Re: [PATCH] ppc64: Fix possible race with set_pte on a present PTE

[Linus Torvalds]

On Mon, 24 May 2004, Linus Torvalds wrote:
>
> We do the accessed bit by clearing the "user readable" thing (or
> something. I forget the exact details, and I'm too lazy to check it out).  

Yup. Lookie here:

	#define __ACCESS_BITS   (_PAGE_ACCESSED | _PAGE_KRE | _PAGE_URE)
	extern inline pte_t pte_mkold(pte_t pte)        { pte_val(pte) &= ~(__ACCESS_BITS); return pte; }

Notice how an "old" pte won't be readable. Then, when we take the page 
fault, we'll do

	extern inline pte_t pte_mkyoung(pte_t pte)      { pte_val(pte) |= __ACCESS_BITS; return pte; }

and now the pte is readable again.

In other words, we absolutely _have_ to do the "pte_mkyoung()" part in the
page fault, or an "old" pte will never become readable again (unless it's
accessed with a write rather than a read, which will then happen to make
it young again).

I'm not quite senile yet.

		Linus
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