Messages in this thread | | | From | Blaisorblade <> | Subject | Re: [patch 11/14] remap_file_pages protection support: pte_present should not trigger on PTE_FILE PROTNONE ptes | Date | Sun, 7 May 2006 19:50:58 +0200 |
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On Saturday 06 May 2006 12:03, Nick Piggin wrote: > Blaisorblade wrote: > > On Tuesday 02 May 2006 05:53, Nick Piggin wrote: > >>blaisorblade@yahoo.it wrote: > >>>From: Paolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it> > >>> > >>>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
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