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

[Muchun Song]

> On Oct 14, 2024, at 16:13, Anshuman Khandual <anshuman.khandual@xxxxxxx> wrote:
> 
> 
> 
> On 10/14/24 08:53, Chen Ridong wrote:
>> From: Chen Ridong <chenridong@xxxxxxxxxx>
>> 
>> 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@xxxxxxxxxx>
>> ---
>> 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

Yes. In this case, @info is NULL and kvfree could handle NULL.
It seems strange but the final behaviour correct.

> 
> - 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".

We could do it like this, which avoids ambiguity (if someone ignores
that kvfree could handle NULL). Something like:

--- a/mm/shrinker.c
+++ b/mm/shrinker.c
@@ -88,13 +88,14 @@ int alloc_shrinker_info(struct mem_cgroup *memcg)
                        goto err;
                info->map_nr_max = shrinker_nr_max;
                if (shrinker_unit_alloc(info, NULL, nid))
-                       goto err;
+                       goto free;
                rcu_assign_pointer(memcg->nodeinfo[nid]->shrinker_info, info);
        }
        mutex_unlock(&shrinker_mutex);

        return ret;
-
+free:
+       kvfree(info);
 err:
        mutex_unlock(&shrinker_mutex);
        free_shrinker_info(memcg);

Thanks.

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