Commit f772b3d9 authored Mar 27, 2007 by Vasily Tarasov Committed by Jens Axboe Mar 27, 2007

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>

parent 703071b5

block/ll_rw_blk.c

+3 −3

Original line number	Diff line number	Diff line
		@@ -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

+1 −1

Original line number	Diff line number	Diff line
		@@ -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;

		/*

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