[PATCH] block: disable block layer bouncing for most memory on 64bit systems

 *    Different hardware can have different requirements as to what pages

 *    it can do I/O directly to. A low level driver can call

 *    blk_queue_bounce_limit to have lower memory pages allocated as bounce

 *    buffers for doing I/O to pages residing above @page. By default

 *    the block layer sets this to the highest numbered "low" memory page.

 *    buffers for doing I/O to pages residing above @page.

 **/

void blk_queue_bounce_limit(request_queue_t *q, u64 dma_addr)

{

	unsigned long bounce_pfn = dma_addr >> PAGE_SHIFT;

	int dma = 0;

	/*

	 * set appropriate bounce gfp mask -- unfortunately we don't have a

	 * full 4GB zone, so we have to resort to low memory for any bounces.

	 * ISA has its own < 16MB zone.

	 */

	if (bounce_pfn < blk_max_low_pfn) {

		BUG_ON(dma_addr < BLK_BOUNCE_ISA);

	q->bounce_gfp = GFP_NOIO;

#if BITS_PER_LONG == 64

	/* Assume anything <= 4GB can be handled by IOMMU.

	   Actually some IOMMUs can handle everything, but I don't

	   know of a way to test this here. */

	if (bounce_pfn < (0xffffffff>>PAGE_SHIFT))

		dma = 1;

	q->bounce_pfn = max_low_pfn;

#else

	if (bounce_pfn < blk_max_low_pfn)

		dma = 1;

	q->bounce_pfn = bounce_pfn;

#endif

	if (dma) {

		init_emergency_isa_pool();

		q->bounce_gfp = GFP_NOIO | GFP_DMA;

	} else

		q->bounce_gfp = GFP_NOIO;

		q->bounce_pfn = bounce_pfn;

	}

}

EXPORT_SYMBOL(blk_queue_bounce_limit);