[PATCH v2] mm, memcg: fix error return value of mem_cgroup_css_alloc()

[Yafang Shao <laoar.shao@gmail.com>]

When I run my memcg testcase which creates lots of memcgs, I found
there're unexpected out of memory logs while there're still enough
available free memory. The error log is,
mkdir: cannot create directory 'foo.65533': Cannot allocate memory

The reason is when we try to create more than MEM_CGROUP_ID_MAX memcgs, an
-ENOMEM errno will be set by mem_cgroup_css_alloc(), but the right errno
should be -EBUSY "Device or resource busy". That is same with
memcg_alloc_cache_id().

As the errno really misled me, we should make it right. After this patch,
the error log will be,
mkdir: cannot create directory 'foo.65533': Device or resource busy

Suggested-by: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
---
 mm/memcontrol.c | 25 ++++++++++++++++++-------
 1 file changed, 18 insertions(+), 7 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index ca194864d802..94319e4dd1bd 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2717,8 +2717,12 @@ static int memcg_alloc_cache_id(void)
 
 	id = ida_simple_get(&memcg_cache_ida,
 			    0, MEMCG_CACHES_MAX_SIZE, GFP_KERNEL);
-	if (id < 0)
+	if (id < 0) {
+		if (id == -ENOSPC)
+			id = -EBUSY;
+
 		return id;
+	}
 
 	if (id < memcg_nr_cache_ids)
 		return id;
@@ -4986,19 +4990,26 @@ static struct mem_cgroup *mem_cgroup_alloc(void)
 	unsigned int size;
 	int node;
 	int __maybe_unused i;
+	long error = -ENOMEM;
 
 	size = sizeof(struct mem_cgroup);
 	size += nr_node_ids * sizeof(struct mem_cgroup_per_node *);
 
 	memcg = kzalloc(size, GFP_KERNEL);
 	if (!memcg)
-		return NULL;
+		return ERR_PTR(error);
 
 	memcg->id.id = idr_alloc(&mem_cgroup_idr, NULL,
 				 1, MEM_CGROUP_ID_MAX,
 				 GFP_KERNEL);
-	if (memcg->id.id < 0)
+	if (memcg->id.id < 0) {
+		if (memcg->id.id == -ENOSPC)
+			error = -EBUSY;
+		else
+			error = memcg->id.id;
+
 		goto fail;
+	}
 
 	memcg->vmstats_local = alloc_percpu(struct memcg_vmstats_percpu);
 	if (!memcg->vmstats_local)
@@ -5042,7 +5053,7 @@ static struct mem_cgroup *mem_cgroup_alloc(void)
 fail:
 	mem_cgroup_id_remove(memcg);
 	__mem_cgroup_free(memcg);
-	return NULL;
+	return ERR_PTR(error);
 }
 
 static struct cgroup_subsys_state * __ref
@@ -5053,8 +5064,8 @@ mem_cgroup_css_alloc(struct cgroup_subsys_state *parent_css)
 	long error = -ENOMEM;
 
 	memcg = mem_cgroup_alloc();
-	if (!memcg)
-		return ERR_PTR(error);
+	if (IS_ERR(memcg))
+		return ERR_CAST(memcg);
 
 	WRITE_ONCE(memcg->high, PAGE_COUNTER_MAX);
 	memcg->soft_limit = PAGE_COUNTER_MAX;
@@ -5104,7 +5115,7 @@ mem_cgroup_css_alloc(struct cgroup_subsys_state *parent_css)
 fail:
 	mem_cgroup_id_remove(memcg);
 	mem_cgroup_free(memcg);
-	return ERR_PTR(-ENOMEM);
+	return ERR_PTR(error);
 }
 
 static int mem_cgroup_css_online(struct cgroup_subsys_state *css)
-- 
2.18.1