Re: [PATCH v3] mm, memcg: fix error return value of mem_cgroup_css_alloc()

[Andrew Morton <akpm@linux-foundation.org>]

On Thu, 9 Apr 2020 16:07:29 +0200 Michal Hocko <mhocko@kernel.org> wrote:

> On Thu 09-04-20 21:59:42, Yafang Shao wrote:
> > On Thu, Apr 9, 2020 at 2:57 PM Michal Hocko <mhocko@kernel.org> wrote:
> > >
> > > On Wed 08-04-20 18:29:56, Andrew Morton wrote:
> > > > On Tue, 7 Apr 2020 14:10:12 -0400 Johannes Weiner <hannes@cmpxchg.org> wrote:
> > > >
> > > > > Given how unlikely this is to affect real setups, I don't think this
> > > > > patch is stable material.
> > > >
> > > > OK, thanks, done.
> > > >
> > > > I didn't notice any acks - is this patch otherwise good to go upstream?
> > >
> > > I will ack it if s@EBUSY@ENOSPC@
> > >
> > 
> > BTW, there's no EBUSY in man page of write(2) neither.
> > But when we echo some procs into cgroup.subtree_control of a non-root
> > cgroup, it will return EBUSY. See also cgroup_subtree_control_write().
> > Pls. correct me if I missed something.
> > 
> > We have to admit that the man page is out of date.
> 
> Or the code is returning something unexpected but nobody has noticed so
> far. Please talk to cgroup maintainers on how to address that.

I agree with your ENOSPC reasoning in
http://lkml.kernel.org/r/20200407063621.GA18914@dhcp22.suse.cz

That means this change, methinks:

--- a/mm/memcontrol.c~mm-memcg-fix-error-return-value-of-mem_cgroup_css_alloc-fix
+++ a/mm/memcontrol.c
@@ -2721,8 +2721,6 @@ static int memcg_alloc_cache_id(void)
 
 	id = ida_simple_get(&memcg_cache_ida,
 			    0, MEMCG_CACHES_MAX_SIZE, GFP_KERNEL);
-	if (id == -ENOSPC)
-		return -EBUSY;
 	if (id < 0)
 		return id;
 
@@ -5004,10 +5002,6 @@ static struct mem_cgroup *mem_cgroup_all
 	memcg->id.id = idr_alloc(&mem_cgroup_idr, NULL,
 				 1, MEM_CGROUP_ID_MAX,
 				 GFP_KERNEL);
-	if (memcg->id.id == -ENOSPC) {
-		error = -EBUSY;
-		goto fail;
-	}
 	if (memcg->id.id < 0) {
 		error = memcg->id.id;
 		goto fail;
_


So the final patch will be as below.  Please review my changelog
alterations.


From: Yafang Shao <laoar.shao@gmail.com>
Subject: mm, memcg: fix error return value of mem_cgroup_css_alloc()

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 "No space left on device", which is an appropriate errno
for userspace's failed mkdir.

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"

[akpm@linux-foundation.org: s/EBUSY/ENOSPC/, per Michal]
Link: http://lkml.kernel.org/r/20200407063621.GA18914@dhcp22.suse.cz
Link: http://lkml.kernel.org/r/1586192163-20099-1-git-send-email-laoar.shao@gmail.com
Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
Suggested-by: Matthew Wilcox <willy@infradead.org>
Acked-by: Michal Hocko <mhocko@kernel.org>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Vladimir Davydov <vdavydov.dev@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/memcontrol.c |   15 +++++++++------
 1 file changed, 9 insertions(+), 6 deletions(-)

--- a/mm/memcontrol.c~mm-memcg-fix-error-return-value-of-mem_cgroup_css_alloc
+++ a/mm/memcontrol.c
@@ -4990,19 +4990,22 @@ static struct mem_cgroup *mem_cgroup_all
 	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) {
+		error = memcg->id.id;
 		goto fail;
+	}
 
 	memcg->vmstats_local = alloc_percpu(struct memcg_vmstats_percpu);
 	if (!memcg->vmstats_local)
@@ -5046,7 +5049,7 @@ static struct mem_cgroup *mem_cgroup_all
 fail:
 	mem_cgroup_id_remove(memcg);
 	__mem_cgroup_free(memcg);
-	return NULL;
+	return ERR_PTR(error);
 }
 
 static struct cgroup_subsys_state * __ref
@@ -5057,8 +5060,8 @@ mem_cgroup_css_alloc(struct cgroup_subsy
 	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;
@@ -5108,7 +5111,7 @@ mem_cgroup_css_alloc(struct cgroup_subsy
 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)
_