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?
