mm/shrinker: fix refcount leak in shrink_slab_memcg()

mainline inclusion
from mainline-v7.1-rc1
category: bugfix

When kmem is disabled for memcg, slab-backed shrinkers are skipped.
However, shrink_slab_memcg() doesn't drop the reference acquired via
shrinker_try_get() before continuing.

Add the missing shrinker_put().

Also, since memcg_kmem_online() and shrinker flags cannot change
dynamically, remove the shrinker from the bitmap to avoid unnecessary
future scans.

Link: https://lkml.kernel.org/r/20260204033553.50039-1-ahacigu.linux@gmail.com
Fixes: 50d09da ("mm: shrinker: make memcg slab shrink lockless")
Signed-off-by: Altan Hacigumus <ahacigu.linux@gmail.com>
Acked-by: Qi Zheng <zhengqi.arch@bytedance.com>
  Link: https://lore.kernel.org/r/20260203073757.135088-1-ahacigu.linux@gmail.com
Reviewed-by: Muchun Song <muchun.song@linux.dev>
Cc: Dave Chinner <david@fromorbit.com>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
(cherry picked from commit 260d70819c37cf59383286f8dc6566eaa32776bb)
Signed-off-by: Wentao Guan <guanwentao@uniontech.com>

Author: ahacigu-linux

mm/shrinker.c

@@ -544,8 +544,11 @@ static unsigned long shrink_slab_memcg(gfp_t gfp_mask, int nid,
 
 			/* Call non-slab shrinkers even though kmem is disabled */
 			if (!memcg_kmem_online() &&
-			    !(shrinker->flags & SHRINKER_NONSLAB))
+			    !(shrinker->flags & SHRINKER_NONSLAB)) {
+				clear_bit(offset, unit->map);
+				shrinker_put(shrinker);
 				continue;
+			}
 
 			ret = do_shrink_slab(&sc, shrinker, priority);
 			if (ret == SHRINK_EMPTY) {