diff mbox series

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

Message ID 20230727080502.77895-5-zhengqi.arch@bytedance.com (mailing list archive)
State Handled Elsewhere, archived
Headers show
Series use refcount+RCU method to implement lockless slab shrink | expand

Commit Message

Qi Zheng July 27, 2023, 8:04 a.m. UTC 
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(-)

Comments

Simon Horman July 28, 2023, 8:13 a.m. UTC | #1 
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.

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;

...
Qi Zheng July 28, 2023, 8:19 a.m. UTC | #2 
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;
> 
> ...
diff mbox series

Patch

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();
 
 	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;
@@ -117,7 +111,6 @@  static ssize_t shrinker_debugfs_scan_write(struct file *file,
 	struct mem_cgroup *memcg = NULL;
 	int nid;
 	char kbuf[72];
-	ssize_t ret;
 
 	read_len = size < (sizeof(kbuf) - 1) ? size : (sizeof(kbuf) - 1);
 	if (copy_from_user(kbuf, buf, read_len))
@@ -146,12 +139,6 @@  static ssize_t shrinker_debugfs_scan_write(struct file *file,
 		return -EINVAL;
 	}
 
-	ret = down_read_killable(&shrinker_rwsem);
-	if (ret) {
-		mem_cgroup_put(memcg);
-		return ret;
-	}
-
 	sc.nid = nid;
 	sc.memcg = memcg;
 	sc.nr_to_scan = nr_to_scan;
@@ -159,7 +146,6 @@  static ssize_t shrinker_debugfs_scan_write(struct file *file,
 
 	shrinker->scan_objects(shrinker, &sc);
 
-	up_read(&shrinker_rwsem);
 	mem_cgroup_put(memcg);
 
 	return size;