Swapfile on ZFS & Deadlock

Benjamin Close Benjamin.Close at clearchain.com
Sat Jun 16 03:22:07 UTC 2007

Kris Kennaway wrote:
> On Fri, Jun 15, 2007 at 11:00:04PM +0930, Benjamin Close wrote:
>> Hi All,
>>    Whilst running out of memory compiling Xorg (scanPCI is a killer) I 
>> discovered a quick way to deadlock the system:
>> dd if=/dev/zero of=somefileonzfs bs=something count=something
>> mdconfig -a -f something
>> swapon /dev/md0
>> Then do something that needs swap.. instant deadlock. The system is 
>> still responsive but all disk access become hung.
>> Known issue? If so is there a way we can warn users/prevent users from 
>> doing it?
> Enable DEBUG_VFS_LOCKS and DEBUG_LOCKS, then break to DDB when the
> deadlock occurs and do 'show lockedvnods'.
Ok, enabled the above and this time got:

swap_pager: indefinite wait buffer: bufobj: 0, blkno: 312865, size: 16384

just before the deadlock:

Show locked vnods returns (hand transcribed)

0xffffff002c3ab5d0: tagz zfs, type VREG
    usecout 1, writecount 1, refcount 2 mountedhere 0
    v_object 0xffffff002f047c80 ref 0 pages 0
       lock type zfs: EXCL (count 1) by thread 0xffffff0030573360 (pid 1188)

Pid 1188 is:

1188   0   0   0   SL   zfs:(&zi   0xffffff000208fd58   [md0]

called doadump and though it went through the motions, savecore didn't 
find anything saved.
Not sure what you need debugging wise, let me know.

System is Intel Core 2 duo, running in SMP amd64, updated Friday 15th June.
The box has 1G physical ram, 1G dedicated swap partition, but needs an 
extra 300M to compile xf86ScanPCI.c (freaky!).
Figured a temp solution would be to allocate a 500M swap file on /var 
which is zfs.


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