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

[Muchun Song <muchun.song@linux.dev>]

> On Oct 16, 2024, at 09:25, chenridong <chenridong@huawei.com> wrote:
> 
> 
> 
> On 2024/10/15 14:55, Anshuman Khandual wrote:
>> On 10/14/24 16:59, Kirill A. Shutemov wrote:
>>> On Mon, Oct 14, 2024 at 03:23:36AM +0000, 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;
>>>>  }
>>> 
>>> NAK. If in the future there going to one more error case after
>>> rcu_assign_pointer() we will end up with double free.
>>> 
>>> This should be safer:
>>> 
>>> diff --git a/mm/shrinker.c b/mm/shrinker.c
>>> index dc5d2a6fcfc4..763fd556bc7d 100644
>>> --- a/mm/shrinker.c
>>> +++ b/mm/shrinker.c
>>> @@ -87,8 +87,10 @@ int alloc_shrinker_info(struct mem_cgroup *memcg)
>>>   if (!info)
>>>   goto err;
>>>   info->map_nr_max = shrinker_nr_max;
>>> - if (shrinker_unit_alloc(info, NULL, nid))
>>> + if (shrinker_unit_alloc(info, NULL, nid)) {
>>> + kvfree(info);
>>>   goto err;
>>> + }
>>>   rcu_assign_pointer(memcg->nodeinfo[nid]->shrinker_info, info);
>>>   }
>>>   mutex_unlock(&shrinker_mutex);
>> Agreed, this is what I mentioned earlier as well.
>> ------------------------------------------------------------------
>> I guess kvfree() should be called just after shrinker_unit_alloc()
>> fails but before calling into "goto err"
>> ------------------------------------------------------------------
> 
> After discussion, it seems that v1 is acceptable.
> Hi, Muchun, do you have any other opinions?

I insist on my opinion, not mixing two different approaches
to do release resources.

Thanks.

> 
> Best regards,
> Ridong