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

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Message-ID: <20240725084232.bj7apjqqowae575c@quack3>
Date: Thu, 25 Jul 2024 10:42:32 +0200
From: Jan Kara <jack@...e.cz>
To: Haifeng Xu <haifeng.xu@...pee.com>
Cc: viro@...iv.linux.org.uk, brauner@...nel.org, jack@...e.cz,
	tj@...nel.org, axboe@...nel.dk, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] fs: don't flush in-flight wb switches for superblocks
 without cgroup writeback

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@...pee.com>
> ---
>  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@...e.com>
SUSE Labs, CR