[PATCH 01/10] mm: update_lru_size warn and reset bad lru_size

[Hugh Dickins]

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 <hughd@xxxxxxxxxx>
---
 include/linux/mm_inline.h |    2 +-
 mm/memcontrol.c           |   24 ++++++++++++++++++++----
 2 files changed, 21 insertions(+), 5 deletions(-)

--- a/include/linux/mm_inline.h
+++ b/include/linux/mm_inline.h
@@ -35,8 +35,8 @@ static __always_inline void del_page_fro
 				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);
 }
 
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1022,22 +1022,38 @@ out:
  * @lru: index of lru list the page is sitting on
  * @nr_pages: positive when adding or negative when removing
  *
- * This function must be called when a page is added to or removed from an
- * lru list.
+ * This function must be called under lru_lock, just before a page is added
+ * to or just after a page is removed from an lru list (that ordering being
+ * so as to allow it to check that lru_size 0 is consistent with list_empty).
  */
 void mem_cgroup_update_lru_size(struct lruvec *lruvec, enum lru_list lru,
 				int nr_pages)
 {
 	struct mem_cgroup_per_zone *mz;
 	unsigned long *lru_size;
+	long size;
+	bool empty;
 
 	if (mem_cgroup_disabled())
 		return;
 
 	mz = container_of(lruvec, struct mem_cgroup_per_zone, lruvec);
 	lru_size = mz->lru_size + lru;
-	*lru_size += nr_pages;
-	VM_BUG_ON((long)(*lru_size) < 0);
+	empty = list_empty(lruvec->lists + lru);
+
+	if (nr_pages < 0)
+		*lru_size += nr_pages;
+
+	size = *lru_size;
+	if (WARN_ONCE(size < 0 || empty != !size,
+		"%s(%p, %d, %d): lru_size %ld but %sempty\n",
+		__func__, lruvec, lru, nr_pages, size, empty ? "" : "not ")) {
+		VM_BUG_ON(1);
+		*lru_size = 0;
+	}
+
+	if (nr_pages > 0)
+		*lru_size += nr_pages;
 }
 
 bool task_in_mem_cgroup(struct task_struct *task, struct mem_cgroup *memcg)