From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752120AbaJSPbk (ORCPT ); Sun, 19 Oct 2014 11:31:40 -0400 Received: from gum.cmpxchg.org ([85.214.110.215]:42269 "EHLO gum.cmpxchg.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751560AbaJSPb1 (ORCPT ); Sun, 19 Oct 2014 11:31:27 -0400 From: Johannes Weiner To: Andrew Morton Cc: Hugh Dickins , Michal Hocko , linux-mm@kvack.org, cgroups@vger.kernel.org, linux-kernel@vger.kernel.org Subject: [patch] mm: memcontrol: update mem_cgroup_page_lruvec() documentation Date: Sun, 19 Oct 2014 11:30:16 -0400 Message-Id: <1413732616-15962-1-git-send-email-hannes@cmpxchg.org> X-Mailer: git-send-email 2.1.2 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 7512102cf64d ("memcg: fix GPF when cgroup removal races with last exit") added a pc->mem_cgroup reset into mem_cgroup_page_lruvec() to prevent a crash where an anon page gets uncharged on unmap, the memcg is released, and then the final LRU isolation on free dereferences the stale pc->mem_cgroup pointer. But since 0a31bc97c80c ("mm: memcontrol: rewrite uncharge API"), pages are only uncharged AFTER that final LRU isolation, which guarantees the memcg's lifetime until then. pc->mem_cgroup now only needs to be reset for swapcache readahead pages. Update the comment and callsite requirements accordingly. Signed-off-by: Johannes Weiner --- mm/memcontrol.c | 16 ++++++++-------- 1 file changed, 8 insertions(+), 8 deletions(-) diff --git a/mm/memcontrol.c b/mm/memcontrol.c index 3a203c7ec6c7..fc1d7ca96b9d 100644 --- a/mm/memcontrol.c +++ b/mm/memcontrol.c @@ -1262,9 +1262,13 @@ struct lruvec *mem_cgroup_zone_lruvec(struct zone *zone, } /** - * mem_cgroup_page_lruvec - return lruvec for adding an lru page + * mem_cgroup_page_lruvec - return lruvec for isolating/putting an LRU page * @page: the page * @zone: zone of the page + * + * This function is only safe when following the LRU page isolation + * and putback protocol: the LRU lock must be held, and the page must + * either be PageLRU() or the caller must have isolated/allocated it. */ struct lruvec *mem_cgroup_page_lruvec(struct page *page, struct zone *zone) { @@ -1282,13 +1286,9 @@ struct lruvec *mem_cgroup_page_lruvec(struct page *page, struct zone *zone) memcg = pc->mem_cgroup; /* - * Surreptitiously switch any uncharged offlist page to root: - * an uncharged page off lru does nothing to secure - * its former mem_cgroup from sudden removal. - * - * Our caller holds lru_lock, and PageCgroupUsed is updated - * under page_cgroup lock: between them, they make all uses - * of pc->mem_cgroup safe. + * Swapcache readahead pages are added to the LRU - and + * possibly migrated - before they are charged. Ensure + * pc->mem_cgroup is sane. */ if (!PageLRU(page) && !PageCgroupUsed(pc) && memcg != root_mem_cgroup) pc->mem_cgroup = memcg = root_mem_cgroup; -- 2.1.2