When investigating a slab cache bloat problem, significant amount of negative dentry cache was seen, but confusingly they neither got shrunk by reclaimer (the host has very tight memory) nor be shrunk by dropping cache. The vmcore shows there are over 14M negative dentry objects on lru, but tracing result shows they were even not scanned at all.
Further investigation shows the memcg's vfs shrinker_map bit is not set. So the reclaimer or dropping cache just skip calling vfs shrinker. So we have to reboot the hosts to get the memory back.
I didn't manage to come up with a reproducer in test environment, and the problem can't be reproduced after rebooting. But it seems there is race between shrinker map bit clear and reparenting by code inspection. The hypothesis is elaborated as below.
The memcg hierarchy on our production environment looks like:
root / \ system user
The main workloads are running under user slice's children, and it creates and removes memcg frequently. So reparenting happens very often under user slice, but no task is under user slice directly.
So with the frequent reparenting and tight memory pressure, the below hypothetical race condition may happen:
CPU A CPU B reparent dst->nr_items == 0 shrinker: total_objects == 0 add src->nr_items to dst set_bit return SHRINK_EMPTY clear_bit child memcg offline replace child's kmemcg_id with parent's (in memcg_offline_kmem()) list_lru_del() between shrinker runs see parent's kmemcg_id dec dst->nr_items reparent again dst->nr_items may go negative due to concurrent list_lru_del()
The second run of shrinker: read nr_items without any synchronization, so it may see intermediate negative nr_items then total_objects may return 0 coincidently
keep the bit cleared dst->nr_items != 0 skip set_bit add scr->nr_item to dst
After this point dst->nr_item may never go zero, so reparenting will not set shrinker_map bit anymore. And since there is no task under user slice directly, so no new object will be added to its lru to set the shrinker map bit either. That bit is kept cleared forever.
How does list_lru_del() race with reparenting? It is because reparenting replaces children's kmemcg_id to parent's without protecting from nlru->lock, so list_lru_del() may see parent's kmemcg_id but actually deleting items from child's lru, but dec'ing parent's nr_items, so the parent's nr_items may go negative as commit 2788cf0c401c ("memcg: reparent list_lrus and free kmemcg_id on css offline") says.
Since it is impossible that dst->nr_items goes negative and src->nr_items goes zero at the same time, so it seems we could set the shrinker map bit iff src->nr_items != 0. We could synchronize list_lru_count_one() and reparenting with nlru->lock, but it seems checking src->nr_items in reparenting is the simplest and avoids lock contention.
Fixes: fae91d6d8be5 ("mm/list_lru.c: set bit in memcg shrinker bitmap on first list_lru item appearance")
8199be001a47 mm: list_lru: set shrinker map bit when child nr_items is not zero
mm/list_lru.c | 10 +++++-----
1 file changed, 5 insertions(+), 5 deletions(-)