提交 dfe0e773 编写于 作者: J Johannes Weiner 提交者: Linus Torvalds

mm: memcontrol: update mem_cgroup_page_lruvec() documentation

Commit 7512102c ("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 commit 0a31bc97 ("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: NJohannes Weiner <hannes@cmpxchg.org>
Cc: Hugh Dickins <hughd@google.com>
Acked-by: NMichal Hocko <mhocko@suse.cz>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>
上级 bc2f2e7f
......@@ -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;
......
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