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

From: Matthew Wilcox <willy@infradead.org>
To: Yafang Shao <laoar.shao@gmail.com>
Cc: hannes@cmpxchg.org, mhocko@kernel.org, vdavydov.dev@gmail.com,
	akpm@linux-foundation.org, linux-mm@kvack.org
Subject: Re: [PATCH] mm, memcg: fix error return value of mem_cgroup_alloc()
Date: Mon, 6 Apr 2020 06:05:33 -0700	[thread overview]
Message-ID: <20200406130533.GZ21484@bombadil.infradead.org> (raw)
In-Reply-To: <1586177647-11889-1-git-send-email-laoar.shao@gmail.com>

On Mon, Apr 06, 2020 at 08:54:07AM -0400, Yafang Shao wrote:
> 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().

I think idr_alloc() is wrong.  I think the right errno to return here is
EBUSY "Device or resource busy".

> -static struct mem_cgroup *mem_cgroup_alloc(void)
> +static struct mem_cgroup *mem_cgroup_alloc(long *error)

The normal way to do this is to return an ERR_PTR().  See
include/linux/err.h.