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Commit dfe0e773 authored by Johannes Weiner's avatar Johannes Weiner Committed by Linus Torvalds
Browse files

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: default avatarJohannes Weiner <hannes@cmpxchg.org>
Cc: Hugh Dickins <hughd@google.com>
Acked-by: default avatarMichal Hocko <mhocko@suse.cz>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent bc2f2e7f

mm/memcontrol.c

+8 −8
Original line number Original line Diff line number Diff line
@@ -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

 * @page: the page

 * @zone: zone of 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)

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;

	memcg = pc->mem_cgroup;





	/*

	/*

	 * Surreptitiously switch any uncharged offlist page to root:

	 * Swapcache readahead pages are added to the LRU - and

	 * an uncharged page off lru does nothing to secure

	 * possibly migrated - before they are charged.  Ensure

	 * its former mem_cgroup from sudden removal.

	 * pc->mem_cgroup is sane.

	 *

	 * Our caller holds lru_lock, and PageCgroupUsed is updated

	 * under page_cgroup lock: between them, they make all uses

	 * of pc->mem_cgroup safe.

	 */

	 */

	if (!PageLRU(page) && !PageCgroupUsed(pc) && memcg != root_mem_cgroup)

	if (!PageLRU(page) && !PageCgroupUsed(pc) && memcg != root_mem_cgroup)

		pc->mem_cgroup = memcg = root_mem_cgroup;

		pc->mem_cgroup = memcg = root_mem_cgroup;