From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pb0-f49.google.com (mail-pb0-f49.google.com [209.85.160.49]) by kanga.kvack.org (Postfix) with ESMTP id 7CE566B003A for ; Tue, 27 May 2014 03:44:13 -0400 (EDT) Received: by mail-pb0-f49.google.com with SMTP id jt11so8717547pbb.8 for ; Tue, 27 May 2014 00:44:13 -0700 (PDT) Received: from fgwmail2.fujitsu.co.jp (fgwmail2.fujitsu.co.jp. [164.71.1.135]) by mx.google.com with ESMTPS id bh4si17772571pad.219.2014.05.27.00.44.11 for (version=TLSv1 cipher=RC4-SHA bits=128/128); Tue, 27 May 2014 00:44:12 -0700 (PDT) Received: from kw-mxq.gw.nic.fujitsu.com (unknown [10.0.237.131]) by fgwmail2.fujitsu.co.jp (Postfix) with ESMTP id E24C344DD87 for ; Tue, 27 May 2014 16:44:10 +0900 (JST) Received: from s2.gw.fujitsu.co.jp (s2.gw.nic.fujitsu.com [10.0.50.92]) by kw-mxq.gw.nic.fujitsu.com (Postfix) with ESMTP id F1B96AC0C06 for ; Tue, 27 May 2014 16:44:09 +0900 (JST) Received: from m1001.s.css.fujitsu.com (m1001.s.css.fujitsu.com [10.240.81.139]) by s2.gw.fujitsu.co.jp (Postfix) with ESMTP id 912CA1DB803C for ; Tue, 27 May 2014 16:44:09 +0900 (JST) Message-ID: <53844220.5040507@jp.fujitsu.com> Date: Tue, 27 May 2014 16:43:28 +0900 From: Kamezawa Hiroyuki MIME-Version: 1.0 Subject: Re: [patch 9/9] mm: memcontrol: rewrite uncharge API References: <1398889543-23671-1-git-send-email-hannes@cmpxchg.org> <1398889543-23671-10-git-send-email-hannes@cmpxchg.org> In-Reply-To: <1398889543-23671-10-git-send-email-hannes@cmpxchg.org> Content-Type: text/plain; charset=ISO-2022-JP Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Johannes Weiner , linux-mm@kvack.org Cc: Michal Hocko , Hugh Dickins , Tejun Heo , cgroups@vger.kernel.org, linux-kernel@vger.kernel.org (2014/05/01 5:25), Johannes Weiner wrote: > The memcg uncharging code that is involved towards the end of a page's > lifetime - truncation, reclaim, swapout, migration - is impressively > complicated and fragile. > > Because anonymous and file pages were always charged before they had > their page->mapping established, uncharges had to happen when the page > type could be known from the context, as in unmap for anonymous, page > cache removal for file and shmem pages, and swap cache truncation for > swap pages. However, these operations also happen well before the > page is actually freed, and so a lot of synchronization is necessary: > > - On page migration, the old page might be unmapped but then reused, > so memcg code has to prevent an untimely uncharge in that case. > Because this code - which should be a simple charge transfer - is so > special-cased, it is not reusable for replace_page_cache(). > > - Swap cache truncation happens during both swap-in and swap-out, and > possibly repeatedly before the page is actually freed. This means > that the memcg swapout code is called from many contexts that make > no sense and it has to figure out the direction from page state to > make sure memory and memory+swap are always correctly charged. > > But now that charged pages always have a page->mapping, introduce > mem_cgroup_uncharge(), which is called after the final put_page(), > when we know for sure that nobody is looking at the page anymore. > > For page migration, introduce mem_cgroup_migrate(), which is called > after the migration is successful and the new page is fully rmapped. > Because the old page is no longer uncharged after migration, prevent > double charges by decoupling the page's memcg association (PCG_USED > and pc->mem_cgroup) from the page holding an actual charge. The new > bits PCG_MEM and PCG_MEMSW represent the respective charges and are > transferred to the new page during migration. > > mem_cgroup_migrate() is suitable for replace_page_cache() as well. > > Swap accounting is massively simplified: because the page is no longer > uncharged as early as swap cache deletion, a new mem_cgroup_swapout() > can transfer the page's memory+swap charge (PCG_MEMSW) to the swap > entry before the final put_page() in page reclaim. > > Finally, because pages are now charged under proper serialization > (anon: exclusive; cache: page lock; swapin: page lock; migration: page > lock), and uncharged under full exclusion, they can not race with > themselves. Because they are also off-LRU during charge/uncharge, > charge migration can not race, with that, either. Remove the crazily > expensive the page_cgroup lock and set pc->flags non-atomically. > > Signed-off-by: Johannes Weiner The whole series seems wonderful to me. Thank you. I'm not sure whether I have enough good eyes now but this seems good. One thing in my mind is batched uncharge rework. Because uncharge() is done in final put_page() path, mem_cgroup_uncharge_start()/mem_cgroup_uncharge_end() placement may not be good enough. swap.c::release_pages() may be good to have mem_cgroup_uncharge_start()/end(). (and you may be able to remove unnecessary calls of mem_cgroup_uncharge_start/end()) Thanks, -Kame -- 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