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 mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 7B754C433EF for ; Fri, 22 Oct 2021 08:03:38 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 63CE660FE7 for ; Fri, 22 Oct 2021 08:03:38 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232344AbhJVIFy (ORCPT ); Fri, 22 Oct 2021 04:05:54 -0400 Received: from smtp-out1.suse.de ([195.135.220.28]:34640 "EHLO smtp-out1.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231984AbhJVIFs (ORCPT ); Fri, 22 Oct 2021 04:05:48 -0400 Received: from relay2.suse.de (relay2.suse.de [149.44.160.134]) by smtp-out1.suse.de (Postfix) with ESMTP id 37FE92197F; Fri, 22 Oct 2021 08:03:30 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1634889810; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=HvhUhgJHwAx55o4YokA22XdtEWzDOauJw0wkGsftkJE=; b=Ci7jMbyro7pQNwCkLt8dItkEIHsVqVyHs4jNAWucpPnB3LJ58r81yzRR7DWgn/zlVOC4th lDfyRSuTDVMiwda1ZAD2UfB+0cbM9uVlafVhIiS9fyXrogMf4tGdQqLERlYFSXliPOhe5f xC3Yg1khDW6b0OvksM2FGzhz0Nap7IE= Received: from suse.cz (unknown [10.100.201.86]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by relay2.suse.de (Postfix) with ESMTPS id 845CCA3B83; Fri, 22 Oct 2021 08:03:29 +0000 (UTC) Date: Fri, 22 Oct 2021 10:03:29 +0200 From: Michal Hocko To: Suren Baghdasaryan Cc: akpm@linux-foundation.org, rientjes@google.com, willy@infradead.org, hannes@cmpxchg.org, guro@fb.com, riel@surriel.com, minchan@kernel.org, christian@brauner.io, hch@infradead.org, oleg@redhat.com, david@redhat.com, jannh@google.com, shakeelb@google.com, luto@kernel.org, christian.brauner@ubuntu.com, fweimer@redhat.com, jengelh@inai.de, linux-api@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, kernel-team@android.com Subject: Re: [PATCH 1/1] mm: prevent a race between process_mrelease and exit_mmap Message-ID: References: <20211022014658.263508-1-surenb@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20211022014658.263508-1-surenb@google.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu 21-10-21 18:46:58, Suren Baghdasaryan wrote: > Race between process_mrelease and exit_mmap, where free_pgtables is > called while __oom_reap_task_mm is in progress, leads to kernel crash > during pte_offset_map_lock call. oom-reaper avoids this race by setting > MMF_OOM_VICTIM flag and causing exit_mmap to take and release > mmap_write_lock, blocking it until oom-reaper releases mmap_read_lock. > Reusing MMF_OOM_VICTIM for process_mrelease would be the simplest way to > fix this race, however that would be considered a hack. Fix this race > by elevating mm->mm_users and preventing exit_mmap from executing until > process_mrelease is finished. Patch slightly refactors the code to adapt > for a possible mmget_not_zero failure. > This fix has considerable negative impact on process_mrelease performance > and will likely need later optimization. I am not sure there is any promise that process_mrelease will run in parallel with the exiting process. In fact the primary purpose of this syscall is to provide a reliable way to oom kill from user space. If you want to optimize process exit resp. its exit_mmap part then you should be using other means. So I would be careful calling this a regression. I do agree that taking the reference count is the right approach here. I was wrong previously [1] when saying that pinning the mm struct is sufficient. I have completely forgot about the subtle sync in exit_mmap. One way we can approach that would be to take exclusive mmap_sem throughout the exit_mmap unconditionally. There was a push back against that though so arguments would have to be re-evaluated. [1] http://lkml.kernel.org/r/YQzZqFwDP7eUxwcn@dhcp22.suse.cz That being said Acked-by: Michal Hocko Thanks! > Fixes: 884a7e5964e0 ("mm: introduce process_mrelease system call") > Signed-off-by: Suren Baghdasaryan > --- > mm/oom_kill.c | 23 ++++++++++++----------- > 1 file changed, 12 insertions(+), 11 deletions(-) > > diff --git a/mm/oom_kill.c b/mm/oom_kill.c > index 831340e7ad8b..989f35a2bbb1 100644 > --- a/mm/oom_kill.c > +++ b/mm/oom_kill.c > @@ -1150,7 +1150,7 @@ SYSCALL_DEFINE2(process_mrelease, int, pidfd, unsigned int, flags) > struct task_struct *task; > struct task_struct *p; > unsigned int f_flags; > - bool reap = true; > + bool reap = false; > struct pid *pid; > long ret = 0; > > @@ -1177,15 +1177,15 @@ SYSCALL_DEFINE2(process_mrelease, int, pidfd, unsigned int, flags) > goto put_task; > } > > - mm = p->mm; > - mmgrab(mm); > - > - /* If the work has been done already, just exit with success */ > - if (test_bit(MMF_OOM_SKIP, &mm->flags)) > - reap = false; > - else if (!task_will_free_mem(p)) { > - reap = false; > - ret = -EINVAL; > + if (mmget_not_zero(p->mm)) { > + mm = p->mm; > + if (task_will_free_mem(p)) > + reap = true; > + else { > + /* Error only if the work has not been done already */ > + if (!test_bit(MMF_OOM_SKIP, &mm->flags)) > + ret = -EINVAL; > + } > } > task_unlock(p); > > @@ -1201,7 +1201,8 @@ SYSCALL_DEFINE2(process_mrelease, int, pidfd, unsigned int, flags) > mmap_read_unlock(mm); > > drop_mm: > - mmdrop(mm); > + if (mm) > + mmput(mm); > put_task: > put_task_struct(task); > put_pid: > -- > 2.33.0.1079.g6e70778dc9-goog -- Michal Hocko SUSE Labs