linux-kernel - Re: [GIT PULL -mm] 00/14 VFS/Unionfs updates/fixes/cleanups

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Message-Id: <200804021654.m32GsRnI020433@agora.fsl.cs.sunysb.edu>
Date:	Wed, 2 Apr 2008 12:54:27 -0400
From:	Erez Zadok <ezk@...sunysb.edu>
To:	Tomas M <tomas@...x.org>
Cc:	Erez Zadok <ezk@...sunysb.edu>, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	viro@....linux.org.uk, hch@...radead.org
Subject: Re: [GIT PULL -mm] 00/14 VFS/Unionfs updates/fixes/cleanups 

In message <47F38E90.20306@...x.org>, Tomas M writes:

> > 1. Based on recommendations we've received while at LSF'08, we've changed
> >    Unionfs to use the vm_operations->fault method, instead of
> >    address_space_operations.
[...]
> May I ask you, why did it took so long?
> Mainly, why did you refused to make the changes last year?
> 
> Thank you
> 
> Tomas M

The current ->fault API requires (IMHO) an ugly hack such as this:

1. get the upper file pointer from vma->vm_file
2. get the lower file from the upper file
3. *override* the vma->vm_file pointer with the lower file
4. call the lower ->fault op with the changed vma->vm_file pointer
5. upon return, *reset* the vma->vm_file pointer back to its original value
   (i.e., pointing to the upper file)

Steps 3 and 5 require this "pointer flipping" that I was sure was
unacceptable to kernel developers who might be reviewing the code.  I didn't
want to use this technique before I've had the sanctioning of the
appropriate kernel maintainers, along with an acceptable path to fix the
->fault API.  LSF was an excellent opportunity to discuss this and many
other related issues in person.

Cheers,
Erez.
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