block: blk_max_pfn is somtimes wrong

There is a small problem in handling page bounce.

At the moment blk_max_pfn equals max_pfn, which is in fact not maximum
possible _number_ of a page frame, but the _amount_ of page frames.  For
example for the 32bit x86 node with 4Gb RAM, max_pfn = 0x100000, but not
0xFFFF.

request_queue structure has a member q->bounce_pfn and queue needs bounce
pages for the pages _above_ this limit.  This routine is handled by
blk_queue_bounce(), where the following check is produced:

	if (q->bounce_pfn >= blk_max_pfn)
		return;

Assume, that a driver has set q->bounce_pfn to 0xFFFF, but blk_max_pfn
equals 0x10000.  In such situation the check above fails and for each bio
we always fall down for iterating over pages tied to the bio.

I want to notice, that for quite a big range of device drivers (ide, md,
...) such problem doesn't happen because they use BLK_BOUNCE_ANY for
bounce_pfn.  BLK_BOUNCE_ANY is defined as blk_max_pfn << PAGE_SHIFT, and
then the check above doesn't fail.  But for other drivers, which obtain
reuired value from drivers, it fails.  For example sata_nv uses
ATA_DMA_MASK or dev->dma_mask.

I propose to use (max_pfn - 1) for blk_max_pfn.  And the same for
blk_max_low_pfn.  The patch also cleanses some checks related with
bounce_pfn.
Signed-off-by: Vasily Tarasov <vtaras@openvz.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

block/ll_rw_blk.c

@@ -1221,7 +1221,7 @@ void blk_recount_segments(request_queue_t *q, struct bio *bio)
 		 * considered part of another segment, since that might
 		 * change with the bounce page.
 		 */
-		high = page_to_pfn(bv->bv_page) >= q->bounce_pfn;
+		high = page_to_pfn(bv->bv_page) > q->bounce_pfn;
 		if (high || highprv)
 			goto new_hw_segment;
 		if (cluster) {
@@ -3658,8 +3658,8 @@ int __init blk_dev_init(void)
 	open_softirq(BLOCK_SOFTIRQ, blk_done_softirq, NULL);
 	register_hotcpu_notifier(&blk_cpu_notifier);
 
-	blk_max_low_pfn = max_low_pfn;
-	blk_max_pfn = max_pfn;
+	blk_max_low_pfn = max_low_pfn - 1;
+	blk_max_pfn = max_pfn - 1;
 
 	return 0;
 }

mm/bounce.c

@@ -204,7 +204,7 @@ static void __blk_queue_bounce(request_queue_t *q, struct bio **bio_orig,
 		/*
 		 * is destination page below bounce pfn?
 		 */
-		if (page_to_pfn(page) < q->bounce_pfn)
+		if (page_to_pfn(page) <= q->bounce_pfn)
 			continue;
 
 		/*