Re: [PATCH v3 7/7] mm: switch deferred split shrinker to list_lru

[Shakeel Butt]

On Wed, Mar 18, 2026 at 03:53:25PM -0400, 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_list_lru_alloc(). 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.
> 
> Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx>

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