Re: [PATCH v5 9/9] mm: switch deferred split shrinker to list_lru

[Usama Arif <usama.arif@linux.dev>]

On 27/05/2026 21:45, Johannes Weiner wrote:
> The deferred split queue handles cgroups in a suboptimal fashion. The
> queue is per-NUMA node or per-cgroup, not the intersection. That means
> on a cgrouped system, a node-restricted allocation entering reclaim
> can end up splitting large pages on other nodes:
> 
>         alloc/unmap
>           deferred_split_folio()
>             list_add_tail(memcg->split_queue)
>             set_shrinker_bit(memcg, node, deferred_shrinker_id)
> 
>         for_each_zone_zonelist_nodemask(restricted_nodes)
>           mem_cgroup_iter()
>             shrink_slab(node, memcg)
>               shrink_slab_memcg(node, memcg)
>                 if test_shrinker_bit(memcg, node, deferred_shrinker_id)
>                   deferred_split_scan()
>                     walks memcg->split_queue
> 
> The shrinker bit adds an imperfect guard rail. As soon as the cgroup
> has a single large page on the node of interest, all large pages owned
> by that memcg, including those on other nodes, will be split.
> 
> list_lru properly sets up per-node, per-cgroup lists. As a bonus, it
> streamlines a lot of the list operations and reclaim walks. It's used
> widely by other major shrinkers already. Convert the deferred split
> queue as well.
> 
> The list_lru per-memcg heads are instantiated on demand when the first
> object of interest is allocated for a cgroup, by calling
> folio_memcg_alloc_deferred(). Add calls to where splittable pages are
> created: anon faults, swapin faults, khugepaged collapse.
> 
> These calls create all possible node heads for the cgroup at once, so
> the migration code (between nodes) doesn't need any special care.
> 
> Reported-by: Mikhail Zaslonko <zaslonko@linux.ibm.com>
> Tested-by: Mikhail Zaslonko <zaslonko@linux.ibm.com>
> Acked-by: Shakeel Butt <shakeel.butt@linux.dev>
> Reviewed-by: Lorenzo Stoakes (Oracle) <ljs@kernel.org>
> Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
> ---
>  include/linux/huge_mm.h    |   7 +-
>  include/linux/memcontrol.h |   4 -
>  include/linux/mmzone.h     |  12 --
>  mm/huge_memory.c           | 364 +++++++++++++------------------------
>  mm/internal.h              |   2 +-
>  mm/khugepaged.c            |   5 +
>  mm/memcontrol.c            |  12 +-
>  mm/memory.c                |   4 +
>  mm/mm_init.c               |  15 --
>  mm/swap_state.c            |  10 +
>  10 files changed, 150 insertions(+), 285 deletions(-)
> 

[...]

> diff --git a/mm/memory.c b/mm/memory.c
> index 135f5c0f57bd..f22e61d8c8de 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -5222,6 +5222,10 @@ static struct folio *alloc_anon_folio(struct vm_fault *vmf)
>  			folio_put(folio);
>  			goto next;
>  		}
> +		if (order > 1 && folio_memcg_alloc_deferred(folio)) {
> +			folio_put(folio);

Ah sorry, should have caught this in the previous version, do we need

count_mthp_stat(order, MTHP_STAT_ANON_FAULT_FALLBACK);

here?

or maybe we just goto next instead of goto fallback and trty next
viable order?


> +			goto fallback;
> +		}
>  		folio_throttle_swaprate(folio, gfp);
>  		/*
>  		 * When a folio is not zeroed during allocation