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 C5628C433FE for ; Tue, 9 Nov 2021 19:26:59 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id B07026128B for ; Tue, 9 Nov 2021 19:26:59 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S243245AbhKIT3p (ORCPT ); Tue, 9 Nov 2021 14:29:45 -0500 Received: from smtp-out1.suse.de ([195.135.220.28]:48542 "EHLO smtp-out1.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S243202AbhKIT3n (ORCPT ); Tue, 9 Nov 2021 14:29:43 -0500 Received: from relay2.suse.de (relay2.suse.de [149.44.160.134]) by smtp-out1.suse.de (Postfix) with ESMTP id 4615B21B19; Tue, 9 Nov 2021 19:26:56 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1636486016; 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=dGLWLZhyTaoqPqSWizn7MZ7b7JNExdc0p33R4AoZluI=; b=XGbQQD7iF3W3OgSIc7k+qW8NoNf27eaW15sdDSxSnftbbymcGMf1yUsP3ebgnYs8kpcER2 VRedNeDLlTDGPupMMX2GkyQu+VOesN9nwXkjL3lMR3Qjk9S7yiEWF342VfbnYstSFwXNib DMo33bC2sgwjtpXYRMh0uFpZsBk2biM= 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 0E5A2A3B84; Tue, 9 Nov 2021 19:26:55 +0000 (UTC) Date: Tue, 9 Nov 2021 20:26:51 +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 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. -- Michal Hocko SUSE Labs