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 CFD1EC433F5 for ; Tue, 16 Nov 2021 13:57:55 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 6A56761B9F for ; Tue, 16 Nov 2021 13:57:55 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.4.1 mail.kernel.org 6A56761B9F 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 EF9B66B0074; Tue, 16 Nov 2021 08:57:54 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id EA7B16B007D; Tue, 16 Nov 2021 08:57:54 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id D98126B007E; Tue, 16 Nov 2021 08:57:54 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0028.hostedemail.com [216.40.44.28]) by kanga.kvack.org (Postfix) with ESMTP id CCF6B6B0074 for ; Tue, 16 Nov 2021 08:57:54 -0500 (EST) Received: from smtpin16.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay03.hostedemail.com (Postfix) with ESMTP id C5C038248D52 for ; Tue, 16 Nov 2021 13:57:46 +0000 (UTC) X-FDA: 78814946532.16.59E25E6 Received: from smtp-out1.suse.de (smtp-out1.suse.de [195.135.220.28]) by imf04.hostedemail.com (Postfix) with ESMTP id 805AB50000AE for ; Tue, 16 Nov 2021 13:57:31 +0000 (UTC) Received: from relay2.suse.de (relay2.suse.de [149.44.160.134]) by smtp-out1.suse.de (Postfix) with ESMTP id 428D7212B8; Tue, 16 Nov 2021 11:29:24 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1637062164; 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=RMic05lzPhLioGF8PuhSrI/0Tr+bIUW81mvQWJAgnnk=; b=q2M1nVDdLKmZ500zVoz3gyiN5+S89CaZfnC160+Ezs0edDkuVfUPhs0fkyKB6Ofh9Q8/dg 5x5czYSTsBKFLISC6cY0lNZS8XsYlV45BV22bb1Owes+qfBqMsofhdoM0yEYC++bAA+KDl uDminKMOzbuoM5K5SFWGuOABHEOUYY4= 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 BCA6EA3B83; Tue, 16 Nov 2021 11:29:23 +0000 (UTC) Date: Tue, 16 Nov 2021 12:29:19 +0100 From: Michal Hocko To: Mina Almasry Cc: Theodore Ts'o , Greg Thelen , Shakeel Butt , Andrew Morton , Hugh Dickins , Roman Gushchin , Johannes Weiner , Tejun Heo , Vladimir Davydov , Muchun Song , riel@surriel.com, linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, cgroups@vger.kernel.org Subject: Re: [PATCH v3 2/4] mm/oom: handle remote ooms Message-ID: References: <20211111234203.1824138-1-almasrymina@google.com> <20211111234203.1824138-3-almasrymina@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Stat-Signature: ap31mmxtcgk14ax1hhx6xzuuandu6qyf Authentication-Results: imf04.hostedemail.com; dkim=pass header.d=suse.com header.s=susede1 header.b=q2M1nVDd; spf=pass (imf04.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-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: 805AB50000AE X-HE-Tag: 1637071051-1766 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 16-11-21 02:17:09, Mina Almasry wrote: > On Tue, Nov 16, 2021 at 1:28 AM Michal Hocko wrote: > > > > On Mon 15-11-21 16:58:19, Mina Almasry wrote: > > > On Mon, Nov 15, 2021 at 2:58 AM Michal Hocko wrote: > > > > > > > > On Fri 12-11-21 09:59:22, Mina Almasry wrote: > > > > > On Fri, Nov 12, 2021 at 12:36 AM Michal Hocko wrote: > > > > > > > > > > > > On Fri 12-11-21 00:12:52, Mina Almasry wrote: > > > > > > > On Thu, Nov 11, 2021 at 11:52 PM Michal Hocko wrote: > > > > > > > > > > > > > > > > On Thu 11-11-21 15:42:01, Mina Almasry wrote: > > > > > > > > > On remote ooms (OOMs due to remote charging), the oom-killer will attempt > > > > > > > > > to find a task to kill in the memcg under oom, if the oom-killer > > > > > > > > > is unable to find one, the oom-killer should simply return ENOMEM to the > > > > > > > > > allocating process. > > > > > > > > > > > > > > > > This really begs for some justification. > > > > > > > > > > > > > > > > > > > > > > I'm thinking (and I can add to the commit message in v4) that we have > > > > > > > 2 reasonable options when the oom-killer gets invoked and finds > > > > > > > nothing to kill: (1) return ENOMEM, (2) kill the allocating task. I'm > > > > > > > thinking returning ENOMEM allows the application to gracefully handle > > > > > > > the failure to remote charge and continue operation. > > > > > > > > > > > > > > For example, in the network service use case that I mentioned in the > > > > > > > RFC proposal, it's beneficial for the network service to get an ENOMEM > > > > > > > and continue to service network requests for other clients running on > > > > > > > the machine, rather than get oom-killed when hitting