Re: [PATCH block/for-linus] writeback: flush inode cgroup wb switches instead of pinning super_block

From: Tejun Heo
Date: Mon Feb 29 2016 - 15:58:44 EST

On Mon, Feb 29, 2016 at 08:54:28PM +0000, Al Viro wrote:
> > This patch removes the problematic super_block pinning and instead
> > makes generic_shutdown_super() flush in-flight wb switches. wb
> > switches are now executed on a dedicated isw_wq so that they can be
> > flushed and isw_nr_in_flight keeps track of the number of in-flight wb
> > switches so that flushing can be avoided in most cases.
> Wait a bloody minute. What's to prevent shrink_dcache_for_umount() from
> dirtying more inodes, triggering more of the same?

Hmmm? The flushing is done after shrink_dcache_for_umount() and
sync_filesystems(). Aren't inodes supposed to stay clean after that?

> > - if (!atomic_inc_not_zero(&inode->i_sb->s_active))
> > - goto out_unlock;
> This would've failed for inodes on superblock in the middle of shutdown;
> what's to do the same for the new variant?

I don't follow. As long as no new writeback operations are initiated
after flushing, none can be in flight for the super_block. Isn't that