Re: INFO: task hung in generic_file_write_iter

[Jan Kara]

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!

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