Re: INFO: task hung in generic_file_write_iter

[Jan Kara]

On Mon 14-01-19 16:13:08, Dmitry Vyukov wrote:
> On Mon, Jan 14, 2019 at 4:11 PM Dmitry Vyukov <dvyukov@xxxxxxxxxx> wrote:
> >
> > On Wed, Jan 9, 2019 at 2:30 PM Jan Kara <jack@xxxxxxx> wrote:
> > >
> > > On Tue 08-01-19 12:49:08, Dmitry Vyukov wrote:
> > > > On Tue, Jan 8, 2019 at 12:24 PM Jan Kara <jack@xxxxxxx> wrote:
> > > > >
> > > > > On Tue 08-01-19 19:04:06, Tetsuo Handa wrote:
> > > > > > On 2019/01/03 2:26, Jan Kara wrote:
> > > > > > > On Thu 03-01-19 01:07:25, Tetsuo Handa wrote:
> > > > > > >> On 2019/01/02 23:40, Jan Kara wrote:
> > > > > > >>> I had a look into this and the only good explanation for this I have is
> > > > > > >>> that sb->s_blocksize is different from (1 << sb->s_bdev->bd_inode->i_blkbits).
> > > > > > >>> If that would happen, we'd get exactly the behavior syzkaller observes
> > > > > > >>> because grow_buffers() would populate different page than
> > > > > > >>> __find_get_block() then looks up.
> > > > > > >>>
> > > > > > >>> However I don't see how that's possible since the filesystem has the block
> > > > > > >>> device open exclusively and blkdev_bszset() makes sure we also have
> > > > > > >>> exclusive access to the block device before changing the block device size.
> > > > > > >>> So changing block device block size after filesystem gets access to the
> > > > > > >>> device should be impossible.
> > > > > > >>>
> > > > > > >>> Anyway, could you perhaps add to your debug patch a dump of 'size' passed
> > > > > > >>> to __getblk_slow() and bdev->bd_inode->i_blkbits? That should tell us
> > > > > > >>> whether my theory is right or not. Thanks!
> > > > > > >>>
> > > > > >
> > > > > > Got two reports. 'size' is 512 while bdev->bd_inode->i_blkbits is 12.
> > > > > >
> > > > > > https://syzkaller.appspot.com/text?tag=CrashLog&x=1237c3ab400000
> > > > > >
> > > > > > [  385.723941][  T439] kworker/u4:3(439): getblk(): executed=9 bh_count=0 bh_state=0 bdev_super_blocksize=512 size=512 bdev_super_blocksize_bits=9 bdev_inode_blkbits=12
> > > > > > (...snipped...)
> > > > > > [  568.159544][  T439] kworker/u4:3(439): getblk(): executed=9 bh_count=0 bh_state=0 bdev_super_blocksize=512 size=512 bdev_super_blocksize_bits=9 bdev_inode_blkbits=12
> > > > >
> > > > > Right, so indeed the block size in the superblock and in the block device
> > > > > gets out of sync which explains why we endlessly loop in the buffer cache
> > > > > code. The superblock uses blocksize of 512 while the block device thinks
> > > > > the set block size is 4096.
> > > > >
> > > > > And after staring into the code for some time, I finally have a trivial
> > > > > reproducer:
> > > > >
> > > > > truncate -s 1G /tmp/image
> > > > > losetup /dev/loop0 /tmp/image
> > > > > mkfs.ext4 -b 1024 /dev/loop0
> > > > > mount -t ext4 /dev/loop0 /mnt
> > > > > losetup -c /dev/loop0
> > > > > l /mnt
> > > > > <hangs>
> > > > >
> > > > > And the problem is that LOOP_SET_CAPACITY ioctl ends up reseting block
> > > > > device block size to 4096 by calling bd_set_size(). I have to think how to
> > > > > best fix this...
> > > > >
> > > > > Thanks for your help with debugging this!
> > > >
> > > > Wow! I am very excited.
> > > > We have 587 open "task hung" reports, I suspect this explains lots of them.
> > > > What would be some pattern that we can use to best-effort distinguish
> > > > most manifestations? Skimming through few reports I see "inode_lock",
> > > > "get_super", "blkdev_put" as common indicators. Anything else?
> > >
> > > Well, there will be always looping task with __getblk_gfp() on its stack
> > > (which should be visible in the stacktrace generated by the stall
> > > detector). Then there can be lots of other processes getting blocked due to
> > > locks and other resources held by this task...
> >
> >
> > Once we have a fix, I plan to do a sweep over existing open "task

I have submitted the fix yesterday to linux-block ([PATCH 0/2] blkdev: Fix
livelock when loop device updates capacity).

> > hung" reports and dup lots of them onto this one. Probably preferring
> > to over-sweep rather then to under-sweep because there are too many of
> > them and lots does not seem to be actionable otherwise.
> > Tetsuo, do you have comments before I start?
> 
> Also, is it possible to add some kind of WARNING for this condition?
> Taking into account how much effort it too to debug, looks like a
> useful check. Or did I ask this already...

There are two things we could do:

1) Warn if we loop in __getblk_slow() more than couple of times (looping
once is normal, looping twice can happen easily due to races).

2) Warn & bail if block size passed to __getblk_slow() does not match the
block device block size.

I'll write the patch.

								Honza
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR