[PATCH] mm: Do not allocate shrinker info with cgroup.memory=nokmem

["Michal Koutný" <mkoutny@suse.com>]

There'd be no work for memcg-aware shrinkers when kernel memory is not
accounted per cgroup, so we can skip allocating per memcg shrinker data.
This saves some memory, avoids holding shrinker_mutex with O(nr_memcgs)
and saves work in shrink_slab_memcg().

Then there are SHRINKER_NONSLAB shrinkers which handle non-kernel memory
so nokmem should not disable their per-memcg behavior. Such shrinkers
(e.g.  deferred_split_shrinker) still need access to per-memcg data (see
also commit 0a432dcbeb32e ("mm: shrinker: make shrinker not depend on
memcg kmem")).

The savings with this patch come on container hosts that create many
superblocks (each with own shrinker) but tracking and processing
per-memcg data is pointless with nokmem (shrink_slab_memcg() is
partially guarded with !memcg_kmem_online already).

The patch uses "boottime" predicate mem_cgroup_kmem_disabled() (not
memcg_kmem_online()) to avoid mistakenly un-MEMCG_AWARE-ing shrinkers
registered before first non-root memcg is mkdir'd.

Signed-off-by: Michal Koutný <mkoutny@suse.com>
---
 mm/shrinker.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/mm/shrinker.c b/mm/shrinker.c
index 4a93fd433689a..7d7302619b7f7 100644
--- a/mm/shrinker.c
+++ b/mm/shrinker.c
@@ -219,6 +219,8 @@ static int shrinker_memcg_alloc(struct shrinker *shrinker)
 
 	if (mem_cgroup_disabled())
 		return -ENOSYS;
+	if (mem_cgroup_kmem_disabled() && !(shrinker->flags & SHRINKER_NONSLAB))
+		return -ENOSYS;
 
 	mutex_lock(&shrinker_mutex);
 	id = idr_alloc(&shrinker_idr, shrinker, 0, 0, GFP_KERNEL);

---
base-commit: cd2e103d57e5615f9bb027d772f93b9efd567224
change-id: 20260225-cgroup-ml-nokmem-shrinker-7da42fbcf8f2

Best regards,
-- 
Michal Koutný <mkoutny@suse.com>