Re: [PATCH 04/35] autonuma: define _PAGE_NUMA_PTE and _PAGE_NUMA_PMD

Konrad Rzeszutek Wilk
2012-06-05T17:27:43Z

On Tue, Jun 05, 2012 at 07:13:54PM +0200, Andrea Arcangeli wrote:
> On Wed, May 30, 2012 at 04:01:51PM -0400, Konrad Rzeszutek Wilk wrote:
> > The only time the _PAGE_PSE (_PAGE_PAT) is set is when
> > _PAGE_PCD | _PAGE_PWT are set. It is this ugly transformation
> > of doing:
> >
> > if (pat_enabled && _PAGE_PWT | _PAGE_PCD)
> > pte = ~(_PAGE_PWT | _PAGE_PCD) | _PAGE_PAT;
> >
> > and then writting the pte with the 7th bit set instead of the
> > 2nd and 3rd to mark it as WC. There is a corresponding reverse too
> > (to read the pte - so the pte_val calls) - so if _PAGE_PAT is
> > detected it will remove the _PAGE_PAT and return the PTE as
> > if it had _PAGE_PWT | _PAGE_PCD.
> >
> > So that little bit of code will need some tweaking - as it does
> > that even if _PAGE_PRESENT is not set. Meaning it would
> > transform your _PAGE_PAT to _PAGE_PWT | _PAGE_PCD. Gah!
>
> It looks like this is disabled in current upstream?
> 8eaffa67b43e99ae581622c5133e20b0f48bcef1

Yup. But it is a temporary bandaid that I hope to fix soon.
>
> > OK. I can whip up a patch to deal with the 'Gah!' case easily if needed.
>
> That would help! But again it looks disabled in Xen?
>
> About linux host (no xen) when I decided to use PSE I checked this part:
>
> /* Set PWT to Write-Combining. All other bits stay the same */
> /*
> * PTE encoding used in Linux:
> * PAT
> * |PCD
> * ||PWT
> * |||
> * 000 WB _PAGE_CACHE_WB
> * 001 WC _PAGE_CACHE_WC
> * 010 UC- _PAGE_CACHE_UC_MINUS
> * 011 UC _PAGE_CACHE_UC
> * PAT bit unused
> */
>
> I need to go read the specs pdf and audit the code against the specs
> to be sure but if my interpretation correct, PAT is never set on linux
> host (novirt) the way the relevant msr are programmed.
>
> If I couldn't use the PSE (/PAT) it'd screw with 32bit because I need
> to poke a bit between _PAGE_BIT_DIRTY and _PAGE_BIT_GLOBAL to avoid
> losing space on the swap entry, and there's just one bit in that range
> (PSE).
>
> _PAGE_UNUSED1 (besides it's used by Xen) wouldn't work unless I change
> the swp entry format for 32bit x86 reducing the max amount of swap
> (conditional to CONFIG_AUTONUMA so it wouldn't be the end of the
> world, plus the amount of swap on 32bit NUMA may not be so important)

Yeah, I concur. I think stick with _PAGE_PAT (/PSE) and I can cook
up the appropiate patch for it on the Xen side.