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

[Robert Jennings <rcj@linux.vnet.ibm.com>]

* Dave Hansen (dave@sr71.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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