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 6C797C05027 for ; Mon, 23 Jan 2023 17:08:49 +0000 (UTC) Received: from boromir.ozlabs.org (localhost [IPv6:::1]) by lists.ozlabs.org (Postfix) with ESMTP id 4P0xQR5YBpz3cdL for ; Tue, 24 Jan 2023 04:08:47 +1100 (AEDT) Authentication-Results: lists.ozlabs.org; dkim=fail reason="signature verification failed" (2048-bit key; unprotected) header.d=google.com header.i=@google.com header.a=rsa-sha256 header.s=20210112 header.b=Rdkh5YvS; dkim-atps=neutral Authentication-Results: lists.ozlabs.org; spf=pass (sender SPF authorized) smtp.mailfrom=google.com (client-ip=2607:f8b0:4864:20::b29; helo=mail-yb1-xb29.google.com; envelope-from=surenb@google.com; receiver=) Authentication-Results: lists.ozlabs.org; dkim=pass (2048-bit key; unprotected) header.d=google.com header.i=@google.com header.a=rsa-sha256 header.s=20210112 header.b=Rdkh5YvS; dkim-atps=neutral Received: from mail-yb1-xb29.google.com (mail-yb1-xb29.google.com [IPv6:2607:f8b0:4864:20::b29]) (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 4P0xPM5gnpz3bVK for ; Tue, 24 Jan 2023 04:07:50 +1100 (AEDT) Received: by mail-yb1-xb29.google.com with SMTP id b1so10413346ybn.11 for ; Mon, 23 Jan 2023 09:07:50 -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=0FW2Lu3pQY4jZR84HYv4JQ4VKgnKPsvFkN66d0OQdfk=; b=Rdkh5YvSKKbAMeVrmVNplinHHWb9L0j8rdsuQhapYgNXgo2gD8OT9rPe2ZaYeKAx5E pquLD54vscJdCDSHhWXmL1EWzDt5pyAG1htz3dnPZFHMU65k7/ecAz8h560ev1Whkpkx GmJi3/qAY1SytqJGcHyL7ZauQwcXJIeewko/PfLsfneDZVVLvC6oLIKSy4xP3e27K+yS i+du5yv0dwEDCaLuKuAjKsXUTksGEyuqnkOl0M4LdMY5aXamVLJ2M8zCnWuE09jvAmuF sB/MZ98fMFzyGzj/8ucdRz1+7hR1rvsj7rFOBbeWzzRB8MHHZmcx/qo6hKDyOuhNc/f8 Lcuw== 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=0FW2Lu3pQY4jZR84HYv4JQ4VKgnKPsvFkN66d0OQdfk=; b=a3tQABM6DVjwd4n78vE5YtdF4076VWyHqGcQ6zWJIYkyIltSGFEX4o1irWHIOLMeYN pz3Ub9rtamzmNnfkhCwwVrkcaWGoJBz7M9HAvmUWZMaCRbaTTXJY79yxO/1w2KTihI/J rShhQ27p71CMKCXqq1CUwcx7E1m3drf6KKLUY8/ocMsSOL69E8EQn4EbyvuY6pJozz/4 VpO5IdMAX4/yHZhaeFk8hcnhXEf23rk9EztlUz1NCucqjQdTBukqD0D5msACojDEUvMw brYGcqiwcGVEfU/bckSCwsW/BqQu5ewQ38OXXwHZGmiJm/yG0u/ZWI/PrG6wHSr73tfa Gm3g== X-Gm-Message-State: AFqh2kp2NCXyAJ78moSjn6hGGJ9QP0ryPZWDRmeFG1uKj1WBI0VxEWgZ O1rrFE6O6xmRi1VuG63HGrlasidpENjb0C3g/VpP5g== X-Google-Smtp-Source: AMrXdXuiJMyjRQftHqmMyvImwNLCOEQZDFoh5cTDm5Yhcc0MUbhCxbDK85e3u9MmqqS9nUopxnOnX6EdJLA1Sjcj6S4= X-Received: by 2002:a25:ceca:0:b0:7e4:115c:9cf6 with SMTP id x193-20020a25ceca000000b007e4115c9cf6mr2777687ybe.316.1674493667105; Mon, 23 Jan 2023 09:07:47 -0800 (PST) MIME-Version: 1.0 References: <20230120170815.yuylbs27r6xcjpq5@revolver> In-Reply-To: From: Suren Baghdasaryan Date: Mon, 23 Jan 2023 09:07:34 -0800 Message-ID: Subject: Re: [PATCH 39/41] kernel/fork: throttle call_rcu() calls in vm_area_free To: Michal Hocko Content-Type: text/plain; charset="UTF-8" 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, 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, arjunroy@google.com, dave@stgolabs.net, minchan@google.com, x86@kernel.org, hughd@google.com, Matthew Wilcox , gurua@google.com, laurent.dufour@fr.ibm.com, linux-arm-kernel@lists.infradead.org, rientjes@google.com, axelrasmussen@google.com, kernel-team@android.com, soheil@google.com, paulmck@kernel.org, jannh@google.com, "Liam R. Howlett" , 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, hughlynch@google.com, linux-kernel@vger.kernel.org, hannes@cmpxchg.org, akpm@linux-foundation.org, tatashin@g oogle.com, mgorman@techsingularity.net Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Sender: "Linuxppc-dev" On Mon, Jan 23, 2023 at 8:55 AM Michal Hocko wrote: > > On Mon 23-01-23 08:22:53, Suren Baghdasaryan wrote: > > On Mon, Jan 23, 2023 at 1:56 AM Michal Hocko wrote: > > > > > > On Fri 20-01-23 09:50:01, Suren Baghdasaryan wrote: > > > > On Fri, Jan 20, 2023 at 9:32 AM Matthew Wilcox wrote: > > > [...] > > > > > The page fault handler (or whatever other reader -- ptrace, proc, etc) > > > > > should have a refcount on the mm_struct, so we can't be in this path > > > > > trying to free VMAs. Right? > > > > > > > > Hmm. That sounds right. I checked process_mrelease() as well, which > > > > operated on mm with only mmgrab()+mmap_read_lock() but it only unmaps > > > > VMAs without freeing them, so we are still good. Michal, do you agree > > > > this is ok? > > > > > > Don't we need RCU procetions for the vma life time assurance? Jann has > > > already shown how rwsem is not safe wrt to unlock and free without RCU. > > > > Jann's case requires a thread freeing the VMA to be blocked on vma > > write lock waiting for the vma real lock to be released by a page > > fault handler. However exit_mmap() means mm->mm_users==0, which in > > turn suggests that there are no racing page fault handlers and no new > > page fault handlers will appear. Is that a correct assumption? If so, > > then races with page fault handlers can't happen while in exit_mmap(). > > Any other path (other than page fault handlers), accesses vma->lock > > under protection of mmap_lock (for read or write, does not matter). > > One exception is when we operate on an isolated VMA, then we don't > > need mmap_lock protection, but exit_mmap() does not deal with isolated > > VMAs, so out of scope here. exit_mmap() frees vm_area_structs under > > protection of mmap_lock in write mode, so races with anything other > > than page fault handler should be safe as they are today. > > I do not see you talking about #PF (RCU + vma read lock protected) with > munmap. It is my understanding that the latter will synchronize over per > vma lock (along with mmap_lock exclusive locking). But then we are back > to the lifetime guarantees, or do I miss anything. munmap() or any VMA-freeing operation other than exit_mmap() will free using call_rcu(), as implemented today. The suggestion is to free VMAs directly, without RCU grace period only when done from exit_mmap(). That' because VMA freeing flood has been seen so far only in the case of exit_mmap() and we assume other cases are not that heavy to cause call_rcu() flood to cause regressions. That assumption might prove false but we can deal with that once we know it needs fixing. > -- > Michal Hocko > SUSE Labs