[dm-devel] [PATCH] dm: Deal with merge_bvec_fn in component devices better

[Mikulas Patocka]

Hi

This was already fixed in 2.6.31 with commit 
8cbeb67ad50f7d68e5e83be2cb2284de8f9c03b5.

See this piece of code in dm.c:

/*
 * If the target doesn't support merge method and some of the devices
 * provided their merge_bvec method (we know this by looking at
 * queue_max_hw_sectors), then we can't allow bios with multiple vector
 * entries.  So always set max_size to 0, and the code below allows
 * just one page.
 */
else if (queue_max_hw_sectors(q) <= PAGE_SIZE >> 9)
        max_size = 0;

Mikulas


On Sun, 1 Sep 2013, Ben Hutchings wrote:

> This is analogous to commit 627a2d3c29427637f4c5d31ccc7fcbd8d312cd71
> ('md: deal with merge_bvec_fn in component devices better.'),
> which does the same for md-devices at the top of the stack.  The
> following explanation is taken from that commit.  Thanks to Neil Brown
> <neilb at suse.de> for the advice.
> 
> If a component device has a merge_bvec_fn then as we never call it
> we must ensure we never need to.  Currently this is done by setting
> max_sector to 1 PAGE, however this does not stop a bio being created
> with several sub-page iovecs that would violate the merge_bvec_fn.
> 
> So instead set max_segments to 1 and set the segment boundary to the
> same as a page boundary to ensure there is only ever one single-page
> segment of IO requested at a time.
> 
> This can particularly be an issue when 'xen' is used as it is
> known to submit multiple small buffers in a single bio.
> 
> References: http://bugs.debian.org/604457
> Signed-off-by: Ben Hutchings <ben at decadent.org.uk>
> ---
> We've been carrying this in Debian for about 3 years and it's about time
> it got reviewed...  It's basically Neil's work so should probably have
> his sign-off as well.
> 
> Ben.
> 
> --- a/drivers/md/dm-table.c
> +++ b/drivers/md/dm-table.c
> @@ -536,13 +536,15 @@ int dm_set_device_limits(struct dm_target *ti, struct dm_dev *dev,
>  		       (unsigned long long) start << SECTOR_SHIFT);
>  
>  	/*
> -	 * Check if merge fn is supported.
> -	 * If not we'll force DM to use PAGE_SIZE or
> -	 * smaller I/O, just to be safe.
> +	 * If we don't call merge_bvec_fn, we must never risk
> +	 * violating it, so limit max_phys_segments to 1 lying within
> +	 * a single page.
>  	 */
> -	if (dm_queue_merge_is_compulsory(q) && !ti->type->merge)
> -		blk_limits_max_hw_sectors(limits,
> -					  (unsigned int) (PAGE_SIZE >> 9));
> +	if (dm_queue_merge_is_compulsory(q) && !ti->type->merge) {
> +		limits->max_segments = 1;
> +		limits->seg_boundary_mask = PAGE_CACHE_SIZE - 1;
> +	}
> +
>  	return 0;
>  }
>  EXPORT_SYMBOL_GPL(dm_set_device_limits);
> 
> -- 
> Ben Hutchings
> The most exhausting thing in life is being insincere. - Anne Morrow Lindberg
>