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

[Christoph Lameter <clameter@sgi.com>]

This issue is a result of commit 656dad312fb41ed95ef08325e9df9bece3aacbbb. 
The intend of moving tests before the check for the highpage was to catch 
some additional errors.

On Mon, 27 Aug 2007, Christoph Lameter wrote:

> On Mon, 27 Aug 2007, Andrew Morton wrote:
> 
> > __GFP_HIGHMEM is not set.
> > 
> > : 	/*
> > : 	 * clear_highpage() will use KM_USER0, so it's a bug to use __GFP_ZERO
> > : 	 * and __GFP_HIGHMEM from hard or soft interrupt context.
> > : 	 */
> > : 	VM_BUG_ON((gfp_flags & __GFP_HIGHMEM) && in_interrupt());
> > 
> > __GFP_HIGHMEM is not set
> > 
> > : 	for (i = 0; i < (1 << order); i++)
> > : 		clear_highpage(page + i);
> > 
> > kmap_atomic() goes boom.
> 
> So the page is not a highmem page. kmap does:
> 
> void *kmap(struct page *page)
> {
>         might_sleep();
>         if (!PageHighMem(page))
>                 return page_address(page);
>         return kmap_high(page);
> }
> 
> -> kmap is fine.
> 
> 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?
> 
> 

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