Received-SPF: pass (google.com: domain of linux-nfs-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Message-ID: <99d0958d-8446-6fe9-a0fb-f562cb04c7c8@bytedance.com>
Date:   Fri, 28 Jul 2023 16:19:01 +0800
MIME-Version: 1.0
User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:102.0)
 Gecko/20100101 Thunderbird/102.12.0
Subject: Re: [PATCH v3 04/49] mm: shrinker: remove redundant shrinker_rwsem in
 debugfs operations
Content-Language: en-US
To:     Simon Horman <simon.horman@corigine.com>
Cc:     akpm@linux-foundation.org, david@fromorbit.com, tkhai@ya.ru,
        vbabka@suse.cz, roman.gushchin@linux.dev, djwong@kernel.org,
        brauner@kernel.org, paulmck@kernel.org, tytso@mit.edu,
        steven.price@arm.com, cel@kernel.org, senozhatsky@chromium.org,
        yujie.liu@intel.com, gregkh@linuxfoundation.org,
        muchun.song@linux.dev, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, x86@kernel.org, kvm@vger.kernel.org,
        xen-devel@lists.xenproject.org, linux-erofs@lists.ozlabs.org,
        linux-f2fs-devel@lists.sourceforge.net, cluster-devel@redhat.com,
        linux-nfs@vger.kernel.org, linux-mtd@lists.infradead.org,
        rcu@vger.kernel.org, netdev@vger.kernel.org,
        dri-devel@lists.freedesktop.org, linux-arm-msm@vger.kernel.org,
        dm-devel@redhat.com, linux-raid@vger.kernel.org,
        linux-bcache@vger.kernel.org,
        virtualization@lists.linux-foundation.org,
        linux-fsdevel@vger.kernel.org, linux-ext4@vger.kernel.org,
        linux-xfs@vger.kernel.org, linux-btrfs@vger.kernel.org,
        Muchun Song <songmuchun@bytedance.com>
References: <20230727080502.77895-1-zhengqi.arch@bytedance.com>
 <20230727080502.77895-5-zhengqi.arch@bytedance.com>
 <ZMN4mjsF1QEsvW7D@corigine.com>
From:   Qi Zheng <zhengqi.arch@bytedance.com>
In-Reply-To: <ZMN4mjsF1QEsvW7D@corigine.com>
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 7bit
Precedence: bulk

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;
> 
> ...