From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754570AbdBVMpX (ORCPT ); Wed, 22 Feb 2017 07:45:23 -0500 Received: from gum.cmpxchg.org ([85.214.110.215]:36572 "EHLO gum.cmpxchg.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752397AbdBVMpO (ORCPT ); Wed, 22 Feb 2017 07:45:14 -0500 Date: Wed, 22 Feb 2017 07:45:01 -0500 From: Johannes Weiner To: Michal Hocko Cc: Andrew Morton , Vladimir Davydov , Chris Down , linux-mm@kvack.org, cgroups@vger.kernel.org, linux-kernel@vger.kernel.org, kernel-team@fb.com Subject: Re: [PATCH] mm: memcontrol: provide shmem statistics Message-ID: <20170222124501.GA9184@cmpxchg.org> References: <20170221164343.32252-1-hannes@cmpxchg.org> <20170222081230.GC5753@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170222081230.GC5753@dhcp22.suse.cz> User-Agent: Mutt/1.7.2 (2016-11-26) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Feb 22, 2017 at 09:12:31AM +0100, Michal Hocko wrote: > On Tue 21-02-17 11:43:43, Johannes Weiner wrote: > > Cgroups currently don't report how much shmem they use, which can be > > useful data to have, in particular since shmem is included in the > > cache/file item while being reclaimed like anonymous memory. > > > > Add a counter to track shmem pages during charging and uncharging. > > Yes this is indeed useful. Accounting shmem to the page cache was a > mistake because this is more than confusing. Sad we cannot fix that. Agreed, this continues to cause confusion with many Linux users :( > I would have just one concern with this patch. You are relying on > PageSwapBacked check but it looks like we are going to implement > MADV_FREE by dropping this flag. I know we do not support MADV_FREE > on shared mappings but if we ever do then the accounting will become > subtly broken. Can/Should we rely on shmem_mapping() check instead? Yes, right now we do MADV_FREE only on private pages, so this patch is safe with Shaohua's changes to how we use PG_swapbacked. Should we support MADV_FREE on shared mappings in the future, using shmem_mapping() for memcg accounting won't work unfortunately, because shared pages are truncated from the page cache before uncharging, and that clears the page->mapping pointer. However, in that case we could probably unaccount the pages from shmem at the time of MADV_FREE, when we clear the PG_swapbacked bit. > Other than that the patch looks good to me. Thanks!