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 030F4C433F5 for ; Fri, 22 Oct 2021 12:04:35 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id D2A79610EA for ; Fri, 22 Oct 2021 12:04:34 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230368AbhJVMGu (ORCPT ); Fri, 22 Oct 2021 08:06:50 -0400 Received: from smtp-out1.suse.de ([195.135.220.28]:50248 "EHLO smtp-out1.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229815AbhJVMGr (ORCPT ); Fri, 22 Oct 2021 08:06:47 -0400 Received: from relay2.suse.de (relay2.suse.de [149.44.160.134]) by smtp-out1.suse.de (Postfix) with ESMTP id D8F9C21983; Fri, 22 Oct 2021 12:04:28 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1634904268; 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=h9x+TRkgC4tzH4TgjCK7sVhgC5F3Aew81sGEOoXA6tA=; b=eZzyWW/49xz88I+SIQPEwgELpiQoeI9/I+zOzwBJVBEH/J14rcGLCeXWMYvCMY8J/TqlWX h5ip56Oqg9Y/ke4gBsG6qvzh4Ip9YH5+aFC0Ga0PC9rAK5PEIa6bVV9Cgmtk0PwFd1nB10 Cxh/zeJeWu4BYF6jGD9MnYbgCMMwGRc= 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 769ACA3B81; Fri, 22 Oct 2021 12:04:27 +0000 (UTC) Date: Fri, 22 Oct 2021 14:04:24 +0200 From: Michal Hocko To: Matthew Wilcox Cc: Suren Baghdasaryan , akpm@linux-foundation.org, rientjes@google.com, 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: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri 22-10-21 12:32:08, Matthew Wilcox wrote: > On Fri, Oct 22, 2021 at 10:03:29AM +0200, Michal Hocko wrote: > > 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. > > I have another reason for wanting to take the mmap_sem throughout > exit_mmap. Liam and I are working on using the Maple tree to replace > the rbtree & vma linked list. It uses lockdep to check that you haven't > forgotten to take a lock (as of two days ago, that mean the mmap_sem > or the RCU read lock) when walking the tree. > > So I'd like to hold it over: > > - unlock_range() > - unmap_vmas() > - free_pgtables() > - while (vma) remove_vma() > > Which is basically the whole of exit_mmap(). I'd like to know more > about why there was pushback on holding the mmap_lock across this > -- we're exiting, so nobody else should have a reference to the mm? https://lore.kernel.org/all/20170724072332.31903-1-mhocko@kernel.org/ -- Michal Hocko SUSE Labs