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

[Christoph Lameter <clameter@sgi.com>]

On Mon, 27 Aug 2007, Andrew Morton wrote:

> So what do we do?
> 
> a) don't call clear_highpage() for non-highmem pages (my fix)

Then we need to be sure that this is the only case where we call 
clear_highpage() from an interrupt. This solves an i386 arch problem in 
core code. Other arches are not broken like that. See f.e. sparc:

void *kmap_atomic(struct page *page, enum km_type type)
{
        unsigned long idx;
        unsigned long vaddr;

        /* even !CONFIG_PREEMPT needs this, for in_atomic in do_page_fault */
        pagefault_disable();
        if (!PageHighMem(page))
                return page_address(page);

        idx = type + KM_TYPE_NR*smp_processor_id();
        vaddr = __fix_to_virt(FIX_KMAP_BEGIN + idx);


> 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.

The check is meaningless if we do not have highmem. kmap_atomic is a 
function to be used in atomic context. I.e. interrupts. Nested by 
definition. It is broken as is since it BUG()s on a legitimate nested 
call.

> 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.

Would that not mean leaving kmap_atomic broken on i386? Before Ingo's 
commit things were fine. Revert the commit and there is no need 
to change core code.

What exactly is the point of checking that a kmap_atomic of a non highmem 
page cannot occur during the kmap_atomic of a highmem page? AFAICT this is 
fine.

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