Re: BUG() in shrink_dcache_for_umount_subtree on nfs4 mount

[Mark Moseley <moseleymark@gmail.com>]

> I'm not 100% sure it's related (but I'm going to guess it is) but on
> these same boxes, they're not actually able to reboot at the end of a
> graceful shutdown. After yielding that bug and continuing with the
> shutdown process, it gets all the way to exec'ing:
>
> reboot -d -f -i
>
> and then just hangs forever. I'm guessing a thread is hung still
> trying to unmount things. On another box, I triggered that bug with a
> umount of one top-level mount that had subtrees. When I umount'd
> another top-level mount with subtrees on that same box, it's blocked
> and unkillable. That second umount also logged another bug to the
> kernel logs.
>
> In both umounts described above, the entries in /proc/mounts go away
> after the umount.
>
> Jeff, are you at liberty to do a graceful shutdown of the box you saw
> that bug on? If so, does it actually reboot?


A bit more info: On the same boxes, freshly booted but with all the
same mounts (even the subtrees) mounted, I don't get that bug, so it
seems to happen just when there's been significant usage within those
mounts. These are all read-only mounts, if it makes a difference.

I was however able to trigger the bug on a box that had been running
(web serving) for about 15 minutes. Here's a snippet from slabinfo
right before umount'ing (let me know if more of it would help):

# grep nfs /proc/slabinfo
nfsd4_delegations      0      0    360   22    2 : tunables    0    0
  0 : slabdata      0      0      0
nfsd4_stateids         0      0    120   34    1 : tunables    0    0
  0 : slabdata      0      0      0
nfsd4_files            0      0    136   30    1 : tunables    0    0
  0 : slabdata      0      0      0
nfsd4_stateowners      0      0    424   38    4 : tunables    0    0
  0 : slabdata      0      0      0
nfs_direct_cache       0      0    136   30    1 : tunables    0    0
  0 : slabdata      0      0      0
nfs_write_data        46     46    704   23    4 : tunables    0    0
  0 : slabdata      2      2      0
nfs_read_data        207    207    704   23    4 : tunables    0    0
  0 : slabdata      9      9      0
nfs_inode_cache    23901  23901   1056   31    8 : tunables    0    0
  0 : slabdata    771    771      0
nfs_page             256    256    128   32    1 : tunables    0    0
  0 : slabdata      8      8      0