Re: [PATCH] fs: don't flush in-flight wb switches for superblocks without cgroup writeback

[Haifeng Xu]

On 2024/7/25 16:42, Jan Kara wrote:
> On Thu 25-07-24 10:39:58, Haifeng Xu wrote:
>> When deactivating any type of superblock, it had to wait for the in-flight
>> wb switches to be completed. wb switches are executed in inode_switch_wbs_work_fn()
>> which needs to acquire the wb_switch_rwsem and races against sync_inodes_sb().
>> If there are too much dirty data in the superblock, the waiting time may increase
>> significantly.
>>
>> For superblocks without cgroup writeback such as tmpfs, they have nothing to
>> do with the wb swithes, so the flushing can be avoided.
>>
>> Signed-off-by: Haifeng Xu <haifeng.xu@xxxxxxxxxx>
>> ---
>>  fs/super.c | 3 ++-
>>  1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/fs/super.c b/fs/super.c
>> index 095ba793e10c..f846f853e957 100644
>> --- a/fs/super.c
>> +++ b/fs/super.c
>> @@ -621,7 +621,8 @@ void generic_shutdown_super(struct super_block *sb)
>>  		sync_filesystem(sb);
>>  		sb->s_flags &= ~SB_ACTIVE;
>>  
>> -		cgroup_writeback_umount();
>> +		if (sb->s_bdi != &noop_backing_dev_info)
>> +			cgroup_writeback_umount();
> 
> So a more obvious check would be:
> 
> 		if (sb->s_bdi->capabilities & BDI_CAP_WRITEBACK)
> 
> even better would be if we'd pass 'sb' into cgroup_writeback_umount() and
> that function would do this check inside so that callers don't have to
> bother... I know there is only one caller so this is not a huge deal but
> still I'd find it cleaner that way.
> 
> 								Honza
> 

Yes, Thanks for you suggestions!