Hello, Jan.

On Wed, Feb 17, 2016 at 09:57:21PM +0100, Jan Kara wrote:

Well, but this has the side-effect that trying to umount a filesystem while
migrations are happening will result in EBUSY error. Without obvious reason
why that happens. As an admin I would be rather upset when umount sometimes
returns EBUSY without apparent reason and you have to basically implement a
loop around umount to make it reliable. So a nack from me for this patch.

I see.  Can you please point me to the s_active check during umount?
I first tried s_umount but couldn't transfer its ownership to the
worker so ended up doing s_active.  I looked at how s_active is used
and couldn't find where it'd block umount.  may_umount() checks
mnt_count, not s_active, so it looked like holding s_active may delay
destruction of the superblock but not prevent umount.

Traditionally, we have used sb->s_count and sb->s_umount semaphore to pin
superblock while writeback code was working on it. That makes umount block
until we can safely unmount the filesystem and thus doesn't result in these
spurious EBUSY errors. But from a quick look this can be problematic for the
cgroup setting.

Alternatively, you could either cancel all the switching work when
unmounting filesystem or maybe just handle I_WB_SWITCH similarly to I_SYNC
- don't grab inode reference when switching is going on, just make
I_WB_SWITCH pin the inode and wait in evict() for it to be clear (similarly
as we call inode_wait_for_writeback() there).

Yeah, this is an alternative but likely more involved.

Thanks.

-- 
tejun