another pagetable initialization crash on xen

[Stefano Stabellini]

Hi Yinghai,
unfortunately I found another pagetable initialization bug on xen
affecting linux 2.6.39-rc0.
The problem is that on xen we need to make sure that all the pagetable pages
are mapped read-only, in fact in xen_set_pte we have this check:

if (pfn >= pgt_buf_start && pfn < pgt_buf_end)
    /* make the pte read-only */

however pgt_buf_end is where the kernel pagetable *currently* ends, so
some kernel pagetable pages allocated after pgt_buf_end might be marked
read-write by mistake.  A simple way to fix the issue would be to use
pgt_buf_top instead:

if (pfn >= pgt_buf_start && pfn < pgt_buf_top)
    /* make the pte read-only */

however after building the kernel pagetable in init_memory_mapping we
only reserve memory between pgt_buf_start and pgt_buf_end:

if (!after_bootmem && pgt_buf_end > pgt_buf_start)
        memblock_x86_reserve_range(pgt_buf_start << PAGE_SHIFT,
                 pgt_buf_end << PAGE_SHIFT, "PGTABLE");

so feature allocations might use memory between the final value of
pgt_buf_end and pgt_buf_top that has been marked read-only in the xen
specific code, causing a crash.
The only way I could find to fix the crash is to reserve also the memory
region between pgt_buf_start and pgt_buf_top on xen, but that would
require an ugly if(xen_domain()) at the of init_memory_mapping or
the introduction of a new pvop function to reserve the pagetable memory.
I don't like the idea, but I couldn't find anything better.
Yinghai, do you have any better suggestions?

Thanks,

Stefano
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