memcg: replace memcg ID idr with xarray

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.

[shakeel.butt@linux.dev: fix error path in mem_cgroup_alloc(), per Dan]
  Link: https://lkml.kernel.org/r/20240815155402.3630804-1-shakeel.butt@linux.dev
Link: https://lkml.kernel.org/r/20240809172618.2946790-1-shakeel.butt@linux.devSigned-off-by: Shakeel Butt <shakeel.butt@linux.dev>
Suggested-by: Matthew Wilcox <willy@infradead.org>
Reviewed-by: Roman Gushchin <roman.gushchin@linux.dev>
Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Reviewed-by: Muchun Song <muchun.song@linux.dev>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/memcontrol.c

@@ -3363,29 +3363,12 @@ 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;
 	}
 }
@@ -3420,7 +3403,7 @@ 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
@@ -3513,17 +3496,17 @@ 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);
@@ -3664,9 +3647,7 @@ 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: