On Wed 17-02-16 16:07:44, Tejun Heo wrote: > 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. Bah, sorry. It's too late here. You are right that s_active will just delay destruction of the superblock until the reference is dropped. So I don't see obvious issues with what you do and I retract my nack. I still feel somewhat uneasy about postponing fs shutdown to a workqueue like this but hopefully there's no hidden catch. Honza > > 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 -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR -- To unsubscribe from this list: send the line "unsubscribe cgroups" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
