Re: [dm-devel] [PATCH 3/3] dm-thin: fix discard_granularity

[Mike Snitzer <snitzer redhat com>]

On Mon, Jul 16 2012 at  2:35pm -0400,
Mikulas Patocka <mpatocka redhat com> wrote:

> dm-thin: fix discard_granularity
> 
> The kernel expects that limits->discard_granularity is a power of two.
> Set this limit only if we use a power of two block size.
> 
> Signed-off-by: Mikulas Patocka <mpatocka redhat com>
> 
> ---
>  drivers/md/dm-thin.c |    3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> Index: linux-3.5-rc6-fast/drivers/md/dm-thin.c
> ===================================================================
> --- linux-3.5-rc6-fast.orig/drivers/md/dm-thin.c	2012-07-16 20:07:49.000000000 +0200
> +++ linux-3.5-rc6-fast/drivers/md/dm-thin.c	2012-07-16 20:08:01.000000000 +0200
> @@ -2502,7 +2502,8 @@ static void set_discard_limits(struct po
>  	 * bios cover a block partially.  A discard that spans a block boundary
>  	 * is not sent to this target.
>  	 */
> -	limits->discard_granularity = pool->sectors_per_block << SECTOR_SHIFT;
> +	if (pool->sectors_per_block_shift >= 0)
> +		limits->discard_granularity = pool->sectors_per_block << SECTOR_SHIFT;
>  	limits->discard_zeroes_data = pool->pf.zero_new_blocks;
>  }

Given the block layer's assumption that discard_granularity is always a
power of 2: thinp should disable discard if the thinp blocksize is a non
power of 2.  So this patch isn't correct (discard support should be
disabled in pool_ctr based on the specified blocksize).

Future work should be:
1) explore/eliminate the block layer's power of 2 constraints
2) divorce thinp's discard limits (for discard passdown) from the pool's
   blocksize