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

[Zheng Liu <gnehzuil.liu@gmail.com>]

On Thu, Sep 13, 2012 at 04:34:55PM +0200, Jan Kara wrote:
> On Thu 13-09-12 20:57:26, Zheng Liu wrote:
> > On Thu, Sep 13, 2012 at 02:07:36PM +0200, Jan Kara wrote:
> > >   Hello,
> > > 
> > > On Thu 13-09-12 18:41:36, Zheng Liu wrote:
> > > > 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