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 A9CA0C433EF for ; Thu, 21 Oct 2021 16:47:35 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 8A92161409 for ; Thu, 21 Oct 2021 16:47:35 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231891AbhJUQtu (ORCPT ); Thu, 21 Oct 2021 12:49:50 -0400 Received: from smtp-out1.suse.de ([195.135.220.28]:55740 "EHLO smtp-out1.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231520AbhJUQts (ORCPT ); Thu, 21 Oct 2021 12:49:48 -0400 Received: from relay2.suse.de (relay2.suse.de [149.44.160.134]) by smtp-out1.suse.de (Postfix) with ESMTP id E86202199D; Thu, 21 Oct 2021 16:47:31 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1634834851; 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=++iA8+uhip7K2PBGESkJUKSkd2b03fx9ioYV7M+4ADc=; b=hW3ymvnHy/zIjqEGz83CLNjmBv0TkMYAecEBj9ZicauP81r1HFWZ1zXmQBDdateXmP6kUp mtbGbyRFsaIHg764GPo8VMrUN+e05/45ZU7+3dDBD60xJXN36r8xEaTA6nM84CnyhJB6Tt gEEGzt8067R6EwrCFARVP0xMDJPvyPQ= 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 1AF66A3B84; Thu, 21 Oct 2021 16:47:31 +0000 (UTC) Date: Thu, 21 Oct 2021 18:47:29 +0200 From: Michal Hocko To: Vasily Averin Cc: Johannes Weiner , Vladimir Davydov , Andrew Morton , Roman Gushchin , Uladzislau Rezki , Vlastimil Babka , Shakeel Butt , Mel Gorman , Tetsuo Handa , cgroups@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, kernel@openvz.org Subject: Re: [PATCH memcg 3/3] memcg: handle memcg oom failures 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 Thu 21-10-21 18:05:28, Vasily Averin wrote: > On 21.10.2021 14:49, Michal Hocko wrote: > > I do understand that handling a very specific case sounds easier but it > > would be better to have a robust fix even if that requires some more > > head scratching. So far we have collected several reasons why the it is > > bad to trigger oom killer from the #PF path. There is no single argument > > to keep it so it sounds like a viable path to pursue. Maybe there are > > some very well hidden reasons but those should be documented and this is > > a great opportunity to do either of the step. > > > > Moreover if it turns out that there is a regression then this can be > > easily reverted and a different, maybe memcg specific, solution can be > > implemented. > > Now I'm agree, > however I still have a few open questions. > > 1) VM_FAULT_OOM may be triggered w/o execution of out_of_memory() > for exampel it can be caused by incorrect vm fault operations, > (a) which can return this error without calling allocator at all. I would argue this to be a bug. How can that particular code tell whether the system is OOM and the oom killer is the a reasonable measure to take? > (b) or which can provide incorrect gfp flags and allocator can fail without execution of out_of_memory. I am not sure I can see any sensible scenario where pagefault oom killer would be an appropriate fix for that. > (c) This may happen on stable/LTS kernels when successful allocation was failed by hit into limit of legacy memcg-kmem contoller. > We'll drop it in upstream kernels, however how to handle it in old kenrels? Triggering the global oom killer for legacy kmem charge failure is clearly wrong. Removing oom killer from #PF would fix that problem. > We can make sure that out_of_memory or alocator was called by set of some per-task flags. I am not sure I see how that would be useful other than reporting a dubious VM_FAULT_OOM usage. I am also not sure how that would be implemented as allocator can be called several times not to mention that the allocation itself could have been done from a different context - e.g. WQ. > Can pagefault_out_of_memory() send itself a SIGKILL in all these cases? In principle it can as sending signal is not prohibited. I would argue it should not though because it is just wrong thing to do in all those cases. > If not -- task will be looped. Yes, but it will be killable from userspace. So this is not an unrecoverable situation. > It is much better than execution of global OOM, however it would be even better to avoid it somehow. How? > You said: "We cannot really kill the task if we could we would have done it by the oom killer already". > However what to do if we even not tried to use oom-killer? (see (b) and (c)) > or if we did not used the allocator at all (see (a)) See above > 2) in your patch we just exit from pagefault_out_of_memory(). and restart new #PF. > We can call schedule_timeout() and wait some time before a new #PF restart. > Additionally we can increase this delay in each new cycle. > It helps to save CPU time for other tasks. > What do you think about? I do not have a strong opinion on this. A short sleep makes sense. I am not sure a more complex implementation is really needed. -- Michal Hocko SUSE Labs