Re: [PATCH v2] mm: memcg: release the css reference when a stock slot empties

On Tue, Aug 18, 2026 at 09:01:35PM +0800, Song Hu wrote:
> 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.
> 
> Fixes: d1a05b6973c7 ("memcg: do not try to drain per-cpu caches without pages")
> Signed-off-by: Song Hu <[email protected]>

Acked-by: Shakeel Butt <[email protected]>