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 BAE56C4332F for ; Tue, 9 Nov 2021 19:41:53 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id A0FC861055 for ; Tue, 9 Nov 2021 19:41:53 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S243802AbhKIToj (ORCPT ); Tue, 9 Nov 2021 14:44:39 -0500 Received: from smtp-out1.suse.de ([195.135.220.28]:49686 "EHLO smtp-out1.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231249AbhKIToh (ORCPT ); Tue, 9 Nov 2021 14:44:37 -0500 Received: from relay2.suse.de (relay2.suse.de [149.44.160.134]) by smtp-out1.suse.de (Postfix) with ESMTP id 4FA3121B0B; Tue, 9 Nov 2021 19:41:50 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1636486910; 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=bzBgy3n+R9OViftZQZmytez+H91+PE5W3Pk0JaAPXFM=; b=K6YTwT+buc/h+Wv/uDk/adZvYMO2s0tx79A81qaE+RtYL4cxHC4ZSLDaSyryb/CeboQr7A j6YYFiqRfrkcon9G8y4jLr7T0XmmdXOc3/iDF+vTG9WWzCi0ExBHajGahzId3KYZFCix4Q 7MfpkwnOrxDR1+sIJeFdVyTHwlApXpg= 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 EBF46A3B81; Tue, 9 Nov 2021 19:41:49 +0000 (UTC) Date: Tue, 9 Nov 2021 20:41:48 +0100 From: Michal Hocko To: Suren Baghdasaryan Cc: Andrew Morton , David Rientjes , Matthew Wilcox , Johannes Weiner , Roman Gushchin , Rik van Riel , Minchan Kim , Christian Brauner , Christoph Hellwig , Oleg Nesterov , David Hildenbrand , Jann Horn , Shakeel Butt , Andy Lutomirski , Christian Brauner , Florian Weimer , Jan Engelhardt , Linux API , linux-mm , LKML , kernel-team , "Kirill A. Shutemov" , Andrea Arcangeli Subject: Re: [PATCH 1/1] mm: prevent a race between process_mrelease and exit_mmap Message-ID: References: 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 Tue 09-11-21 20:26:56, Michal Hocko wrote: > On Tue 09-11-21 11:01:02, Suren Baghdasaryan wrote: > [...] > > Discussing how the patch I want to post works for maple trees that > > Matthew is working on, I've got a question: > > > > IIUC, according to Michal's post here: > > https://lore.kernel.org/all/20170725154514.GN26723@dhcp22.suse.cz, > > unmap_vmas() can race with other mmap_lock read holders (including > > oom_reap_task_mm()) with no issues. > > Maple tree patchset requires rcu read lock or the mmap semaphore be > > held (read or write side) when walking the tree, including inside > > unmap_vmas(). When asked, he told me that he is not sure why it's > > currently "safe" to walk the vma->vm_next list in unmap_vmas() while > > another thread is reaping the mm. > > Michal (or maybe someone else), could you please clarify why > > unmap_vmas() can safely race with oom_reap_task_mm()? Or maybe my > > understanding was wrong? > > I cannot really comment on the mapple tree part. But the existing > synchronization between oom reaper and exit_mmap is based on > - oom_reaper takes mmap_sem for reading > - exit_mmap sets MMF_OOM_SKIP and takes the exclusive mmap_sem before > unmap_vmas. > > The oom_reaper therefore can either unmap the address space if the lock > is taken before exit_mmap or it would it would bale out on MMF_OOM_SKIP > if it takes the lock afterwards. So the reaper cannot race with > unmap_vmas. Forgot to mention, that _if_ we can get rid of the nasty unlock;lock pattern in exit_mmap and simply take the exclusive mmap_sem there for unmap_vmas onward then we could get rid of the MMF_OOM_SKIP as well because oom_reaper would simply have no vmas to iterate through so the whole thing would become much more easier to follow. -- Michal Hocko SUSE Labs