[Devel] [PATCH] block: blk_max_pfn is somtimes wrong

[Vasily Tarasov]

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 at openvz.org>

---

--- ./block/ll_rw_blk.c.max_pfn	2007-01-10 03:35:11.000000000 +0300
+++ ./block/ll_rw_blk.c	2007-02-08 14:42:48.000000000 +0300
@@ -1221,7 +1221,7 @@ void blk_recount_segments(request_queue_
 		 * 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.max_pfn	2006-11-30 00:57:37.000000000 +0300
+++ ./mm/bounce.c	2007-02-08 14:49:35.000000000 +0300
@@ -204,7 +204,7 @@ static void __blk_queue_bounce(request_q
 		/*
 		 * is destination page below bounce pfn?
 		 */
-		if (page_to_pfn(page) < q->bounce_pfn)
+		if (page_to_pfn(page) <= q->bounce_pfn)
 			continue;
 
 		/*