[Xen-devel] Re: [PATCH] xen: partially revert "xen: set max_pfn_mapped to the last pfn mapped"

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Tue, 7 Jun 2011, Konrad Rzeszutek Wilk wrote:
> On Tue, Jun 07, 2011 at 03:35:05PM +0100, Stefano Stabellini wrote:
> > We only need to set max_pfn_mapped to the last pfn mapped on x86_64 to
> > make sure that cleanup_highmap doesn't remove important mappings at
> > _end.
> > 
> > We don't need to do this on x86_32 because cleanup_highmap is not called
> > on x86_32. Besides lowering max_pfn_mapped on x86_32 has the unwanted
> > side effect of limiting the amount of memory available for the 1:1
> > kernel pagetable allocation.
> > 
> > This patch reverts the x86_32 part of the original patch.
> > 
> > Signed-off-by: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>
> > 
> > diff --git a/arch/x86/xen/mmu.c b/arch/x86/xen/mmu.c
> > index dc708dc..afe1d54 100644
> > --- a/arch/x86/xen/mmu.c
> > +++ b/arch/x86/xen/mmu.c
> > @@ -1599,6 +1599,11 @@ static void __init xen_map_identity_early(pmd_t 
> > *pmd, unsigned long max_pfn)
> >             for (pteidx = 0; pteidx < PTRS_PER_PTE; pteidx++, pfn++) {
> >                     pte_t pte;
> >  
> > +#ifdef CONFIG_X86_32
> > +                   if (pfn > max_pfn_mapped)
> > +                           max_pfn_mapped = pfn;
> > +#endif
> > +
> >                     if (!pte_none(pte_page[pteidx]))
> >                             continue;
> >  
> > @@ -1766,7 +1771,9 @@ pgd_t * __init xen_setup_kernel_pagetable(pgd_t *pgd,
> >     initial_kernel_pmd =
> >             extend_brk(sizeof(pmd_t) * PTRS_PER_PMD, PAGE_SIZE);
> >  
> > -   max_pfn_mapped = PFN_DOWN(__pa(xen_start_info->mfn_list));
> > +   max_pfn_mapped = PFN_DOWN(__pa(xen_start_info->pt_base) +
> > +                             xen_start_info->nr_pt_frames * PAGE_SIZE +
> > +                             512*1024);
> 
> The x86_64 has about the same max_pfn_mapped value, but it does not have
> that extra 512kbytes. What is that for? Perhaps we should provide a comment
> to explain what that extra memory mapped is for?

The main difference between x86_64 and x86_32 is not the initial value
of max_pfn_mapped here, but the fact that max_pfn_mapped is updated in
xen_map_identity_early. In fact I get about 4000 pages of difference
with/without this patch.

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