Re: [PATCH v2] mm: shrinker: avoid memleak in alloc_shrinker_info

[Anshuman Khandual <anshuman.khandual@arm.com>]

On 10/14/24 08:53, Chen Ridong wrote:
> From: Chen Ridong <chenridong@huawei.com>
> 
> A memleak was found as bellow:
> 
> unreferenced object 0xffff8881010d2a80 (size 32):
>   comm "mkdir", pid 1559, jiffies 4294932666
>   hex dump (first 32 bytes):
>     00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
>     40 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  @...............
>   backtrace (crc 2e7ef6fa):
>     [<ffffffff81372754>] __kmalloc_node_noprof+0x394/0x470
>     [<ffffffff813024ab>] alloc_shrinker_info+0x7b/0x1a0
>     [<ffffffff813b526a>] mem_cgroup_css_online+0x11a/0x3b0
>     [<ffffffff81198dd9>] online_css+0x29/0xa0
>     [<ffffffff811a243d>] cgroup_apply_control_enable+0x20d/0x360
>     [<ffffffff811a5728>] cgroup_mkdir+0x168/0x5f0
>     [<ffffffff8148543e>] kernfs_iop_mkdir+0x5e/0x90
>     [<ffffffff813dbb24>] vfs_mkdir+0x144/0x220
>     [<ffffffff813e1c97>] do_mkdirat+0x87/0x130
>     [<ffffffff813e1de9>] __x64_sys_mkdir+0x49/0x70
>     [<ffffffff81f8c928>] do_syscall_64+0x68/0x140
>     [<ffffffff8200012f>] entry_SYSCALL_64_after_hwframe+0x76/0x7e
> 
> In the alloc_shrinker_info function, when shrinker_unit_alloc return
> err, the info won't be freed. Just fix it.
> 
> Fixes: 307bececcd12 ("mm: shrinker: add a secondary array for shrinker_info::{map, nr_deferred}")
> Signed-off-by: Chen Ridong <chenridong@huawei.com>
> ---
>  mm/shrinker.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/mm/shrinker.c b/mm/shrinker.c
> index dc5d2a6fcfc4..92270413190d 100644
> --- a/mm/shrinker.c
> +++ b/mm/shrinker.c
> @@ -97,6 +97,7 @@ int alloc_shrinker_info(struct mem_cgroup *memcg)
>  
>  err:
>  	mutex_unlock(&shrinker_mutex);
> +	kvfree(info);
>  	free_shrinker_info(memcg);
>  	return -ENOMEM;
>  }

There are two scenarios when "goto err:" gets called

- When shrinker_info allocations fails, no kvfree() is required
	- but after this change kvfree() would be called even
	  when the allocation had failed originally, which does
	  not sound right

- shrinker_unit_alloc() fails, kvfree() is actually required

I guess kvfree() should be called just after shrinker_unit_alloc()
fails but before calling into "goto err".

But curious, should not both kvzalloc_node()/kvfree() be avoided
while inside mutex lock to avoid possible lockdep issues ?