[RFC PATCH 7/9] ARM64: mm: HugeTLB support.

[Will Deacon]

Hi Steve,

On Tue, Apr 30, 2013 at 05:30:46PM +0100, Steve Capper wrote:
> Add huge page support to ARM64, different huge page sizes are
> supported depending on the size of normal pages:
> 
> PAGE_SIZE is 4K:
>    2MB - (pmds) these can be allocated at any time.
> 1024MB - (puds) usually allocated on bootup with the command line
>          with something like: hugepagesz=1G hugepages=6
> 
> PAGE_SIZE is 64K:
>  512MB - (pmds), usually allocated on bootup via command line.

[...]

> diff --git a/arch/arm64/include/asm/pgtable-hwdef.h b/arch/arm64/include/asm/pgtable-hwdef.h
> index 75fd13d..c3cac68 100644
> --- a/arch/arm64/include/asm/pgtable-hwdef.h
> +++ b/arch/arm64/include/asm/pgtable-hwdef.h
> @@ -53,6 +53,7 @@
>  #define PTE_TYPE_MASK		(_AT(pteval_t, 3) << 0)
>  #define PTE_TYPE_FAULT		(_AT(pteval_t, 0) << 0)
>  #define PTE_TYPE_PAGE		(_AT(pteval_t, 3) << 0)
> +#define PTE_TYPE_HUGEPAGE	(_AT(pmdval_t, 1) << 0)

This breaks PROT_NONE mappings, where you get:

	pte = pte_mkhuge(pte_modify(pte, newprot));

The pte_modify will clear the valid bit and set the prot_none bit (in order
to create a present, faulting entry) but then your pte_mkhuge will come in
and clobber that with a valid block entry.

Will