Re: [PATCH v3] mm: unqueue deferred split folios outside the lruvec lock

[Shakeel Butt <shakeel.butt@linux.dev>]

On Mon, Oct 19, 2026 at 04:06:46PM -0700, Joanne Koong wrote:
> move_folios_to_lru(), folio_batch_move_lru() and folios_put_refs() call
> folio_unqueue_deferred_split() on folios where the refcount has dropped
> to zero. In these paths, folio_unqueue_deferred_split() is called with
> the lruvec lock often held (move_folios_to_lru() always holds it and
> folio_batch_move_lru()/folios_put_refs() usually hold it for every folio
> in the batch except the first). However, it's not needed to hold the
> lruvec lock.
> 
> The lruvec lock protects folio->lru and the reclaim LRU lists whereas
> the deferred split queue lives on folio->_deferred_list in
> deferred_split_lru. The deferred_split_lru is a list_lru with its own
> per-memcg per-node lock.
> 
> There are already other existing callers that call
> folio_unqueue_deferred_split() without holding the lruvec lock (eg
> __folio_put(), shrink_folio_list(), __folio_migrate_mapping(), ...).
> 
> Add folio_batch_unqueue_deferred_split() and use it in
> move_folios_to_lru(), folio_batch_move_lru(), and folios_put_refs() to
> unqueue each batch of zero-refcount folios from the deferred split list
> after the lruvec lock is released and before
> mem_cgroup_uncharge_folios(). This helps shorten the lruvec lock's
> critical sections, which run with IRQs disabled and are highly
> contended.
> 
> This came out of the discussion on v2 of the mTHP shrinker series [1],
> where queueing small mTHP folios on the deferred split queue was found
> to regress Barry's fault microbenchmark. With [1] applied and
> khugepaged/max_ptes_none=0 so that every folio is queued, perf lock
> contention showed that the extra cost came from waiting on the lruvec
> lock in the free path.
> 
> Test setup: 2-socket Intel Skylake, 40 cores / 80 threads, memory bound
> to node 0 so that all threads contend on one lruvec, with each of 80
> threads faulting in and unmapping 16 MB of anonymous memory 200 times.
> With only 16K mTHP enabled, this patch reduces the queueing cost from
> 15% runtime to noise, and lruvec lock wait time (summed across threads)
> drops from 491 seconds to 38 seconds. Most of the remaining contention
> moves to the deferred split list_lru lock, but runtime still drops back
> to the unqueued baseline. This test uses a worst-case configuration ([1]
> is expected to only queue folios of 2M and larger).
> 
> [1] https://lore.kernel.org/linux-mm/CAJnrk1a2PpUGTsCgS7jct2KET4K2zAL+_h+te8G1MaU9bXs99w@mail.gmail.com/
> 
> Reviewed-by: Lance Yang <lance.yang@linux.dev>
> Signed-off-by: Joanne Koong <joannelkoong@gmail.com>

Acked-by: Shakeel Butt <shakeel.butt@linux.dev>