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 X-Spam-Level: X-Spam-Status: No, score=-5.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id E7049C433EF for ; Tue, 21 Sep 2021 07:50:25 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id CC44060EE7 for ; Tue, 21 Sep 2021 07:50:25 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230317AbhIUHvw (ORCPT ); Tue, 21 Sep 2021 03:51:52 -0400 Received: from smtp-out1.suse.de ([195.135.220.28]:60946 "EHLO smtp-out1.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230136AbhIUHvs (ORCPT ); Tue, 21 Sep 2021 03:51:48 -0400 Received: from relay2.suse.de (relay2.suse.de [149.44.160.134]) by smtp-out1.suse.de (Postfix) with ESMTP id 9938D21D59; Tue, 21 Sep 2021 07:50:19 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1632210619; 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=FTiZUwoZLD7CoTiPY7EAa8Ut/2BuzZbrnfa7NhOfH4g=; b=WaTWtd/zyEaKy32pCO/ZFivMBrGBsNSSYsOmLfz2N2vNrCNSqUdoJXcC5wAJfcHU1tEjFe hLxCmBUc61a4RKr1Zs4/cThVcZfwuJIJ/KE+0hwgql2dctCRltou2hlafJSjc+G8Co/py7 lAsLJeP1KoKdl9BY+Hob9NW/qbqRgbY= 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 4584EA3B9D; Tue, 21 Sep 2021 07:50:19 +0000 (UTC) Date: Tue, 21 Sep 2021 09:50:18 +0200 From: Michal Hocko To: Sultan Alsawaf Cc: Andrew Morton , David Rientjes , Mel Gorman , linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] mm: Mark the OOM reaper thread as freezable Message-ID: References: <20210918233920.9174-1-sultan@kerneltoast.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 Mon 20-09-21 15:29:46, Sultan Alsawaf wrote: > On Mon, Sep 20, 2021 at 10:30:12PM +0200, Michal Hocko wrote: > > We usually tend to provide Fixes where there has been something fixed. > > It seems just confusing if it is used for non functional changes, > > cleanups etc. Thera are gray zones of course. > > Got it, thanks. So no tag would be used in such a case? > > > I am not sure I follow. My understanding is that we need to make sure > > oom_reaper is not running after the quiescent state as it is changing > > user space address space. For that I believe we need to freeze the > > kthread at a proper moment. That is currently the entry point and maybe > > we can extend that even to the reaping loop but I haven't really > > explored that. PF_FREEZER_SKIP would skip over the reaper and that could > > result in it racing with the snapshotting no? > > Kthreads cannot be implicitly frozen; it's not like PREEMPT. From what I've read > in the freezer code, two things must occur for a kthread to freeze: the kthread > must have PF_NOFREEZE unset (via set_freezable(), as is done in the patch I've > submitted here), and the kthread must have an explicit call into the freezer, > such as via wait_event_freezable(). > > Right now, oom_reaper is totally ignored by the freezer because PF_NOFREEZE is > set by default in all kthreads. As such, oom_reaper can keep running while > system-wide freezing occurs. If you think this can mangle snapshots, then > perhaps there is a real bug here after all. OK, now I am getting your point. Sorry for being dense here. Process freezing has always been kinda muddy to me as I've said earlier. I have completely misunderstood your earlier PF_NOFREEZE remark. -- Michal Hocko SUSE Labs