linux-kernel - Re: [PATCH block/for-4.5-fixes] writeback: keep superblock pinned during cgroup writeback association switches

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Date:	Wed, 17 Feb 2016 23:30:09 +0100
From:	Jan Kara <jack@...e.cz>
To:	Tejun Heo <tj@...nel.org>
Cc:	Jan Kara <jack@...e.cz>, Jens Axboe <axboe@...nel.dk>,
	Tahsin Erdogan <tahsin@...gle.com>, cgroups@...r.kernel.org,
	Theodore Ts'o <tytso@....edu>,
	Nauman Rafique <nauman@...gle.com>,
	linux-kernel@...r.kernel.org, Jan Kara <jack@...e.com>
Subject: Re: [PATCH block/for-4.5-fixes] writeback: keep superblock pinned
 during cgroup writeback association switches

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