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Message-Id: <55B22931-34E1-4DAF-B392-A48EC2A9EE1A@linux.dev>
Date: Wed, 16 Oct 2024 22:22:57 +0800
From: Muchun Song <muchun.song@...ux.dev>
To: Vlastimil Babka <vbabka@...e.cz>
Cc: chenridong <chenridong@...wei.com>,
Anshuman Khandual <anshuman.khandual@....com>,
Ridong Chen <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 Oct 16, 2024, at 20:13, Vlastimil Babka <vbabka@...e.cz> wrote:
>
> 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().
Qi told me that the @info will not visible immediately yesterday.
So non-rcu-based kvfree is safe. Even if this fix could
work properly, it’s a bit strange for me to use
rcu_assign_pointer to assign the @info without full initialization to it.
Muchun,
Thanks.
>
>>>
>>> I think this is concise.
>>>
>>> Best regards,
>>> Ridong
>>
>>
>
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