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

Hugh Dickins
2016-04-05T20:40:07Z

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@google.com>
---
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)