kjournald hang on ext3 to ext3 copy

[Andreas Dilger]

On Jun 16, 2007  08:17 -0500, John Marconi wrote:
> I am running into a situation in which one of my ext3 filesystems is 
> getting hung during normal usage.  There are three ext3 filesystems on a 
> CompactFLASH.  One is mounted as / and one as /tmp.  In my test, I am 
> copying a 100 MB file from /root to /tmp repeatedly.  While doing this 
> test, I eventually see the copying stop, and any attempts to access /tmp 
> fail - if I even do ls /tmp the command will hang.
> 
> I suspect kjournald because of the following ps output:
> PID      PPID   WCHAN:20      PCPU  %MEM  PSR  COMM
> 8847    99 start_this_handle        1.1  0.0  28     pdflush
> 8853    99 schedule_timeout       0.2  0.0   7     pdflush
>  188     1 kswapd                       0.0  0.0  19   kswapd0
> 8051     1 mtd_blktrans_thread   0.0  0.0  22   mtdblockd
> 8243     1 kjournald                    0.0  0.0   0   kjournald
> 8305     1 schedule_timeout        0.0  0.0   2   udevd
> 8378     1 kjournald                    0.0  0.0   0   kjournald
> 8379     1 journal_commit_trans 16.6  0.0   0   kjournald
> 8437     1 schedule_timeout       0.0  0.0   0   evlogd
> 8527     1 syslog                        0.0  0.0   1   klogd
> 8534     1 schedule_timeout       0.0  0.0   0   portmap
> 8569     1 schedule_timeout       0.0  0.0   0   rngd
> 8639     1 schedule_timeout       0.1  0.0  24   sshd
> 8741  8639 schedule_timeout    0.0  0.0   0     sshd
> 8743  8741 wait                        0.0  0.0   9       bash
> 8857  8743 schedule_timeout    4.9  0.0   7         cp
> 8664     1 schedule_timeout       0.0  0.0   0   xinetd
> 8679     1 schedule_timeout       0.0  0.0   0   evlnotifyd
> 8689     1 schedule_timeout       0.0  0.0   0   evlactiond
> 8704     1 wait                           0.0  0.0   1   bash
> 8882  8704 -                            0.0  0.0   2     ps
> 
> If I run ps repeatedly, I always see process 8379 in 
> journal_commit_transaction, and it is always taking between 12% and 20% 
> of processor 0 up.  This process never completes.  I also see process 
> 8847 in start_this_handle forever as well - so I believe they are related. 
> 
> This system is using a 2.6.14 kernel.

Please try to reproduce with a newer kernel, as this kind of problem
might have been fixed already.


Two tips for debugging this kind of issue:
- you need to have detailed stack traces (e.g. sysrq-t) of all the
  interesting processes

- if a process is stuck inside a large function (e.g. 8379 in example)
  you need to provide the exact line number.  this can be found by compiling
  the kernel with CONFIG_DEBUG_INFO (-g flag to gcc) and then doing
  "gdb vmlinux" and "p *(journal_commit_transaction+{offset})", where the
  byte offset is printed in the sysrq-t output, and then include the code
  surrounding that line from the source file

- a process stuck in "start_this_handle()" is often just an innocent
  bystander.  It is waiting for the currently committing transaction to
  complete before it can start a new filesystem-modifying operation (handle).
  That said, the journal handle acts like a lock and has been the cause of
  many deadlock problems (e.g. process 1 holds lock, waits for handle;
  process 2 holds transaction open waiting for lock).  pdflush might be one
  of the "process 1" kind of tasks, and some other process is holding the
  transaction open preventing it from completing.

Cheers, Andreas
--
Andreas Dilger
Principal Software Engineer
Cluster File Systems, Inc.