Re: [patch] mm: fail GFP_DMA allocations when ZONE_DMA is not configured

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

On Thu, 14 Apr 2011 14:46:56 -0700 (PDT)
David Rientjes <rientjes@google.com> wrote:

> The page allocator will improperly return a page from ZONE_NORMAL even 
> when __GFP_DMA is passed if CONFIG_ZONE_DMA is disabled.  The caller 
> expects DMA memory, perhaps for ISA devices with 16-bit address 
> registers, and may get higher memory resulting in undefined behavior.
> 
> This patch causes the page allocator to return NULL in such circumstances 
> with a warning emitted to the kernel log on the first occurrence.
> 
> Signed-off-by: David Rientjes <rientjes@google.com>
> ---
>  mm/page_alloc.c |    4 ++++
>  1 files changed, 4 insertions(+), 0 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -2225,6 +2225,10 @@ __alloc_pages_nodemask(gfp_t gfp_mask, unsigned int order,
>  
>  	if (should_fail_alloc_page(gfp_mask, order))
>  		return NULL;
> +#ifndef CONFIG_ZONE_DMA
> +	if (WARN_ON_ONCE(gfp_mask & __GFP_DMA))
> +		return NULL;
> +#endif

Worried.  We have a large number of drivers which use GFP_DMA and I bet
some of them didn't really need to set it, and can use DMA32 memory. 
They will now break.

What is drivers/pci/intel-iommu.c doing with GFP_DMA btw?

How commonly are people disabling ZONE_DMA?

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