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SubjectRe: 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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