On Wed, Jan 27, 2010 at 05:26:13PM +0000, Ian Campbell wrote:
> On Mon, 2010-01-25 at 20:02 +0000, Jeremy Fitzhardinge wrote:
> >
> > IanC, Pasi, myself and others explored a number of other ways to try
> > and fix it in the Xen pvops code, but they all turned out to be very
> > expensive, just not work (they just pushed the race around), or
> > require new pvops just for this case.
>
> Just to brainstorm a bit more:
>
> There's no way a kunmap_atomic pvop would be acceptable? it would at
> least make the API symmetrical.
>
> What about a hypercall which would set a PTE with the writable bit set
> atomically depending on the pinned status of the referenced page? (I
> haven't even vaguely thought this idea through).
>
> Is there some way we can disable HIGHPTE at runtime even if
> CONFIG_HIGHPTE=y? Looks like that might be relatively self-contained in
> pte_alloc_one(). All the actual uses of high PTEs goes through
> kmap_atomic which explicitly tests for PageHighmem so by ensuring PTEs
> are never high at allocation time we would skip all those paths.
> Something like the untested patch below, but not so skanky, obviously.
>
> This last would be nice since it also remove the
> crippling-for-virtualisation overhead, so it would potentially benefit
> KVM and VMI as well...
>
> > Given that HIGHPTE is generally a bad idea and should be deprecated
> > (any machine big enough to need it should definitely be running a
> > 64-bit kernel), I've left it on the backburner hoping for some
> > inspiration to strike. So far it has not.
>
> Unfortunately distros seem to be using it for their native kernels and
> since pvops means they won't have a separate xen kernel I think we need
> to figure something out.
>
> Ian.
>
> diff --git a/arch/x86/mm/pgtable.c b/arch/x86/mm/pgtable.c
> index 65215ab..49f8e83 100644
> --- a/arch/x86/mm/pgtable.c
> +++ b/arch/x86/mm/pgtable.c
> @@ -28,7 +28,10 @@ pgtable_t pte_alloc_one(struct mm_struct *mm, unsigned
> long address)
> struct page *pte;
>
> #ifdef CONFIG_HIGHPTE
> - pte = alloc_pages(PGALLOC_GFP | __GFP_HIGHMEM, 0);
> + if (is_xen_domain())
> + pte = alloc_pages(PGALLOC_GFP, 0);
> + else
> + pte = alloc_pages(PGALLOC_GFP | __GFP_HIGHMEM, 0);
> #else
> pte = alloc_pages(PGALLOC_GFP, 0);
> #endif
>
I just tried this patch, but it fails to compile:
arch/x86/mm/pgtable.c: In function 'pte_alloc_one':
arch/x86/mm/pgtable.c:19: error: implicit declaration of function
'is_xen_domain'
make[2]: *** [arch/x86/mm/pgtable.o] Error 1
make[1]: *** [arch/x86/mm] Error 2
make: *** [arch/x86] Error 2
I tried grepping around for that function but didn't find it from any header..
-- Pasi
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