[PATCH v2] memcg: replace memcg ID idr with xarray

[Shakeel Butt <shakeel.butt@xxxxxxxxx>]

At the moment memcg IDs are managed through IDR which requires external
synchronization mechanisms and makes the allocation code a bit awkward.
Let's switch to xarray and make the code simpler.

Signed-off-by: Shakeel Butt <shakeel.butt@xxxxxxxxx>
Suggested-by: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Reviewed-by: Roman Gushchin <roman.gushchin@xxxxxxxxx>
Reviewed-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>
Reviewed-by: Muchun Song <muchun.song@xxxxxxxxx>
Acked-by: Michal Hocko <mhocko@xxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---

Changes since v1:
- Fix error path in mem_cgroup_alloc (Dan Carpenter)

 mm/memcontrol.c | 39 ++++++++++-----------------------------
 1 file changed, 10 insertions(+), 29 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index df84683a0e1c..e8e03a5e1e5e 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -3408,29 +3408,12 @@ static void memcg_wb_domain_size_changed(struct mem_cgroup *memcg)
  */
 
 #define MEM_CGROUP_ID_MAX	((1UL << MEM_CGROUP_ID_SHIFT) - 1)
-static DEFINE_IDR(mem_cgroup_idr);
-static DEFINE_SPINLOCK(memcg_idr_lock);
-
-static int mem_cgroup_alloc_id(void)
-{
-	int ret;
-
-	idr_preload(GFP_KERNEL);
-	spin_lock(&memcg_idr_lock);
-	ret = idr_alloc(&mem_cgroup_idr, NULL, 1, MEM_CGROUP_ID_MAX + 1,
-			GFP_NOWAIT);
-	spin_unlock(&memcg_idr_lock);
-	idr_preload_end();
-	return ret;
-}
+static DEFINE_XARRAY_ALLOC1(mem_cgroup_ids);
 
 static void mem_cgroup_id_remove(struct mem_cgroup *memcg)
 {
 	if (memcg->id.id > 0) {
-		spin_lock(&memcg_idr_lock);
-		idr_remove(&mem_cgroup_idr, memcg->id.id);
-		spin_unlock(&memcg_idr_lock);
-
+		xa_erase(&mem_cgroup_ids, memcg->id.id);
 		memcg->id.id = 0;
 	}
 }
@@ -3465,7 +3448,7 @@ static inline void mem_cgroup_id_put(struct mem_cgroup *memcg)
 struct mem_cgroup *mem_cgroup_from_id(unsigned short id)
 {
 	WARN_ON_ONCE(!rcu_read_lock_held());
-	return idr_find(&mem_cgroup_idr, id);
+	return xa_load(&mem_cgroup_ids, id);
 }
 
 #ifdef CONFIG_SHRINKER_DEBUG
@@ -3558,17 +3541,17 @@ static struct mem_cgroup *mem_cgroup_alloc(struct mem_cgroup *parent)
 	struct mem_cgroup *memcg;
 	int node, cpu;
 	int __maybe_unused i;
-	long error = -ENOMEM;
+	long error;
 
 	memcg = kzalloc(struct_size(memcg, nodeinfo, nr_node_ids), GFP_KERNEL);
 	if (!memcg)
-		return ERR_PTR(error);
+		return ERR_PTR(-ENOMEM);
 
-	memcg->id.id = mem_cgroup_alloc_id();
-	if (memcg->id.id < 0) {
-		error = memcg->id.id;
+	error = xa_alloc(&mem_cgroup_ids, &memcg->id.id, NULL,
+			 XA_LIMIT(1, MEM_CGROUP_ID_MAX), GFP_KERNEL);
+	if (error)
 		goto fail;
-	}
+	error = -ENOMEM;
 
 	memcg->vmstats = kzalloc(sizeof(struct memcg_vmstats),
 				 GFP_KERNEL_ACCOUNT);
@@ -3709,9 +3692,7 @@ static int mem_cgroup_css_online(struct cgroup_subsys_state *css)
 	 * publish it here at the end of onlining. This matches the
 	 * regular ID destruction during offlining.
 	 */
-	spin_lock(&memcg_idr_lock);
-	idr_replace(&mem_cgroup_idr, memcg, memcg->id.id);
-	spin_unlock(&memcg_idr_lock);
+	xa_store(&mem_cgroup_ids, memcg->id.id, memcg, GFP_KERNEL);
 
 	return 0;
 offline_kmem:
-- 
2.43.5