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 15:12:58 -0700 (PDT)
Christoph Lameter <clameter@sgi.com> wrote:

> On Mon, 27 Aug 2007, Andrew Morton wrote:
> 
> > > Nesting (at least allocate a 
> > > regular page while highmem page is being mapped) needs to work in order to 
> > > be able to allocate a page from an interrupt contexts.
> > 
> > yup, but interrupt-level code should use the reserved-for-interrupt kmap
> > slots (KM_IRQ0, etc).
> 
> We are not using any kmap slot since we are allocating a non highmem page!

Right.  So a get_zeroed_page() from IRQ context happens to be not buggy,
even though it goes BUG.

So what do we do?

a) don't call clear_highpage() for non-highmem pages (my fix)

b) don't do the is-the-pte-zero check in kmap_atomic_prot() if the page
   isn't highmem

   This is bad because it'll disable the check completely if the machine
   doesn't physically have highmem, or if the page happened to be a lowmem
   one.

c) don't do the is-the-pte-zero check if __GFP_HIGHMEM wasn't set.

   OK, but we don't pass the gfp_t into kmap_atomic.  So we need to do
   this at the caller site.  That's what a) does.

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