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

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Date:	Wed, 24 Aug 2011 12:33:17 +0400
From:	Pavel Emelyanov <xemul@...allels.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	Cyrill Gorcunov <gorcunov@...il.com>,
	Al Viro <viro@...IV.linux.org.uk>,
	James Bottomley <jbottomley@...allels.com>,
	Tejun Heo <tj@...nel.org>,
	Glauber Costa <glommer@...allels.com>,
	"containers@...ts.osdl.org" <containers@...ts.osdl.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Serge Hallyn <serge.hallyn@...onical.com>,
	Nathan Lynch <ntl@...ox.com>,
	Oren Laadan <orenl@...columbia.edu>,
	Daniel Lezcano <dlezcano@...ibm.com>,
	LINUXFS-ML <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH resend] vfs: Add ->statfs callback for pipefs

> 
> 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

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