Re: quotacheck deadlock?

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Thu, Jul 20, 2017 at 08:38:46AM -0400, Brian Foster wrote:
> On Wed, Jul 19, 2017 at 11:58:04PM -0700, Darrick J. Wong wrote:
> > Hi,
> > 
> > I ran the following sequence of commands on 4.13-rc1:
> > 
> > # mkfs.xfs -f /dev/sdf
> > # xfs_db -x -c 'sb 0' -c 'addr rootino' -c 'write -d core.uid 4294967295' /dev/sdf
> > # mount /dev/sdf -o usrquota
> > 
> > The kernel reports that it's starting quotacheck, but never finishes.
> > echo t > /proc/sysrq produces this for the hung mount command:
> > 
> > mount           R  running task        0   988    895 0x00000000
> > Call Trace:
> >  ? sched_clock_cpu+0xa8/0xe0
> >  ? xfs_qm_flush_one+0x3c/0x120 [xfs]
> >  ? lock_acquire+0xac/0x200
> >  ? lock_acquire+0xac/0x200
> >  ? xfs_qm_flush_one+0x3c/0x120 [xfs]
> >  ? xfs_qm_dquot_walk+0xa1/0x170 [xfs]
> >  ? get_lock_stats+0x19/0x60
> >  ? get_lock_stats+0x19/0x60
> >  ? xfs_qm_dquot_walk+0xa1/0x170 [xfs]
> >  ? xfs_qm_dquot_walk+0x125/0x170 [xfs]
> >  ? radix_tree_gang_lookup+0xd1/0xf0
> >  ? xfs_qm_shrink_count+0x20/0x20 [xfs]
> >  ? xfs_qm_dquot_walk+0xbb/0x170 [xfs]
> >  ? kfree+0x23f/0x2d0
> >  ? kvfree+0x2a/0x40
> >  ? xfs_bulkstat+0x315/0x680 [xfs]
> >  ? xfs_qm_get_rtblks+0xa0/0xa0 [xfs]
> >  ? xfs_qm_quotacheck+0x2bd/0x360 [xfs]
> >  ? xfs_qm_mount_quotas+0x106/0x1f0 [xfs]
> >  ? xfs_mountfs+0x6f2/0xb00 [xfs]
> >  ? xfs_fs_fill_super+0x483/0x610 [xfs]
> >  ? mount_bdev+0x180/0x1b0
> >  ? xfs_finish_flags+0x150/0x150 [xfs]
> >  ? xfs_fs_mount+0x15/0x20 [xfs]
> >  ? mount_fs+0x14/0x80
> >  ? vfs_kern_mount+0x67/0x170
> >  ? do_mount+0x195/0xd00
> >  ? kmem_cache_alloc_trace+0x231/0x2a0
> >  ? SyS_mount+0x95/0xe0
> >  ? entry_SYSCALL_64_fastpath+0x1f/0xbe
> > 
> > Any thoughts?  I'm not sure what's going on for sure, other than the
> > call stack looks funny and it's midnight so I'm going to sleep. :)
> > 
> 
> It looks like a problem with the loop in xfs_qm_dquot_walk(). The next
> lookup index is calculated as:
> 
> 	 next_index = be32_to_cpu(dqp->q_core.d_id) + 1;
> 
> ... each time through the loop. With the uid written above, the +1
> overflows the 32-bit next_index back to zero and the lookup starts over.
> I suppose a simple fix might be to do something like the following.
> Thoughts?
> 
> --- 8< ---
> 
> diff --git a/fs/xfs/xfs_qm.c b/fs/xfs/xfs_qm.c
> index 6ce948c..f013c893 100644
> --- a/fs/xfs/xfs_qm.c
> +++ b/fs/xfs/xfs_qm.c
> @@ -111,6 +111,8 @@ xfs_qm_dquot_walk(
>  			skipped = 0;
>  			break;
>  		}
> +		if (!next_index)
> +			break;

Well, this /does/ fix the quotacheck lockup... but leads me straight
into the next problem, which is that xfs_quota -x -c 'report -i' just
goes into an infinite loop:

root                3          0          0     00 [--------]
#4294967295           1          0          0     00 [--------]
<repeats>

That said, the userland APIs *chown/set*uid return -EINVAL if you pass
in a userid of -1U, so one could argue that it's not a valid id anyway.
Via stat(), the kernel squashes -1U down to 65534 (nobody), which
implies that (Linux, anyway) doesn't consider -1U to be a valid id.
ISTR XFS treats uids as a mostly opaque value that we get from and pass
to the VFS without a whole lot of interpretation...?

--D

>  	}
>  
>  	if (skipped) {
> --
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