linux-kernel - Re: [PATCH v2] mm: shrinker: avoid memleak in alloc_shrinker

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Message-ID: <270ca4d5-b35f-4533-87c9-dc15e7b00f6f@suse.cz>
Date: Wed, 16 Oct 2024 14:13:39 +0200
From: Vlastimil Babka <vbabka@...e.cz>
To: Muchun Song <muchun.song@...ux.dev>, chenridong <chenridong@...wei.com>
Cc: Anshuman Khandual <anshuman.khandual@....com>,
 Chen Ridong <chenridong@...weicloud.com>, akpm@...ux-foundation.org,
 david@...morbit.com, zhengqi.arch@...edance.com, roman.gushchin@...ux.dev,
 linux-mm@...ck.org, linux-kernel@...r.kernel.org, wangweiyang2@...wei.com
Subject: Re: [PATCH v2] mm: shrinker: avoid memleak in alloc_shrinker_info

On 10/14/24 11:20, Muchun Song wrote:
> 
> 
>> On Oct 14, 2024, at 17:04, chenridong <chenridong@...wei.com> wrote:
>> 
>> 
>> 
>> On 2024/10/14 16:43, Muchun Song wrote:
>>>> On Oct 14, 2024, at 16:13, Anshuman Khandual <anshuman.khandual@....com> wrote:
>>>> 
>>>> 
>>>> 
>>>> On 10/14/24 08:53, Chen Ridong wrote:
>>>>> From: Chen Ridong <chenridong@...wei.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@...wei.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
>>> 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.

If the info is immediately visible, is the failure cleanup
free_shrinker_info() safe? It uses kvfree(info) and not kvfree_rcu(), and
shrinker_unit_free() is also doing kfree().

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