linux-kernel - Re: [PATCH v3 04/49] mm: shrinker: remove redundant shrinker

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Date:   Fri, 28 Jul 2023 10:13:14 +0200
From:   Simon Horman <simon.horman@...igine.com>
To:     Qi Zheng <zhengqi.arch@...edance.com>
Cc:     akpm@...ux-foundation.org, david@...morbit.com, tkhai@...ru,
        vbabka@...e.cz, roman.gushchin@...ux.dev, djwong@...nel.org,
        brauner@...nel.org, paulmck@...nel.org, tytso@....edu,
        steven.price@....com, cel@...nel.org, senozhatsky@...omium.org,
        yujie.liu@...el.com, gregkh@...uxfoundation.org,
        muchun.song@...ux.dev, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, x86@...nel.org, kvm@...r.kernel.org,
        xen-devel@...ts.xenproject.org, linux-erofs@...ts.ozlabs.org,
        linux-f2fs-devel@...ts.sourceforge.net, cluster-devel@...hat.com,
        linux-nfs@...r.kernel.org, linux-mtd@...ts.infradead.org,
        rcu@...r.kernel.org, netdev@...r.kernel.org,
        dri-devel@...ts.freedesktop.org, linux-arm-msm@...r.kernel.org,
        dm-devel@...hat.com, linux-raid@...r.kernel.org,
        linux-bcache@...r.kernel.org,
        virtualization@...ts.linux-foundation.org,
        linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org,
        linux-xfs@...r.kernel.org, linux-btrfs@...r.kernel.org,
        Muchun Song <songmuchun@...edance.com>
Subject: Re: [PATCH v3 04/49] mm: shrinker: remove redundant shrinker_rwsem
 in debugfs operations

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

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