mm: memcg: release the css reference when a stock slot empties

consume_stock() can drive a stock slot's nr_pages to zero while its
cached[] pointer stays set, so the slot keeps pinning the css reference
that refill_stock() took.  The offlining drain only flushes slots with
cached pages, so the reference is never released unless the slot happens
to be displaced by an unrelated charge or by CPU hotplug, and the memcg
lingers in the dying state - up to NR_MEMCG_STOCK (7) of them per CPU
under container churn.

Keeping the slot populated past the last page only saves a
css_get()/css_put() pair on the next charge of the same memcg, and costs
more than that: the offlining drain has to know about empty slots, and
refill_stock() cannot reuse them either, so a charge under a different
memcg evicts a live batch through the drain_idx rotation instead.

Drop the reference in consume_stock() when the slot empties.  Empty slots
stop existing, so is_memcg_drain_needed() and the drain path stay as they
are, and refill_stock() reuses emptied slots directly.  The cost is one
refcount pair per emptied slot, at most once per MEMCG_CHARGE_BATCH pages.

Link: https://lore.kernel.org/20260818130135.154315-1-husong@kylinos.cn
Fixes: d1a05b6973c7 ("memcg: do not try to drain per-cpu caches without pages")
Signed-off-by: Song Hu <husong@kylinos.cn>
Acked-by: Michal Hocko <mhocko@suse.com>
Acked-by: Shakeel Butt <shakeel.butt@linux.dev>
Reviewed-by: Joshua Hahn <joshua.hahnjy@gmail.com>
Cc: Audra Mitchell <audra@redhat.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Nico Pache <npache@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

0 files changed, 0 insertions, 0 deletions