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=-15.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER, INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED autolearn=ham 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 4AE21C433DB for ; Tue, 16 Feb 2021 17:19:43 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id E9F5064E00 for ; Tue, 16 Feb 2021 17:19:42 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org E9F5064E00 Authentication-Results: mail.kernel.org; dmarc=fail (p=quarantine dis=none) header.from=suse.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id 894D56B0005; Tue, 16 Feb 2021 12:19:42 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id 844556B0006; Tue, 16 Feb 2021 12:19:42 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 735136B006C; Tue, 16 Feb 2021 12:19:42 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0237.hostedemail.com [216.40.44.237]) by kanga.kvack.org (Postfix) with ESMTP id 5D2276B0005 for ; Tue, 16 Feb 2021 12:19:42 -0500 (EST) Received: from smtpin17.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay03.hostedemail.com (Postfix) with ESMTP id 258AB8248047 for ; Tue, 16 Feb 2021 17:19:42 +0000 (UTC) X-FDA: 77824793004.17.burn48_410158d27645 Received: from filter.hostedemail.com (10.5.16.251.rfc1918.com [10.5.16.251]) by smtpin17.hostedemail.com (Postfix) with ESMTP id 04FAC180D07A2 for ; Tue, 16 Feb 2021 17:19:41 +0000 (UTC) X-HE-Tag: burn48_410158d27645 X-Filterd-Recvd-Size: 5628 Received: from mx2.suse.de (mx2.suse.de [195.135.220.15]) by imf45.hostedemail.com (Postfix) with ESMTP for ; Tue, 16 Feb 2021 17:19:41 +0000 (UTC) X-Virus-Scanned: by amavisd-new at test-mx.suse.de DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1613495980; 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=rOgv2eEZRaweG7NJSebRdSvYOOr4gqJRXTJSspBSCOc=; b=h74MeufS56pS+vSQUpiTiDJslgiawZx4tJtn7aauwv7pBJhcNTfCnkvkZcET1ccH9wgS0S uSIXIOcmQcAWyo9lIedoNUMHCxP1lJvUJ71ea01Qe4mg3wVOss7YoIzXmifMbWipNTDnwz bmUSS78TXUvirG7wUzA62/J22oR4h+8= Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id 12DE6B7A4; Tue, 16 Feb 2021 17:19:40 +0000 (UTC) Date: Tue, 16 Feb 2021 18:19:39 +0100 From: Michal Hocko To: Johannes Weiner Cc: Muchun Song , vdavydov.dev@gmail.com, akpm@linux-foundation.org, cgroups@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 4/4] mm: memcontrol: fix swap uncharge on cgroup v2 Message-ID: References: <20210212170159.32153-1-songmuchun@bytedance.com> <20210212170159.32153-4-songmuchun@bytedance.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: 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 16-02-21 11:59:00, Johannes Weiner wrote: > Hello Muchun, > > On Sat, Feb 13, 2021 at 01:01:59AM +0800, Muchun Song wrote: > > The swap charges the actual number of swap entries on cgroup v2. > > If a swap cache page is charged successful, and then we uncharge > > the swap counter. It is wrong on cgroup v2. Because the swap > > entry is not freed. > > The patch makes sense to me. But this code is a bit tricky, we should > add more documentation to how it works and what the problem is. > > How about this for the changelog? > > --- > mm: memcontrol: fix swap undercounting for shared pages in cgroup2 > > When shared pages are swapped in partially, we can have some page > tables referencing the in-memory page and some referencing the swap > slot. Cgroup1 and cgroup2 handle these overlapping lifetimes slightly > differently due to the nature of how they account memory and swap: > > Cgroup1 has a unified memory+swap counter that tracks a data page > regardless whether it's in-core or swapped out. On swapin, we transfer > the charge from the swap entry to the newly allocated swapcache page, > even though the swap entry might stick around for a while. That's why > we have a mem_cgroup_uncharge_swap() call inside mem_cgroup_charge(). > > Cgroup2 tracks memory and swap as separate, independent resources and > thus has split memory and swap counters. On swapin, we charge the > newly allocated swapcache page as memory, while the swap slot in turn > must remain charged to the swap counter as long as its allocated too. > > The cgroup2 logic was broken by commit 2d1c498072de ("mm: memcontrol: > make swap tracking an integral part of memory control"), because it > accidentally removed the do_memsw_account() check in the branch inside > mem_cgroup_uncharge() that was supposed to tell the difference between > the charge transfer in cgroup1 and the separate counters in cgroup2. > > As a result, cgroup2 currently undercounts consumed swap when shared > pages are partially swapped back in. This in turn allows a cgroup to > consume more swap than its configured limit intends. > > Add the do_memsw_account() check back to fix this problem. Yes this clarfies both the issue and the subtlety of the accounting. Thanks a lot Johannes! This is a great example of how changelogs should really look. > --- > > > Fixes: 2d1c498072de ("mm: memcontrol: make swap tracking an integral part of memory control") > > Signed-off-by: Muchun Song > > Acked-by: Johannes Weiner > > > --- > > mm/memcontrol.c | 2 +- > > 1 file changed, 1 insertion(+), 1 deletion(-) > > > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c > > index c737c8f05992..be6bc5044150 100644 > > --- a/mm/memcontrol.c > > +++ b/mm/memcontrol.c > > @@ -6753,7 +6753,7 @@ int mem_cgroup_charge(struct page *page, struct mm_struct *mm, gfp_t gfp_mask) > > memcg_check_events(memcg, page); > > local_irq_enable(); > > > > - if (PageSwapCache(page)) { > > + if (!cgroup_subsys_on_dfl(memory_cgrp_subsys) && PageSwapCache(page)) { > > It's more descriptive to use do_memsw_account() here, IMO. > > We should also add a comment. How about this above the branch? > > /* > * Cgroup1's unified memory+swap counter has been charged with the > * new swapcache page, finish the transfer by uncharging the swap > * slot. The swap slot would also get uncharged when it dies, but > * for shared pages it can stick around indefinitely and we'd count > * the page twice the entire time. > * > * Cgroup2 has separate resource counters for memory and swap, > * so this is a non-issue here. Memory and swap charge lifetimes > * correspond 1:1 to page and swap slot lifetimes: we charge the > * page to memory here, and uncharge swap when the slot is freed. > */ Yes very helpful. With the changelog update and the comment Acked-by: Michal Hocko Thanks! -- Michal Hocko SUSE Labs