the remote memcg > > > > > > > limit. But, this is not a hard requirement, the network service could > > > > > > > fork a process that does the remote charging to guard against the > > > > > > > remote charge bringing down the entire process. > > > > > > > > > > > > This all belongs to the changelog so that we can discuss all potential > > > > > > implication and do not rely on any implicit assumptions. > > > > > > > > > > Understood. Maybe I'll wait to collect more feedback and upload v4 > > > > > with a thorough explanation of the thought process. > > > > > > > > > > > E.g. why does > > > > > > it even make sense to kill a task in the origin cgroup? > > > > > > > > > > > > > > > > The behavior I saw returning ENOMEM for this edge case was that the > > > > > code was forever looping the pagefault, and I was (seemingly > > > > > incorrectly) under the impression that a suggestion to forever loop > > > > > the pagefault would be completely fundamentally unacceptable. > > > > > > > > Well, I have to say I am not entirely sure what is the best way to > > > > handle this situation. Another option would be to treat this similar to > > > > ENOSPACE situation. This would result into SIGBUS IIRC. > > > > > > > > The main problem with OOM killer is that it will not resolve the > > > > underlying problem in most situations. Shmem files would likely stay > > > > laying around and their charge along with them. Killing the allocating > > > > task has problems on its own because this could be just a DoS vector by > > > > other unrelated tasks sharing the shmem mount point without a gracefull > > > > fallback. Retrying the page fault is hard to detect. SIGBUS might be > > > > something that helps with the latest. The question is how to communicate > > > > this requerement down to the memcg code to know that the memory reclaim > > > > should happen (Should it? How hard we should try?) but do not invoke the > > > > oom killer. The more I think about this the nastier this is. > > > > > > So actually I thought the ENOSPC suggestion was interesting so I took > > > the liberty to prototype it. The changes required: > > > > > > 1. In out_of_memory() we return false if !oc->chosen && > > > is_remote_oom(). This gets bubbled up to try_charge_memcg() as > > > mem_cgroup_oom() returning OOM_FAILED. > > > 2. In try_charge_memcg(), if we get an OOM_FAILED we again check > > > is_remote_oom(), if it is a remote oom, return ENOSPC. > > > 3. The calling code would return ENOSPC to the user in the no-fault > > > path, and SIGBUS the user in the fault path with no changes. > > > > I think this should be implemented at the caller side rather than > > somehow hacked into the memcg core. It is the caller to know what to do. > > The caller can use gfp flags to control the reclaim behavior. > > > > Hmm I'm a bit struggling to envision this. So would it be acceptable > at the call sites where we doing a remote charge, such as > shmem_add_to_page_cache(), if we get ENOMEM from the > mem_cgroup_charge(), and we know we're doing a remote charge (because > current's memcg != the super block memcg), then we return ENOSPC from > shmem_add_to_page_cache()? I believe that will return ENOSPC to the > userspace in the non-pagefault path and SIGBUS in the pagefault path. > Or you had something else in mind? Yes, exactly. I meant that all this special casing would be done at the shmem layer as it knows how to communicate this usecase. [...] > > And just a small clarification. Tmpfs is fundamentally problematic from > > the OOM handling POV. The nuance here is that the OOM happens in a > > different memcg and thus a different resource domain. If you kill a task > > in the target memcg then you effectively DoS that workload. If you kill > > the allocating task then it is DoSed by anybody allowed to write to that > > shmem. All that without a graceful fallback. > > I don't know if this addresses your concern, but I'm limiting the > memcg= use to processes that can enter that memcg. Therefore they > would be able to allocate memory in that memcg anyway by entering it. > So if they wanted to intentionally DoS that memcg they can already do > it without this feature. Can you elaborate some more? How do you enforce that the mount point cannot be accessed by anybody outside of that constraint? -- Michal Hocko SUSE Labs