Re: Page aging broken in 2.6

[Linus Torvalds]

On Fri, 26 Dec 2003, Benjamin Herrenschmidt wrote:
> 
> in mm/rmap.c, in page_referenced(), we do that twice:
> 
>                 if (ptep_test_and_clear_young(pte))
>                         referenced++;
> 
> And we never flush the TLB entry. 
> 
> I don't know if x86 (or other archs really using page tables) will
> actually set the referenced bit again in the PTE if it's already set
> in the TLB, if not, then x86 needs a flush too.

This was very much done on purpose. The theory is, that if you're low on
memory and have a lot of pages mapped, you will see enough TLB trashing
for this to not matter.

And if you aren't low on memory, or don't have a lot of pages mapped, it 
_also_ doesn't matter.

> ppc and ppc64 need a flush to evict the entry from the hash table or
> we'll never set the _PAGE_ACCESSED bit anymore.

Yeah, all hail bad MMU's.

Hash tables may need some kind of "not very urgent TLB flush" thing, so 
that it doesn't penalize sane architectures.

		Linus
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