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

[Muchun Song]

> On Oct 14, 2024, at 17:04, chenridong <chenridong@xxxxxxxxxx> wrote:
> 
> 
> 
> On 2024/10/14 16:43, Muchun Song wrote:
>>> 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 ?
> How about:
> 
> diff --git a/mm/shrinker.c b/mm/shrinker.c
> index dc5d2a6fcfc4..7baee7f00497 100644
> --- a/mm/shrinker.c
> +++ b/mm/shrinker.c
> @@ -87,9 +87,9 @@ int alloc_shrinker_info(struct mem_cgroup *memcg)
>                 if (!info)
>                         goto err;
>                 info->map_nr_max = shrinker_nr_max;
> +               rcu_assign_pointer(memcg->nodeinfo[nid]->shrinker_info, info);
>                 if (shrinker_unit_alloc(info, NULL, nid))
>                         goto err;
> -               rcu_assign_pointer(memcg->nodeinfo[nid]->shrinker_info, info);
>         }
>         mutex_unlock(&shrinker_mutex);

No. We should make sure the @info is fully initialized before others
could see it. That's why rcu_assign_pointer is used here.

> 
> I think this is concise.
> 
> Best regards,
> Ridong