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Date:   Wed, 9 Jan 2019 14:30:06 +0100
From:   Jan Kara <jack@...e.cz>
To:     Dmitry Vyukov <dvyukov@...gle.com>
Cc:     Jan Kara <jack@...e.cz>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        syzbot <syzbot+9933e4476f365f5d5a1b@...kaller.appspotmail.com>,
        Linux-MM <linux-mm@...ck.org>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Michal Hocko <mhocko@...nel.org>,
        Andi Kleen <ak@...ux.intel.com>, jlayton@...hat.com,
        LKML <linux-kernel@...r.kernel.org>,
        Matthew Wilcox <mawilcox@...rosoft.com>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
        tim.c.chen@...ux.intel.com,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: INFO: task hung in generic_file_write_iter

On Tue 08-01-19 12:49:08, Dmitry Vyukov wrote:
> On Tue, Jan 8, 2019 at 12:24 PM Jan Kara <jack@...e.cz> 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...

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR

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