[PATCH] mm, memcg: fix error return value of mem_cgroup_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 -ENOSPC, as explained above the function idr_alloc().

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': No space left on device

Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
---
 mm/memcontrol.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index ca19486..9c85470 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -4980,7 +4980,7 @@ static void mem_cgroup_free(struct mem_cgroup *memcg)
 	__mem_cgroup_free(memcg);
 }
 
-static struct mem_cgroup *mem_cgroup_alloc(void)
+static struct mem_cgroup *mem_cgroup_alloc(long *error)
 {
 	struct mem_cgroup *memcg;
 	unsigned int size;
@@ -4997,8 +4997,10 @@ static struct mem_cgroup *mem_cgroup_alloc(void)
 	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) {
+		*error = memcg->id.id;
 		goto fail;
+	}
 
 	memcg->vmstats_local = alloc_percpu(struct memcg_vmstats_percpu);
 	if (!memcg->vmstats_local)
@@ -5052,7 +5054,7 @@ static struct mem_cgroup *mem_cgroup_alloc(void)
 	struct mem_cgroup *memcg;
 	long error = -ENOMEM;
 
-	memcg = mem_cgroup_alloc();
+	memcg = mem_cgroup_alloc(&error);
 	if (!memcg)
 		return ERR_PTR(error);
 
-- 
1.8.3.1