Re: [dm-devel] [PATCH 1/2 v2] dm: discard support for the linear target

[Christoph Hellwig <hch lst de>]

On Sat, Jun 26, 2010 at 04:31:24PM -0400, Mike Snitzer wrote:
> Allow discards to be passed through to a single device linear mapping.
> Introduce DM_TARGET_SUPPORTS_DISCARDS target features flag that each
> target must set once discard support is added.
> 
> Verify table's underlying devices support discards prior to setting the
> associated DM device as capable of discards (via QUEUE_FLAG_DISCARD).
> 
> Signed-off-by: Mike Snitzer <snitzer redhat com>
> ---
>  drivers/md/dm-linear.c        |    1 +
>  drivers/md/dm-table.c         |   51 ++++++++++++++++++++++++++++++++++
>  drivers/md/dm.c               |   60 ++++++++++++++++++++++++++++++++--------
>  drivers/md/dm.h               |    1 +
>  include/linux/device-mapper.h |    1 +
>  5 files changed, 102 insertions(+), 12 deletions(-)

> +static int device_discard_incapable(struct dm_target *ti, struct dm_dev *dev,
> +				    sector_t start, sector_t len, void *data)
> +{
> +	struct block_device *bdev = dev->bdev;
> +	struct request_queue *q = bdev_get_queue(bdev);
> +
> +	WARN_ON(!q);
> +	return (!q || !blk_queue_discard(q));
> +}

How could a NULL queue happen here?

Otherwise it looks okay to me, but my DM knowledge is rather limited.