From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S965046AbcBDBjV (ORCPT ); Wed, 3 Feb 2016 20:39:21 -0500 Received: from mail-pf0-f182.google.com ([209.85.192.182]:36645 "EHLO mail-pf0-f182.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755602AbcBDBjT (ORCPT ); Wed, 3 Feb 2016 20:39:19 -0500 Date: Wed, 3 Feb 2016 17:39:08 -0800 (PST) From: Hugh Dickins X-X-Sender: hugh@eggly.anvils To: Johannes Weiner cc: Vladimir Davydov , Mateusz Guzik , Andrew Morton , Michal Hocko , linux-mm@kvack.org, cgroups@vger.kernel.org, linux-kernel@vger.kernel.org, kernel-team@fb.com, Greg Thelen , Hugh Dickins Subject: Re: [PATCH 1/3] mm: migrate: do not touch page->mem_cgroup of live pages In-Reply-To: <20160203183547.GA4007@cmpxchg.org> Message-ID: References: <1454109573-29235-1-git-send-email-hannes@cmpxchg.org> <1454109573-29235-2-git-send-email-hannes@cmpxchg.org> <20160203131748.GB15520@mguzik> <20160203140824.GJ21016@esperanza> <20160203183547.GA4007@cmpxchg.org> User-Agent: Alpine 2.11 (LSU 23 2013-08-11) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, 3 Feb 2016, Johannes Weiner wrote: > CCing Hugh and Greg, they have worked on the memcg migration code most > recently. AFAIK the only reason newpage->mem_cgroup had to be set up > that early in migration was because of the way dirty accounting used > to work. But Hugh took memcg out of the equation there, so moving > mem_cgroup_migrate() to the end should be safe, as long as the pages > are still locked and off the LRU. Yes, that should be safe now: Vladimir's patch looks okay to me, fixing the immediate irq issue. But it would be nicer, if mem_cgroup_migrate() were called solely from migrate_page_copy() - deleting the other calls in mm/migrate.c, including that from migrate_misplaced_transhuge_page() (which does some rewinding on error after its migrate_page_copy(): but just as you now let a successfully migrated old page be uncharged when it's freed, so you can leave a failed new_page to be uncharged when it's freed, no extra code needed). And (even more off-topic), I'm slightly sad to see that the lrucare arg which mem_cgroup_migrate() used to have (before I renamed it and you renamed it back!) has gone, so mem_cgroup_migrate() now always demands lrucare of commit_charge(). I'd hoped that with your separation of new from old charge, mem_cgroup_migrate() would never need lrucare; but that's not true for the fuse case, though true for everyone else. Maybe just not worth bothering about? Or the reintroduction of some unnecessary zone->lru_lock-ing in page migration, which we ought to try to avoid? Or am I wrong, and even fuse doesn't need it? That early return "if (newpage->mem_cgroup)": isn't mem_cgroup_migrate() a no-op for fuse, or is there some corner case by which newpage can be on LRU but its mem_cgroup unset? Hugh