mm/list_lru: make the case where mlru is NULL as unlikely

In the following memcg_list_lru_alloc() function, mlru here is almost
always NULL, so in most cases this should save a function call, mark mlru
as unlikely to optimize the code, and reusing the mlru for the next
attempt when the tree insertion fails.

        do {
                xas_lock_irqsave(&xas, flags);
                if (!xas_load(&xas) && !css_is_dying(&pos->css)) {
                        xas_store(&xas, mlru);
                        if (!xas_error(&xas))
                                mlru = NULL;
                }
                xas_unlock_irqrestore(&xas, flags);
        } while (xas_nomem(&xas, GFP_KERNEL));
>       if (mlru)
                kfree(mlru);

Link: https://lkml.kernel.org/r/20250227082223.1173847-1-jingxiangzeng.cas@gmail.com
Signed-off-by: Zeng Jingxiang <linuszeng@tencent.com>
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202412290924.UTP7GH2Z-lkp@intel.com/
Suggested-by: Johannes Weiner <hannes@cmpxchg.org>
Reviewed-by: Muchun Song <muchun.song@linux.dev>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Chengming Zhou <chengming.zhou@linux.dev>
Cc: Jingxiang Zeng <linuszeng@tencent.com>
Cc: Kairui Song <kasong@tencent.com>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/list_lru.c b/mm/list_lru.c
index 7d69434c70e0accf515ad8ee4ac6383167d2235b..490473af312265208f038ef663dbf449af00f6bc 100644 (file)
--- a/mm/list_lru.c
+++ b/mm/list_lru.c
@@ -510,7 +510,7 @@ int memcg_list_lru_alloc(struct mem_cgroup *memcg, struct list_lru *lru,
                         gfp_t gfp)
 {
        unsigned long flags;
-       struct list_lru_memcg *mlru;
+       struct list_lru_memcg *mlru = NULL;
        struct mem_cgroup *pos, *parent;
        XA_STATE(xas, &lru->xa, 0);
 
@@ -535,9 +535,11 @@ int memcg_list_lru_alloc(struct mem_cgroup *memcg, struct list_lru *lru,
                        parent = parent_mem_cgroup(pos);
                }
 
-               mlru = memcg_init_list_lru_one(lru, gfp);
-               if (!mlru)
-                       return -ENOMEM;
+               if (!mlru) {
+                       mlru = memcg_init_list_lru_one(lru, gfp);
+                       if (!mlru)
+                               return -ENOMEM;
+               }
                xas_set(&xas, pos->kmemcg_id);
                do {
                        xas_lock_irqsave(&xas, flags);
@@ -548,10 +550,11 @@ int memcg_list_lru_alloc(struct mem_cgroup *memcg, struct list_lru *lru,
                        }
                        xas_unlock_irqrestore(&xas, flags);
                } while (xas_nomem(&xas, gfp));
-               if (mlru)
-                       kfree(mlru);
        } while (pos != memcg && !css_is_dying(&pos->css));
 
+       if (unlikely(mlru))
+               kfree(mlru);
+
        return xas_error(&xas);
 }
 #else