires special casing remove_vma for those two different paths > (exit_mmap and remove_mt). If you ask me that sounds like a suboptimal > code to even not handle potential large munmap which might very well be > a rare thing as you say. But haven't we learned that sooner or later we > will find out there is somebody that cares afterall? Anyway, this is not > something I care about all that much. It is just weird to special case > exit_mmap, if you ask me. Up to Suren to decide which way he wants to > go. I just really didn't like the initial implementation of batching > based on a completely arbitrary batch limit and lazy freeing. I would prefer to go with the simplest sufficient solution. A potential issue with a large munmap might prove to be real but I think we know how to easily fix that with batching if the issue ever materializes (I'll have a fix ready implementing Michal's suggestion). So, I suggest going with Liam's/Matthew's solution and converting to Michal's solution if regression shows up anywhere else. Would that be acceptable? > -- > Michal Hocko > SUSE Labs