[dm-devel] [PATCH 3/3] dm-thin: fix discard_granularity
Mike Snitzer
snitzer at redhat.com
Tue Jul 17 15:31:41 UTC 2012
On Mon, Jul 16 2012 at 2:35pm -0400,
Mikulas Patocka <mpatocka at 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 at 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
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