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 40218C433EF for ; Tue, 9 Nov 2021 19:37:20 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id E668760EE9 for ; Tue, 9 Nov 2021 19:37:19 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.4.1 mail.kernel.org E668760EE9 Authentication-Results: mail.kernel.org; dmarc=fail (p=reject dis=none) header.from=google.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=kvack.org Received: by kanga.kvack.org (Postfix) id 7DD206B0095; Tue, 9 Nov 2021 14:37:19 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id 7663D6B0098; Tue, 9 Nov 2021 14:37:19 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 5E1E46B0099; Tue, 9 Nov 2021 14:37:19 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0088.hostedemail.com [216.40.44.88]) by kanga.kvack.org (Postfix) with ESMTP id 497FC6B0095 for ; Tue, 9 Nov 2021 14:37:19 -0500 (EST) Received: from smtpin26.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay04.hostedemail.com (Postfix) with ESMTP id F41597BE56 for ; Tue, 9 Nov 2021 19:37:18 +0000 (UTC) X-FDA: 78790400556.26.CE9BDB5 Received: from mail-yb1-f172.google.com (mail-yb1-f172.google.com [209.85.219.172]) by imf07.hostedemail.com (Postfix) with ESMTP id 700FD1000F4E for ; Tue, 9 Nov 2021 19:37:18 +0000 (UTC) Received: by mail-yb1-f172.google.com with SMTP id v7so417979ybq.0 for ; Tue, 09 Nov 2021 11:37:18 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20210112; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=dQprVm+3qzu3dZEgfzO0O6Slbj/WjUJyb02QjMT9Eas=; b=SrO7kkz0GdY4VvmfhvtWo2O0vzZDq5YOBSr9XsZJEV3iK1f3RmY36DnsqZg+NgboLx wtNxrG4NX2VdfMmljAXeEkr4cGAq3Z4rl+XNbdMrZ5aQ44K6lr0tBW6kLFUrM/9zoRc+ urTz8KcHWEAaylDlPGfuTQZhVvffy8iZr0bZp+jNzaZahSrwBunJuphW6xi0PgdbC/Vt K7nBwjZ0/thjuEhFeSnF4EN6FzKWb5NocwhE0V2YMQEjDJrTD4CPjTl5LgVJGKNTrcJG z6rNnLYs5BODTzogjYlDR3/qd7HUkFuOjuCbn+LAJ5HXpPP2xSFZq7HJ7sY9nVXvVSTf 7H+g== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=dQprVm+3qzu3dZEgfzO0O6Slbj/WjUJyb02QjMT9Eas=; b=K6KHTcB1khf/jB7U014Mbpoh9o/AKPdAQKdQBzjeKI5Bb9A1GmAfTH4inVr3yhiKnK oglWJb8QCCvkxw9pLbh06tfxoHcrMZzPekAcxoP3lW0yh6gIh1MCzayICRdBW4pubXF3 U0gZx4tS7/O6mUMyOAzGDzNdaExL/SGyDbp8MlCQaroyxBnO60QFDS6r1rD80GB8SRJF 2PmRyWiQe4WV0ddOcIxzuy60ZF6lTeEJuLdGR3JzpQx3lngZVC/uvr+3/71Ye0JR2hNZ d2Xdt8ikXV6M+y4+a6HmjMJY510ns0ycmqmQSuaB39Cltc2f3lbnrEhjV/I+8+0G2ZAL pntg== X-Gm-Message-State: AOAM530+/ML3QBnaCgC64PhzqhwxbfpgS1s6mW7wxCiICaAuuugQlfen ofNVmaNeaXnXmWgafd0/efJHIpFzvSi9JT+gV+kGbw== X-Google-Smtp-Source: ABdhPJxU75C9fcuW8uYWKXj+MUN9NRQwAfY7318XA4p/9zFwRGBY+kpjk9UHdNU9CUXomL9Q5fmwM8CO8aELT+WipXs= X-Received: by 2002:a25:d4d5:: with SMTP id m204mr12272621ybf.418.1636486637539; Tue, 09 Nov 2021 11:37:17 -0800 (PST) MIME-Version: 1.0 References: In-Reply-To: From: Suren Baghdasaryan Date: Tue, 9 Nov 2021 11:37:06 -0800 Message-ID: Subject: Re: [PATCH 1/1] mm: prevent a race between process_mrelease and exit_mmap To: Michal Hocko 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 Content-Type: text/plain; charset="UTF-8" Authentication-Results: imf07.hostedemail.com; dkim=pass header.d=google.com header.s=20210112 header.b=SrO7kkz0; spf=pass (imf07.hostedemail.com: domain of surenb@google.com designates 209.85.219.172 as permitted sender) smtp.mailfrom=surenb@google.com; dmarc=pass (policy=reject) header.from=google.com X-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: 700FD1000F4E X-Stat-Signature: us7d1nn7n6fduyxykmcrrr7hc48en5tx X-HE-Tag: 1636486638-419054 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, Nov 9, 2021 at 11:26 AM 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. I see. So, it's the combination of MMF_OOM_SKIP and mmap_lock working as a barrier which prevent them from racing with each other... I wasn't sure how https://lore.kernel.org/all/20170724072332.31903-1-mhocko@kernel.org/ was implementing this synchronization because it would take mmap_sem write side after unmap_vmas() and IIUC there was no "mmap_lock_write(); mmap_unlock_write();" sequence in exit_mmap at that time. I'll need to checkout the old sources to figure this out. Thanks, Suren. > > -- > Michal Hocko > SUSE Labs