Re: [PATCH 6/7] ext4: endless truncate due to nonlocked dio readers V2

On Thu, Sep 13, 2012 at 04:34:55PM +0200, Jan Kara wrote:

On Thu 13-09-12 20:57:26, Zheng Liu wrote:

quoted

On Thu, Sep 13, 2012 at 02:07:36PM +0200, Jan Kara wrote:

quoted

  Hello,

On Thu 13-09-12 18:41:36, Zheng Liu wrote:

quoted

Could you please provide more detailed workload to convince me?  I
am thinking about whether we really need to disable dioread_nolock
feature in here.  In our benchmarks, we don't see this problem.

  I just did:

# Create file
dd if=/dev/zero of=/mnt/file bs=1M count=30
sync
# Start 10 DIO dio readers in parallel reading the file in a loop
for (( i = 0; i < 10; i++ )); do
  while true; do
    dd if=/mnt/file bs=4k iflag=direct of=/dev/null
  done &
done
sleep 1

# Try to truncate the file - never finishes.
truncate -s 16 /mnt/file

It is pretty easy to hit this. Besides being a DOS attack vector (but I
won't be too concerned about this - there are plenty of ways how local
process can screw you) I can easily imagine some application to get bitten
by this. 

Hi Jan,

Thanks for your explanation, but in my desktop I cannot reproduce this
problem.  The size of `file' is 16.  Am I missing something?

  Hum, on my test machine with 3.6-rc1 it does not... Maybe for your
desktop you need a larger sleep before running truncate so that readers
have time to start up? Also I suppose you have ext4 mounted with
dioread_nolock mount option?

Yes, it can be reproduced after increasing sleep time.  Thanks.

Regards,
Zheng