Re: [PATCH 1/3] sd: Make discard granularity match logical block size when LBPRZ=1

[Johannes Thumshirn <jthumshirn@suse.de>]

On Fri, 2015-11-13 at 16:46 -0500, Martin K. Petersen wrote:
> A device may report an OPTIMAL UNMAP GRANULARITY and UNMAP
> GRANULARITY
> ALIGNMENT in the Block Limits VPD. These parameters describe the
> device's internal provisioning allocation units. By default the block
> layer will round and align any discard requests based on these
> limits.
> 
> If a device reports LBPRZ=1 to guarantee zeroes after discard,
> however,
> it is imperative that the block layer does not leave out any parts of
> the requested block range. Otherwise the device can not do the
> required
> zeroing of any partial allocation units and this can lead to data
> corruption.
> 
> Since the dm thinp personality relies on the block layer's current
> behavior and is unable to deal with partial discard blocks we work
> around the problem by setting the granularity to match the logical
> block
> size when LBPRZ is enabled.
> 
> Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>
> ---
>  drivers/scsi/sd.c | 23 ++++++++++++++++++-----
>  1 file changed, 18 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/scsi/sd.c b/drivers/scsi/sd.c
> index 3f370228bf31..c322969026bc 100644
> --- a/drivers/scsi/sd.c
> +++ b/drivers/scsi/sd.c
> @@ -636,11 +636,24 @@ static void sd_config_discard(struct scsi_disk
> *sdkp, unsigned int mode)
>  	unsigned int max_blocks = 0;
>  
>  	q->limits.discard_zeroes_data = 0;
> -	q->limits.discard_alignment = sdkp->unmap_alignment *
> -		logical_block_size;
> -	q->limits.discard_granularity =
> -		max(sdkp->physical_block_size,
> -		    sdkp->unmap_granularity * logical_block_size);
> +
> +	/*
> +	 * When LBPRZ is reported, discard alignment and granularity
> +	 * must be fixed to the logical block size. Otherwise the
> block
> +	 * layer will drop misaligned portions of the request which
> can
> +	 * lead to data corruption. If LBPRZ is not set, we honor
> the
> +	 * device preference.
> +	 */
> +	if (sdkp->lbprz) {
> +		q->limits.discard_alignment = 0;
> +		q->limits.discard_granularity = 1;
> +	} else {
> +		q->limits.discard_alignment = sdkp->unmap_alignment
> *
> +			logical_block_size;
> +		q->limits.discard_granularity =
> +			max(sdkp->physical_block_size,
> +			    sdkp->unmap_granularity *
> logical_block_size);
> +	}
>  
>  	sdkp->provisioning_mode = mode;
>  

Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
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