Re: [patch 1/1] alloc_pages(): permit get_zeroed_page(GFP_ATOMIC) from interrupt context

[Andrew Morton <akpm@linux-foundation.org>]

On Mon, 27 Aug 2007 14:20:57 -0700 (PDT)
Christoph Lameter <clameter@sgi.com> wrote:

> kmap_atomic() does:
> 
> void *kmap_atomic_prot(struct page *page, enum km_type type, pgprot_t prot)
> {
>         enum fixed_addresses idx;
>         unsigned long vaddr;
> 
>         /* even !CONFIG_PREEMPT needs this, for in_atomic in do_page_fault */
>         pagefault_disable();
> 
>         idx = type + KM_TYPE_NR*smp_processor_id();
>         BUG_ON(!pte_none(*(kmap_pte-idx)));
> 
>         if (!PageHighMem(page))
>                 return page_address(page);
> 
>         vaddr = __fix_to_virt(FIX_KMAP_BEGIN + idx);
>         set_pte(kmap_pte-idx, mk_pte(page, prot));
>         arch_flush_lazy_mmu_mode();
> 
>         return (void*) vaddr;
> }
> 
> Move the check for highmem to the beginning of the function? Why 
> should kmap_atomic fail for a non highmem page?

For test coverage, mainly.  If someone is testing highmem-enabled code on
a 512MB machine, we want them to get told about any highmem-handling bugs,
even though they don't have highmem.

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