Re: [v8 PATCH 05/13] mm: vmscan: use kvfree_rcu instead of call_rcu

On Mon, Mar 8, 2021 at 6:54 AM Paul E. McKenney [off-list ref] wrote:

On Sun, Mar 07, 2021 at 10:13:04PM -0800, Shakeel Butt wrote:

quoted

On Tue, Feb 16, 2021 at 4:13 PM Yang Shi [off-list ref] wrote:

quoted

Using kvfree_rcu() to free the old shrinker_maps instead of call_rcu().
We don't have to define a dedicated callback for call_rcu() anymore.

Signed-off-by: Yang Shi <redacted>
---
 mm/vmscan.c | 7 +------
 1 file changed, 1 insertion(+), 6 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 2e753c2516fa..c2a309acd86b 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c

@@ -192,11 +192,6 @@ static inline int shrinker_map_size(int nr_items)
        return (DIV_ROUND_UP(nr_items, BITS_PER_LONG) * sizeof(unsigned long));
 }

-static void free_shrinker_map_rcu(struct rcu_head *head)
-{
-       kvfree(container_of(head, struct memcg_shrinker_map, rcu));
-}
-
 static int expand_one_shrinker_map(struct mem_cgroup *memcg,
                                   int size, int old_size)
 {

@@ -219,7 +214,7 @@ static int expand_one_shrinker_map(struct mem_cgroup *memcg,
                memset((void *)new->map + old_size, 0, size - old_size);

                rcu_assign_pointer(memcg->nodeinfo[nid]->shrinker_map, new);
-               call_rcu(&old->rcu, free_shrinker_map_rcu);
+               kvfree_rcu(old);

Please use kvfree_rcu(old, rcu) instead of kvfree_rcu(old). The single
param can call synchronize_rcu().

Especially given that you already have the ->rcu field that the
two-argument form requires.

The reason for using the single-argument form is when you have lots of
little data structures, such that getting rid of that rcu_head structure
is valuable enough to be worth the occasional call to synchronize_rcu().
However, please note that this call to synchronize_rcu() happens only
under OOM conditions.

Thanks, Shakeel and Paul. I didn't realize the difference. Will use
the two params form in the new version.

                                                        Thanx, Paul