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SubjectRe: [PATCH v3 04/49] mm: shrinker: remove redundant shrinker_rwsem in debugfs operations
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Hi Simon,

On 2023/7/28 16:13, Simon Horman wrote:
> On Thu, Jul 27, 2023 at 04:04:17PM +0800, Qi Zheng wrote:
>> The debugfs_remove_recursive() will wait for debugfs_file_put() to return,
>> so the shrinker will not be freed when doing debugfs operations (such as
>> shrinker_debugfs_count_show() and shrinker_debugfs_scan_write()), so there
>> is no need to hold shrinker_rwsem during debugfs operations.
>>
>> Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
>> Reviewed-by: Muchun Song <songmuchun@bytedance.com>
>> ---
>> mm/shrinker_debug.c | 14 --------------
>> 1 file changed, 14 deletions(-)
>>
>> diff --git a/mm/shrinker_debug.c b/mm/shrinker_debug.c
>> index 3ab53fad8876..f1becfd45853 100644
>> --- a/mm/shrinker_debug.c
>> +++ b/mm/shrinker_debug.c
>> @@ -55,11 +55,6 @@ static int shrinker_debugfs_count_show(struct seq_file *m, void *v)
>> if (!count_per_node)
>> return -ENOMEM;
>>
>> - ret = down_read_killable(&shrinker_rwsem);
>> - if (ret) {
>> - kfree(count_per_node);
>> - return ret;
>> - }
>> rcu_read_lock();
>
> Hi Qi Zheng,
>
> As can be seen in the next hunk, this function returns 'ret'.
> However, with this change 'ret' is uninitialised unless
> signal_pending() returns non-zero in the while loop below.

Thanks for your feedback, the 'ret' should be initialized to 0,
will fix it.

Thanks,
Qi

>
> This is flagged in a clan-16 W=1 build.
>
> mm/shrinker_debug.c:87:11: warning: variable 'ret' is used uninitialized whenever 'do' loop exits because its condition is false [-Wsometimes-uninitialized]
> } while ((memcg = mem_cgroup_iter(NULL, memcg, NULL)) != NULL);
> ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> mm/shrinker_debug.c:92:9: note: uninitialized use occurs here
> return ret;
> ^~~
> mm/shrinker_debug.c:87:11: note: remove the condition if it is always true
> } while ((memcg = mem_cgroup_iter(NULL, memcg, NULL)) != NULL);
> ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> 1
> mm/shrinker_debug.c:77:7: warning: variable 'ret' is used uninitialized whenever 'if' condition is true [-Wsometimes-uninitialized]
> if (!memcg_aware) {
> ^~~~~~~~~~~~
> mm/shrinker_debug.c:92:9: note: uninitialized use occurs here
> return ret;
> ^~~
> mm/shrinker_debug.c:77:3: note: remove the 'if' if its condition is always false
> if (!memcg_aware) {
> ^~~~~~~~~~~~~~~~~~~
> mm/shrinker_debug.c:52:9: note: initialize the variable 'ret' to silence this warning
> int ret, nid;
> ^
> = 0
>
>>
>> memcg_aware = shrinker->flags & SHRINKER_MEMCG_AWARE;
>> @@ -92,7 +87,6 @@ static int shrinker_debugfs_count_show(struct seq_file *m, void *v)
>> } while ((memcg = mem_cgroup_iter(NULL, memcg, NULL)) != NULL);
>>
>> rcu_read_unlock();
>> - up_read(&shrinker_rwsem);
>>
>> kfree(count_per_node);
>> return ret;
>
> ...

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