linux-kernel - Re: Definition of x86 _PAGE_SPECIAL and sharing _PAGE

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Date:	Thu, 28 Aug 2008 10:32:30 +1000
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	Ingo Molnar <mingo@...e.hu>, Hugh Dickens <hugh@...itas.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Memory Management List <linux-mm@...ck.org>
Subject: Re: Definition of x86 _PAGE_SPECIAL and sharing _PAGE_UNUSED1

On Thursday 28 August 2008 05:02, Jeremy Fitzhardinge wrote:
> _PAGE_SPECIAL is overloading _PAGE_UNUSED1.  Does it really leave
> _PAGE_UNUSED1 available for other uses, or does it become an exclusive
> user of that flag.  Under what circumstances can they be shared?
>
> arch/x86/mm/pageattr-test.c is now using _PAGE_UNUSED1 as the flag used
> to make sure that huge pages are shattered properly (previously it used
> _PAGE_GLOBAL).  Is that going to clash with _PAGE_SPECIAL?

Ah... pity it was hidden away there and not put into the include file.


> In other words, should we drop _PAGE_UNUSED1 altogether, or at least
> define how the its different users can coexist?

I don't feel strongly about it. But you should put your definition in
pgtable.h (and possibly explain how it coexists with _SPECIAL).


> Am I right in supposing that _PAGE_SPECIAL can only be set on user pages?

Yes.


> (Also, "SPECIAL" is awfully generic.  Was there really no more
> descriptive name for this?)

I thought it was about on par with its counterpart, which is "normal".
Either way, I don't think a casual reader would get an adequate idea
of how it works in one word. normal ~= refcounted, special ~= !refcounted
I guess, but it is slightly more than that and besides, normal was there
first, and I think Linus coined it... if you can convince him to change
it then you have my blessing to change special into whatever you want.
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