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 B2E10C433F5 for ; Tue, 9 Nov 2021 19:26:59 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 5414361251 for ; Tue, 9 Nov 2021 19:26:59 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.4.1 mail.kernel.org 5414361251 Authentication-Results: mail.kernel.org; dmarc=fail (p=quarantine dis=none) header.from=suse.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=kvack.org Received: by kanga.kvack.org (Postfix) id D18E76B008C; Tue, 9 Nov 2021 14:26:58 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id CC85B6B0093; Tue, 9 Nov 2021 14:26:58 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id BDE646B0095; Tue, 9 Nov 2021 14:26:58 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0093.hostedemail.com [216.40.44.93]) by kanga.kvack.org (Postfix) with ESMTP id AD5D56B008C for ; Tue, 9 Nov 2021 14:26:58 -0500 (EST) Received: from smtpin12.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay02.hostedemail.com (Postfix) with ESMTP id 612037DCBC for ; Tue, 9 Nov 2021 19:26:58 +0000 (UTC) X-FDA: 78790374516.12.3B72FE3 Received: from smtp-out1.suse.de (smtp-out1.suse.de [195.135.220.28]) by imf28.hostedemail.com (Postfix) with ESMTP id CD99A9000E08 for ; Tue, 9 Nov 2021 19:26:57 +0000 (UTC) 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: X-Rspamd-Server: rspam01 X-Rspamd-Queue-Id: CD99A9000E08 X-Stat-Signature: z5anwcjnerimq5eb58b4dg7utf4skr1a Authentication-Results: imf28.hostedemail.com; dkim=pass header.d=suse.com header.s=susede1 header.b=XGbQQD7i; spf=pass (imf28.hostedemail.com: domain of mhocko@suse.com designates 195.135.220.28 as permitted sender) smtp.mailfrom=mhocko@suse.com; dmarc=pass (policy=quarantine) header.from=suse.com X-HE-Tag: 1636486017-460933 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: 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