[PATCH] Direct I/O bio size regression

[David Chinner <dgc@sgi.com>]

The change introduced here in 2.6.15:

http://kernel.org/git/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=defd94b75409b983f94548ea2f52ff5787ddb848

sets the request queue max_sector size unconditionally to 1024 sectors in
blk_queue_max_sectors() even if the underlying hardware can support a larger
number of sectors.

Hence when building direct I/O bios, we have the situation where:

	- dio_new_bio() limits bio vector size artifically to
	  1024 sectors / page size because bio_get_nr_vecs()
	  is used q->max_sectors to size the new bio; and
	- dio_bio_add_page() limits the total bio size to 1024
	  sectors because bio_add_page() now uses q->max_sectors
	  to limit the size of the bio.
	  
Therefore, we can't build direct I/Os larger than 1024 sectors even
if the hardware supports large I/Os.  This is a regression as before
this mod we were able to issue direct I/Os limited by either the
maximum number of vectors in an bio or the hardware limits.

The patch below (against 2.6.16) allows direct I/O to build bios as
large as the underlying hardware will allow.

Signed-off-by: Dave Chinner <dgc@sgi.com>
---
 bio.c |    4 ++--
 1 files changed, 2 insertions(+), 2 deletions(-)

Index: 2.6.x-xfs-new/fs/bio.c
===================================================================
--- 2.6.x-xfs-new.orig/fs/bio.c	2006-02-06 11:57:50.000000000 +1100
+++ 2.6.x-xfs-new/fs/bio.c	2006-04-24 15:46:16.849484424 +1000
@@ -304,7 +304,7 @@ int bio_get_nr_vecs(struct block_device 
 	request_queue_t *q = bdev_get_queue(bdev);
 	int nr_pages;
 
-	nr_pages = ((q->max_sectors << 9) + PAGE_SIZE - 1) >> PAGE_SHIFT;
+	nr_pages = ((q->max_hw_sectors << 9) + PAGE_SIZE - 1) >> PAGE_SHIFT;
 	if (nr_pages > q->max_phys_segments)
 		nr_pages = q->max_phys_segments;
 	if (nr_pages > q->max_hw_segments)
@@ -446,7 +446,7 @@ int bio_add_page(struct bio *bio, struct
 		 unsigned int offset)
 {
 	struct request_queue *q = bdev_get_queue(bio->bi_bdev);
-	return __bio_add_page(q, bio, page, len, offset, q->max_sectors);
+	return __bio_add_page(q, bio, page, len, offset, q->max_hw_sectors);
 }
 
 struct bio_map_data {