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 <hughd@google.com>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Andres Lagar-Cavilla <andreslc@google.com>
Cc: Yang Shi <yang.shi@linaro.org>
Cc: Ning Qu <quning@gmail.com>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Andres Lagar-Cavilla <andreslc@google.com>
Cc: Konstantin Khlebnikov <koct9i@gmail.com>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/include/linux/mm_inline.h b/include/linux/mm_inline.h
index 712e8c3..d8cea81 100644 (file)
--- 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);
 }
 

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 011dac8..6a01997 100644 (file)
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1023,22 +1023,38 @@ 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)