From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pf0-f169.google.com (mail-pf0-f169.google.com [209.85.192.169]) by kanga.kvack.org (Postfix) with ESMTP id 0FD444403D8 for ; Wed, 3 Feb 2016 20:39:20 -0500 (EST) Received: by mail-pf0-f169.google.com with SMTP id o185so25879896pfb.1 for ; Wed, 03 Feb 2016 17:39:20 -0800 (PST) Received: from mail-pf0-x233.google.com (mail-pf0-x233.google.com. [2607:f8b0:400e:c00::233]) by mx.google.com with ESMTPS id af6si12822583pad.226.2016.02.03.17.39.19 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Wed, 03 Feb 2016 17:39:19 -0800 (PST) Received: by mail-pf0-x233.google.com with SMTP id w123so26014409pfb.0 for ; Wed, 03 Feb 2016 17:39:19 -0800 (PST) Date: Wed, 3 Feb 2016 17:39:08 -0800 (PST) From: 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> MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: owner-linux-mm@kvack.org List-ID: 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 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 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org