From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756809AbbLQQJv (ORCPT ); Thu, 17 Dec 2015 11:09:51 -0500 Received: from gum.cmpxchg.org ([85.214.110.215]:48532 "EHLO gum.cmpxchg.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751447AbbLQQJs (ORCPT ); Thu, 17 Dec 2015 11:09:48 -0500 Date: Thu, 17 Dec 2015 11:09:25 -0500 From: Johannes Weiner To: Vladimir Davydov Cc: Andrew Morton , Michal Hocko , linux-mm@kvack.org, cgroups@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v2 1/7] mm: memcontrol: charge swap to cgroup2 Message-ID: <20151217160925.GA24124@cmpxchg.org> References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.24 (2015-08-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Dec 17, 2015 at 03:29:54PM +0300, Vladimir Davydov wrote: > In the legacy hierarchy we charge memsw, which is dubious, because: > > - memsw.limit must be >= memory.limit, so it is impossible to limit > swap usage less than memory usage. Taking into account the fact that > the primary limiting mechanism in the unified hierarchy is > memory.high while memory.limit is either left unset or set to a very > large value, moving memsw.limit knob to the unified hierarchy would > effectively make it impossible to limit swap usage according to the > user preference. > > - memsw.usage != memory.usage + swap.usage, because a page occupying > both swap entry and a swap cache page is charged only once to memsw > counter. As a result, it is possible to effectively eat up to > memory.limit of memory pages *and* memsw.limit of swap entries, which > looks unexpected. > > That said, we should provide a different swap limiting mechanism for > cgroup2. > > This patch adds mem_cgroup->swap counter, which charges the actual > number of swap entries used by a cgroup. It is only charged in the > unified hierarchy, while the legacy hierarchy memsw logic is left > intact. > > The swap usage can be monitored using new memory.swap.current file and > limited using memory.swap.max. > > Note, to charge swap resource properly in the unified hierarchy, we have > to make swap_entry_free uncharge swap only when ->usage reaches zero, > not just ->count, i.e. when all references to a swap entry, including > the one taken by swap cache, are gone. This is necessary, because > otherwise swap-in could result in uncharging swap even if the page is > still in swap cache and hence still occupies a swap entry. At the same > time, this shouldn't break memsw counter logic, where a page is never > charged twice for using both memory and swap, because in case of legacy > hierarchy we uncharge swap on commit (see mem_cgroup_commit_charge). This was actually an oversight when rewriting swap accounting. It should have always been uncharged when the swap slot is released. > Signed-off-by: Vladimir Davydov Acked-by: Johannes Weiner