Re: [PATCH resend] vfs: Add ->statfs callback for pipefs

[Pavel Emelyanov <xemul@parallels.com>]

> 
> OK, I give up - how does it work?
> 
> <goes off and reads the kernel>
> 
> So it appears that the statfs call would previously return -ENOSYS, but
> with this change the statfs will succeed and userspace can then inspect
> f_type.  Yes?  I will fix your changelog by adding a description along these
> lines.

Yes, the primary intention is to get the f_type.

> And I'll ask you to fix it further by telling us why we want to do
> this?  The kernel has had this issue for a long time - why does it now
> matter?

This is required by checkpoint-restart in the userspace to make it
possible to distinguish pipes from fifos. In more details:

When we dump information about task's open files we use the /proc/pid/fd
directoy's symlinks and the fact that opening any of them gives us exactly
the same dentry->inode pair as the original process has. Now if a task 
we're dumping has opened pipe and fifo we need to detect this and act
accordingly. Knowing that an fd with type S_ISFIFO resides on a pipefs
is the most precise way.

Thanks,
Pavel