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

[Jan Kara]

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

-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR