Re: [patch 11/14] remap_file_pages protection support: pte_present should not trigger on PTE_FILE PROTNONE ptes

[Blaisorblade]

On Saturday 06 May 2006 12:03, Nick Piggin wrote:
> Blaisorblade wrote:
> > On Tuesday 02 May 2006 05:53, Nick Piggin wrote:
> >>blaisorblade@xxxxxxxx wrote:
> >>>From: Paolo 'Blaisorblade' Giarrusso <blaisorblade@xxxxxxxx>
> >>>
> >>>pte_present(pte) implies that pte_pfn(pte) is valid. Normally even with
> >>> a _PAGE_PROTNONE pte this holds, but not when such a PTE is installed
> >>> by the new install_file_pte; previously it didn't store protections,
> >>> only file offsets, with the patches it also stores protections, and can
> >>> set _PAGE_PROTNONE|_PAGE_FILE.
> >
> > What could be done is to set a PTE with "no protection", use another bit
> > rather than _PAGE_PROTNONE. This wastes one more bit but doable.

> I see.

> I guess your problem is that you're overloading the pte protection bits
> for present ptes as protection bits for not present (file) ptes. I'd rather
> you just used a different encoding for file pte protections then.

Yes, this is what I said above, so we agree; and indeed this overloading was 
decided when the present problem didn't trigger, so it can now change. As 
detailed in the patch description, the previous PageReserved handling 
prevented freeing page 0 and hided this.

> "Wasting" a bit seems much more preferable for this very uncommon case (for
> most people) rather than bloating pte_present check, which is called in
> practically every performance critical inner loop).

Yes, I thought about this problem, I wasn't sure how hard it was.

> That said, if the patch is i386/uml specific then I don't have much say in
> it.

It's presently specific, but will probably extend. Implementations for some 
other archs were already sent and I've collected them (will send 
afterwards,I've avoided excess bloat).

> If Ingo/Linus and Jeff/Yourself, respectively, accept the patch, then 
> fine.

> But I think you should drop the comment from the core code. It seems wrong.

Yep, forgot there, thanks for reminding, I've now removed it.
-- 
Inform me of my mistakes, so I can keep imitating Homer Simpson's "Doh!".
Paolo Giarrusso, aka Blaisorblade (Skype ID "PaoloGiarrusso", ICQ 215621894)
http://www.user-mode-linux.org/~blaisorblade

Chiacchiera con i tuoi amici in tempo reale! 
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