linux-kernel - Re: [PATCH 1/2] mm: implement remap_pfn_range with apply_to_page

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Date:	Thu, 13 Nov 2008 21:22:55 -0800
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Nick Piggin <nickpiggin@...oo.com.au>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Memory Management List <linux-mm@...ck.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"Pallipadi, Venkatesh" <venkatesh.pallipadi@...el.com>
Subject: Re: [PATCH 1/2] mm: implement remap_pfn_range with apply_to_page_range

Nick Piggin wrote:
> On Friday 14 November 2008 13:56, Jeremy Fitzhardinge wrote:
>   
>> Nick Piggin wrote:
>>     
>>> This isn't performance critical to anyone?
>>>       
>> The only difference should be between having the specialized code and an
>> indirect function call, no?
>>     
>
> Indirect function call per pte. It's going to be slower surely.
>   

Yes, though changing the calling convention to handle (up to) a whole 
page worth of ptes in one call would be fairly simple I think.

> It is accepted practice to (carefully) duplicate the page table walking
> functions in memory management code. I don't think that's a problem,
> there is already so many instances of them (just be sure to stick to
> exactly the same form and variable names, and any update or bugfix to
> any of them is trivially applicable to all).
>   

I think that's pretty awful practice, frankly, and I'd much prefer there 
to be a single iterator function which everyone uses.  The open-coded 
iterators everywhere just makes it completely impractical to even think 
about other kinds of pagetable structures.  (Of course we have at least 
two "general purpose" pagetable walkers now...)

    J
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