linux-kernel - Re: [PATCH 1/2] vmsplice: unmap gifted pages for recipient

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Date:	Tue, 8 Oct 2013 14:48:19 -0500
From:	Robert Jennings <rcj@...ux.vnet.ibm.com>
To:	Dave Hansen <dave@...1.net>
Cc:	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	linux-mm@...ck.org, Alexander Viro <viro@...iv.linux.org.uk>,
	Rik van Riel <riel@...hat.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Matt Helsley <matt.helsley@...il.com>,
	Anthony Liguori <anthony@...emonkey.ws>,
	Michael Roth <mdroth@...ux.vnet.ibm.com>,
	Lei Li <lilei@...ux.vnet.ibm.com>,
	Leonardo Garcia <lagarcia@...ux.vnet.ibm.com>,
	Vlastimil Babka <vbabka@...e.cz>
Subject: Re: [PATCH 1/2] vmsplice: unmap gifted pages for recipient

* Dave Hansen (dave@...1.net) wrote:
> On 10/07/2013 01:21 PM, Robert C Jennings wrote:
> > +					} else {
> > +						if (vma)
> > +							zap_page_range(vma,
> > +								user_start,
> > +								(user_end -
> > +								 user_start),
> > +								NULL);
> > +						vma = find_vma_intersection(
> > +								current->mm,
> > +								useraddr,
> > +								(useraddr +
> > +								 PAGE_SIZE));
> > +						if (!IS_ERR_OR_NULL(vma)) {
> > +							user_start = useraddr;
> > +							user_end = (useraddr +
> > +								    PAGE_SIZE);
> > +						} else
> > +							vma = NULL;
> > +					}
> 
> This is pretty unspeakably hideous.  Was there truly no better way to do
> this?

I was hoping to find a better way to coalesce pipe buffers and zap
entire VMAs (and it needs better documentation but your argument is with
structure and I agree). I would love suggestions for improving this but
that is not to say that I've abandoned it; I'm still looking for ways
to make this cleaner.

Doing find_vma() on a single page in the VMA rather than on each and
then zapping once provides a 50% runtime reduction for the writer when
tested with a 256MB vmsplice operation.  Based on the result I felt that
coalescing was justfied but the implementation is ugly.

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