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

Christoph Lameter
2007-08-27T21:32:21Z

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?