From mboxrd@z Thu Jan 1 00:00:00 1970 From: Johannes Weiner Subject: Re: [PATCH v2] mm/vmscan: check references from all memcgs for swapbacked memory Date: Thu, 6 Oct 2022 11:32:45 -0400 Message-ID: References: <20221005173713.1308832-1-yosryahmed@google.com> Mime-Version: 1.0 Return-path: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=cmpxchg-org.20210112.gappssmtp.com; s=20210112; h=in-reply-to:content-disposition:mime-version:references:message-id :subject:cc:to:from:date:from:to:cc:subject:date:message-id:reply-to; bh=HD/3Y6U1aB12XT2sHowJ6F+GbBNpOcL+Q3nS96b8W9o=; b=4WqNJOFbpojr+On41IcjBSg7LLLaApk/OvMwSZrD9xiRQdqxbAuydQWa8UeYIvCyhO 71w8L7bjt2nKU93JMcwDPianc+XdxnPNXX4B+xQCTOIoQHO//929+nqYjQTJ4rlCrGvX SxjZjTK/AM3XuEUPiQi7xuK8HCzqJ358fzfCO8zx1wSic9jk8EDKXu1vMKOBhSJO5PH/ BkZrwgkqQfWTQrK2S+LIPDWTylL0Gn34/3zbK+msE1WdaqKmGo1IWGlwGh23B1zzkwvI ltbqb0bdy6Ex5VCWcVz5xASQbvnvcPLjQVq/AbT5Z7ixy5cZebpN23fMPkSxgtbzE6yn 5DaA== Content-Disposition: inline In-Reply-To: List-ID: Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: Yosry Ahmed Cc: Yu Zhao , Andrew Morton , Michal Hocko , Roman Gushchin , Shakeel Butt , Muchun Song , Greg Thelen , David Rientjes , Cgroups , Linux-MM On Thu, Oct 06, 2022 at 12:30:45AM -0700, Yosry Ahmed wrote: > On Wed, Oct 5, 2022 at 9:19 PM Johannes Weiner wrote: > > > > On Wed, Oct 05, 2022 at 03:13:38PM -0600, Yu Zhao wrote: > > > On Wed, Oct 5, 2022 at 3:02 PM Yosry Ahmed wrote: > > > > > > > > On Wed, Oct 5, 2022 at 1:48 PM Yu Zhao wrote: > > > > > > > > > > On Wed, Oct 5, 2022 at 11:37 AM Yosry Ahmed wrote: > > > > > > > > > > > > During page/folio reclaim, we check if a folio is referenced using > > > > > > folio_referenced() to avoid reclaiming folios that have been recently > > > > > > accessed (hot memory). The rationale is that this memory is likely to be > > > > > > accessed soon, and hence reclaiming it will cause a refault. > > > > > > > > > > > > For memcg reclaim, we currently only check accesses to the folio from > > > > > > processes in the subtree of the target memcg. This behavior was > > > > > > originally introduced by commit bed7161a519a ("Memory controller: make > > > > > > page_referenced() cgroup aware") a long time ago. Back then, refaulted > > > > > > pages would get charged to the memcg of the process that was faulting them > > > > > > in. It made sense to only consider accesses coming from processes in the > > > > > > subtree of target_mem_cgroup. If a page was charged to memcg A but only > > > > > > being accessed by a sibling memcg B, we would reclaim it if memcg A is > > > > > > is the reclaim target. memcg B can then fault it back in and get charged > > > > > > for it appropriately. > > > > > > > > > > > > Today, this behavior still makes sense for file pages. However, unlike > > > > > > file pages, when swapbacked pages are refaulted they are charged to the > > > > > > memcg that was originally charged for them during swapping out. Which > > > > > > means that if a swapbacked page is charged to memcg A but only used by > > > > > > memcg B, and we reclaim it from memcg A, it would simply be faulted back > > > > > > in and charged again to memcg A once memcg B accesses it. In that sense, > > > > > > accesses from all memcgs matter equally when considering if a swapbacked > > > > > > page/folio is a viable reclaim target. > > > > > > > > > > > > Modify folio_referenced() to always consider accesses from all memcgs if > > > > > > the folio is swapbacked. > > > > > > > > > > It seems to me this change can potentially increase the number of > > > > > zombie memcgs. Any risk assessment done on this? > > > > > > > > Do you mind elaborating the case(s) where this could happen? Is this > > > > the cgroup v1 case in mem_cgroup_swapout() where we are reclaiming > > > > from a zombie memcg and swapping out would let us move the charge to > > > > the parent? > > > > > > The scenario is quite straightforward: for a page charged to memcg A > > > and also actively used by memcg B, if we don't ignore the access from > > > memcg B, we won't be able to reclaim it after memcg A is deleted. > > > > This patch changes the behavior of limit-induced reclaim. There is no > > limit reclaim on A after it's been deleted. And parental/global > > reclaim has always recognized outside references. > > Do you mind elaborating on the parental reclaim part? > > I am looking at the code and it looks like memcg reclaim of a parent > (limit-induced or proactive) will only consider references coming from > its subtree, even when reclaiming from its dead children. It looks > like as long as sc->target_mem_cgroup is set, we ignore outside > references (relative to sc->target_mem_cgroup). Yes, I was referring to outside of A. As of today, any siblings of A can already pin its memory after it's dead. I suppose your patch would add cousins to that list. It doesn't seem like a categorial difference to me. > If that is true, maybe we want to keep ignoring outside references for > swap-backed pages if the folio is charged to a dead memcg? My > understanding is that in this case we will uncharge the page from the > dead memcg and charge the swapped entry to the parent, reducing the > number of refs on the dead memcg. Without this check, this patch might > prevent the charge from being moved to the parent in this case. WDYT? I don't think it's worth it. Keeping the semantics simple and behavior predictable is IMO more valuable. It also wouldn't fix the scrape-before-rmdir issue Yu points out, which I think is the more practical concern. In light of that, it might be best to table the patch for now. (Until we have reparent-on-delete for anon and file pages...)