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

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Date:	Wed, 27 Aug 2008 17:47:34 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Nick Piggin <nickpiggin@...oo.com.au>
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

Nick Piggin wrote:
> Ah... pity it was hidden away there and not put into the include file.
>   

Yes.  I just prepped a patch to bring it out into the light.

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

Yes, that was my plan, but without knowing how _SPECIAL is used, it's a
bit tricky.  Is there a comment somewhere which describes who sets it
and when?  From a quick look, it seems it's set on newly added user
pages which aren't COWed.  Can they be shared file-backed pages? 
Anonymous pages?  Device pages?

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

OK, that won't clash with CPA tests at all, since they're kernel only.

>> (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.
>   

It's only used in a couple of places, so giving it a longer name
wouldn't cost much.  _PAGE_USER_UNCOUNTED?  But ugh, a lot of cross-arch
churnpatch to do it.

    J

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