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 690B8C05027 for ; Mon, 23 Jan 2023 18:47:19 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231851AbjAWSrS (ORCPT ); Mon, 23 Jan 2023 13:47:18 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:57224 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231538AbjAWSrQ (ORCPT ); Mon, 23 Jan 2023 13:47:16 -0500 Received: from mail-yw1-x112c.google.com (mail-yw1-x112c.google.com [IPv6:2607:f8b0:4864:20::112c]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 2335B4497 for ; Mon, 23 Jan 2023 10:47:15 -0800 (PST) Received: by mail-yw1-x112c.google.com with SMTP id 00721157ae682-4a263c4ddbaso185983377b3.0 for ; Mon, 23 Jan 2023 10:47:15 -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=EEZWpRCqDnFRlmuW2ysCODSPytTJZRpx7aAYLqFc7io=; b=XUYoLqkWsCaBJ04HPvri04PJii+UnFNZfkttGO5DhZ8O2cDkfdoED9gTGKUpCW9h1a JBcTELRdCohSIRirMa+mImJm53I336wxROakhil5KRb9AkilXbhbOf8RDwpNO5TUNRN6 5v1+iN4dIzrHw8ZxFeWt3klOFn3U+XFIWQpG90bfiWFj3EU8yyCx+D4tyHkyiB5MXlEH hHlT66itrO9FizYgORvZRn5EsQbfXCI4h1yRuSQMhdrtoQQExigoat0ejKguLIelNJmb HKo6I8eKidhP2zQ97/alMJnO+/yx1DXIiS5235nyc/LfXHwRMP9rsCWu2SLVdJz56lXN bCFA== 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=EEZWpRCqDnFRlmuW2ysCODSPytTJZRpx7aAYLqFc7io=; b=NgyJnUbHQDxTFWssESNMn2hKPUbV22YOMBtSCMCYzaCwhlKE1+3eBPuK7u/1JqMJmj +rztLEtEdrM/MIqRML47rc9zDGdbBJ5MLoSu3Qu6M+2f7AaGqJc6dkBn9NDD1Or0PzqC /mvgKy/o8/QkxxBsuksFSJgB8EVrDo4oUaEy8KlGrnkqa+IE80Nd9bE7xUKIet+dfvvB TTQrMQTmSzdw4OgGuVFQl0ACmtqP7grQHaRN1f+c5V+h7djzzb2llkORmAMylh7kOp5P DTIi49W/0XhRtX7yJOp+DCpIECwoF73HUIpRLPY4LCdv0EioIj1mbmEfz9RvBoJNeA0n 100A== X-Gm-Message-State: AFqh2kphd+qFgmllala8cZ7oLuBoaWpXaTj4qu3UOBP6wQLK2g6/HdDs ZXhgv1Io92bJS6OhJ2IT/IKsJWsxh9GCnbynBsdLpw== X-Google-Smtp-Source: AMrXdXulFktynQi9i95G9lw3C1GDLWNlJVNyrZN3qPgr/W6spDmnmjN0faCW1cv674kzUn8t1U2hk+KJr7613dnjJc4= X-Received: by 2002:a81:708c:0:b0:479:b6a1:d9a4 with SMTP id l134-20020a81708c000000b00479b6a1d9a4mr2744223ywc.263.1674499634132; Mon, 23 Jan 2023 10:47:14 -0800 (PST) MIME-Version: 1.0 References: <20230120170815.yuylbs27r6xcjpq5@revolver> In-Reply-To: From: Suren Baghdasaryan Date: Mon, 23 Jan 2023 10:47:02 -0800 Message-ID: Subject: Re: [PATCH 39/41] kernel/fork: throttle call_rcu() calls in vm_area_free To: Matthew Wilcox Cc: Michal Hocko , "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 10:23 AM Matthew Wilcox wrote: > > On Mon, Jan 23, 2023 at 09:46:20AM -0800, Suren Baghdasaryan wrote: > > On Mon, Jan 23, 2023 at 9:16 AM Michal Hocko wrote: > > > > > > On Mon 23-01-23 09:07:34, Suren Baghdasaryan wrote: > > > > 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(). > > > > > > OK, I have clearly missed that. This makes more sense but it also adds > > > some more complexity and assumptions - a harder to maintain code in the > > > end. Whoever wants to touch this scheme in future would have to > > > re-evaluate all of them. So, I would just avoid that special casing if > > > that is feasible. > > > > Ok, I understand your point. > > > > > > > > Dealing with the flood of call_rcu during exit_mmap is a trivial thing > > > to deal with as proposed elsewhere (just batch all of them in a single > > > run). This will surely add some more code but at least the locking would > > > consistent. > > > > 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); More precisely, it's: mmget(mm); vma = lock_vma_under_rcu(mm, addr); -> calls vma_read_trylock(vma) mmput(mm); vma->a = b; vma_read_unlock(vma); To be fair, vma_read_unlock() does not take mm as a parameter, so one could have an impression that mm doesn't need to be pinned at the time of its call. > > then that's something we'd catch in review -- you obviously can't use > the mm after you've dropped your reference to it. > > Having all this extra code to solve two problems badly is a very poor > choice. We have two distinct problems, each of which has a simple, > efficient solution. >