mm: update_lru_size warn and reset bad lru_size

Though debug kernels have a VM_BUG_ON to help protect from misaccounting
lru_size, non-debug kernels are liable to wrap it around: and then the
vast unsigned long size draws page reclaim into a loop of repeatedly
doing nothing on an empty list, without even a cond_resched().

That soft lockup looks confusingly like an over-busy reclaim scenario,
with lots of contention on the lru_lock in shrink_inactive_list(): yet
has a totally different origin.

Help differentiate with a custom warning in
mem_cgroup_update_lru_size(), even in non-debug kernels; and reset the
size to avoid the lockup.  But the particular bug which suggested this
change was mine alone, and since fixed.

Make it a WARN_ONCE: the first occurrence is the most informative, a
flurry may follow, yet even when rate-limited little more is learnt.

Signed-off-by: Hugh Dickins <[email protected]>
Cc: "Kirill A. Shutemov" <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Cc: Andres Lagar-Cavilla <[email protected]>
Cc: Yang Shi <[email protected]>
Cc: Ning Qu <[email protected]>
Cc: Mel Gorman <[email protected]>
Cc: Andres Lagar-Cavilla <[email protected]>
Cc: Konstantin Khlebnikov <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Johannes Weiner <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

1 files changed, 1 insertions, 1 deletions

diff --git a/include/linux/mm_inline.h b/include/linux/mm_inline.h
index 712e8c37a200..d8cea81ab1ac 100644
--- a/include/linux/mm_inline.h
+++ b/include/linux/mm_inline.h
@@ -35,8 +35,8 @@ static __always_inline void del_page_from_lru_list(struct page *page,
 				struct lruvec *lruvec, enum lru_list lru)
 {
 	int nr_pages = hpage_nr_pages(page);
-	mem_cgroup_update_lru_size(lruvec, lru, -nr_pages);
 	list_del(&page->lru);
+	mem_cgroup_update_lru_size(lruvec, lru, -nr_pages);
 	__mod_zone_page_state(lruvec_zone(lruvec), NR_LRU_BASE + lru, -nr_pages);
 }