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=-2.3 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED,USER_AGENT_MUTT autolearn=unavailable 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 CC5C8C282DE for ; Thu, 23 May 2019 19:41:38 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id A61D02168B for ; Thu, 23 May 2019 19:41:38 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="FtnxFgFC" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2390553AbfEWTlh (ORCPT ); Thu, 23 May 2019 15:41:37 -0400 Received: from bombadil.infradead.org ([198.137.202.133]:47918 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2389233AbfEWTlf (ORCPT ); Thu, 23 May 2019 15:41:35 -0400 DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=bombadil.20170209; h=In-Reply-To:Content-Type:MIME-Version :References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Id: List-Help:List-Unsubscribe:List-Subscribe:List-Post:List-Owner:List-Archive; bh=Y4CF8VQ482x9+gF5CsOg/mXlQPERa3qs/eCs0oVRAJ8=; b=FtnxFgFCfEdc3mEAZAw8IsXF4 jQpoZBQ3b80gdnApy4UED7IQOONKMYS3liEa9vqS1CyVy5xj44NkYARQJZJzFhXCmbAN9cLJAJ608 4GU5MnZJHC16lhg91q/FopB9whBsFo1JoMQ5lrKNF57bReSLM1haCrprcPTP9OeZtlZ37LsiQNprS bwFpdZ2W/fn+2lbz0zu+AO4SigjcCkqknzEFb0W/ZgQhQLl05fTV6EKHV4e154+YQOMRD5Cq56KQj Yj4UsROp60Rm9vYLIgIjw6l/ItZUFWDZyYwz4+ryzdVyOrM1ewvkWJoDv7Aiqlj+C4h5XpDfleN1E JsFu9hqfA==; Received: from willy by bombadil.infradead.org with local (Exim 4.90_1 #2 (Red Hat Linux)) id 1hTtak-000431-T7; Thu, 23 May 2019 19:41:30 +0000 Date: Thu, 23 May 2019 12:41:30 -0700 From: Matthew Wilcox To: Johannes Weiner Cc: Shakeel Butt , Andrew Morton , Linux MM , linux-fsdevel , LKML , Kernel Team Subject: Re: xarray breaks thrashing detection and cgroup isolation Message-ID: <20190523194130.GA4598@bombadil.infradead.org> References: <20190523174349.GA10939@cmpxchg.org> <20190523183713.GA14517@bombadil.infradead.org> <20190523190032.GA7873@bombadil.infradead.org> <20190523192117.GA5723@cmpxchg.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190523192117.GA5723@cmpxchg.org> User-Agent: Mutt/1.9.2 (2017-12-15) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, May 23, 2019 at 03:21:17PM -0400, Johannes Weiner wrote: > On Thu, May 23, 2019 at 12:00:32PM -0700, Matthew Wilcox wrote: > > On Thu, May 23, 2019 at 11:49:41AM -0700, Shakeel Butt wrote: > > > On Thu, May 23, 2019 at 11:37 AM Matthew Wilcox wrote: > > > > On Thu, May 23, 2019 at 01:43:49PM -0400, Johannes Weiner wrote: > > > > > I noticed that recent upstream kernels don't account the xarray nodes > > > > > of the page cache to the allocating cgroup, like we used to do for the > > > > > radix tree nodes. > > > > > > > > > > This results in broken isolation for cgrouped apps, allowing them to > > > > > escape their containment and harm other cgroups and the system with an > > > > > excessive build-up of nonresident information. > > > > > > > > > > It also breaks thrashing/refault detection because the page cache > > > > > lives in a different domain than the xarray nodes, and so the shadow > > > > > shrinker can reclaim nonresident information way too early when there > > > > > isn't much cache in the root cgroup. > > > > > > > > > > I'm not quite sure how to fix this, since the xarray code doesn't seem > > > > > to have per-tree gfp flags anymore like the radix tree did. We cannot > > > > > add SLAB_ACCOUNT to the radix_tree_node_cachep slab cache. And the > > > > > xarray api doesn't seem to really support gfp flags, either (xas_nomem > > > > > does, but the optimistic internal allocations have fixed gfp flags). > > > > > > > > Would it be a problem to always add __GFP_ACCOUNT to the fixed flags? > > > > I don't really understand cgroups. > > > > > Also some users of xarray may not want __GFP_ACCOUNT. That's the > > > reason we had __GFP_ACCOUNT for page cache instead of hard coding it > > > in radix tree. > > > > This is what I don't understand -- why would someone not want > > __GFP_ACCOUNT? For a shared resource? But the page cache is a shared > > resource. So what is a good example of a time when an allocation should > > _not_ be accounted to the cgroup? > > We used to cgroup-account every slab charge to cgroups per default, > until we changed it to a whitelist behavior: > > commit b2a209ffa605994cbe3c259c8584ba1576d3310c > Author: Vladimir Davydov > Date: Thu Jan 14 15:18:05 2016 -0800 > > Revert "kernfs: do not account ino_ida allocations to memcg" > > Currently, all kmem allocations (namely every kmem_cache_alloc, kmalloc, > alloc_kmem_pages call) are accounted to memory cgroup automatically. > Callers have to explicitly opt out if they don't want/need accounting > for some reason. Such a design decision leads to several problems: > > - kmalloc users are highly sensitive to failures, many of them > implicitly rely on the fact that kmalloc never fails, while memcg > makes failures quite plausible. Doesn't apply here. The allocation under spinlock is expected to fail, and then we'll use xas_nomem() with the caller's specified GFP flags which may or may not include __GFP_ACCOUNT. > - A lot of objects are shared among different containers by design. > Accounting such objects to one of containers is just unfair. > Moreover, it might lead to pinning a dead memcg along with its kmem > caches, which aren't tiny, which might result in noticeable increase > in memory consumption for no apparent reason in the long run. These objects are in the slab of radix_tree_nodes, and we'll already be accounting page cache nodes to the cgroup, so accounting random XArray nodes to the cgroups isn't going to make the problem worse. > - There are tons of short-lived objects. Accounting them to memcg will > only result in slight noise and won't change the overall picture, but > we still have to pay accounting overhead. XArray nodes are generally not short-lived objects.