fix /proc/vmcore seek

[Frederic Weisbecker]

On Fri, Sep 03, 2010 at 01:58:44PM -0700, Andrew Morton wrote:
> On Thu, 2 Sep 2010 14:20:19 +0200
> Arnd Bergmann <arnd at arndb.de> wrote:
> 
> > Commit 73296bc611 "procfs: Use generic_file_llseek in /proc/vmcore" broke
> > seeking on /proc/vmcore. This changes it back to use default_llseek
> > in order to restore the original behaviour.
> 
> "broke" isn't a useful description of a bug :(
> 
> A bit of list-trolling indicates that this change caused makedumpfile
> to hang during kexec dumping, yes?  If so, why did this happen?  How
> did the switch to generic_file_llseek() cause the /proc/vmcore seek
> behaviour to change?
> 
> I'm not seeing a lot of difference between default_llseek() and
> generic_file_llseek() apart from file->f_mapping->host versus
> file->f_path.dentry->d_inode.  Was it that, or what?
> 
> etcetera.  Better patch descriptions, please!


Yeah, that's the problem, we don't know yet why it broke. I was about
to investigate and try to find the problem before applying it but
got distracted by other things.

I'll try to find the issue.

Thanks.