Re: [PATCH v3] memcg: cache obj_stock by memcg, not by objcg pointer

[Harry Yoo <harry@kernel.org>]

On 5/19/26 8:41 AM, Shakeel Butt wrote:
> On Mon, May 18, 2026 at 03:28:27PM -0700, Shakeel Butt wrote:
>> Commit 01b9da291c49 ("mm: memcontrol: convert objcg to be per-memcg
>> per-node type") split a memcg's single obj_cgroup into one per NUMA
>> node, but the per-CPU obj_stock_pcp still keys cached_objcg by
>> pointer. Cross-NUMA workloads now see a drain on every refill and a
>> miss on every consume that targets a sibling per-node objcg of the
>> same memcg, producing the 67.7% stress-ng switch-mq regression
>> reported by LKP.
>>
>> stock->nr_bytes are fungible across per-node objcgs of one memcg.
>> Treat the cache as keyed by memcg in __consume_obj_stock() and
>> __refill_obj_stock() so siblings share the reserve. Compare via
>> READ_ONCE(objcg->memcg) directly: pointer-compare only, no deref, so
>> the rcu_read_lock contract on obj_cgroup_memcg() does not apply.
>>
>> Sharing the reserve without re-caching means bytes funded by one
>> per-node objcg's slow path can be consumed/freed under a different
>> sibling, leaving sub-page residue on whichever sibling was cached at
>> drain time. The pre-existing obj_cgroup_release() path would WARN and
>> silently drop that residue, leaking up to nr_node_ids * (PAGE_SIZE - 1)
>> bytes per memcg lifecycle from the page_counter. Forward the residue
>> into a per-node objcg of the same (post-reparent) memcg at release time
>> instead, so it can be reconciled later via a refill atomic_xchg or
>> another release; the chain terminates at root_mem_cgroup, whose
>> page_counter has no enforced limit.
>>
>> Please note that this is temporary fix and will be reverted when
>> per-node kmem accounting is introduced.

... because once per-node kmem accounting is introduced,
"stock->nr_bytes are fungible across per-node objcgs of one memcg"
no longer holds?

And the follow-up plain is to revert this and address it with a 
multi-objcg percpu stock [1], similar to a multi-memcg percpu charge 
cache we have now, right? (regardless of per-node kmem accounting's 
progress)

If this temporary fix imposes other potential correctness issues, would 
it make sense to land [1] in mainline before the next LTS release and 
skip this temporary fix?

[1] https://lore.kernel.org/oe-lkp/agtPMpQK2jXdQAY4@linux.dev

-- 
Cheers,
Harry / Hyeonggon