Messages in this thread Patch in this message | | | From | Johannes Weiner <> | Subject | [patch] mm: memcontrol: update mem_cgroup_page_lruvec() documentation | Date | Sun, 19 Oct 2014 11:30:16 -0400 |
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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 <hannes@cmpxchg.org> --- 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
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