Re: [PATCH] Direct I/O bio size regression

[Jens Axboe <axboe@suse.de>]

On Mon, Apr 24 2006, David Chinner wrote:
> 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 {

Clearly correct, I'll make sure this gets merged right away.

-- 
Jens Axboe

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