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

[Qi Zheng]

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@xxxxxxxxxxxxx>
> > Reviewed-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
> > ---
> >   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;
>
> ...