[Drbd-dev] [PATCH 23/23] block: remove the discard_zeroes_data flag

[Bart Van Assche]

On Thu, 2017-03-23 at 10:33 -0400, Christoph Hellwig wrote:
> Now that we use the proper REQ_OP_WRITE_ZEROES operation everywhere we can
> kill this hack.
> 
> [ ... ]
>
> diff --git a/Documentation/ABI/testing/sysfs-block b/Documentation/ABI/testing/sysfs-block
> index 2da04ce6aeef..dea212db9df3 100644
> --- a/Documentation/ABI/testing/sysfs-block
> +++ b/Documentation/ABI/testing/sysfs-block
> @@ -213,14 +213,8 @@ What:		/sys/block/<disk>/queue/discard_zeroes_data
>  Date:		May 2011
>  Contact:	Martin K. Petersen <martin.petersen at oracle.com>
>  Description:
> -		Devices that support discard functionality may return
> -		stale or random data when a previously discarded block
> -		is read back. This can cause problems if the filesystem
> -		expects discarded blocks to be explicitly cleared. If a
> -		device reports that it deterministically returns zeroes
> -		when a discarded area is read the discard_zeroes_data
> -		parameter will be set to one. Otherwise it will be 0 and
> -		the result of reading a discarded area is undefined.
> +		Will always return 0.  Don't rely on any specific behavior
> +		for discards, and don't read this file.
>  
>  What:		/sys/block/<disk>/queue/write_same_max_bytes
>  Date:		January 2012
>
> [ ... ]
>
> --- a/block/blk-sysfs.c
> +++ b/block/blk-sysfs.c
> @@ -208,7 +208,7 @@ static ssize_t queue_discard_max_store(struct request_queue *q,
>  
>  static ssize_t queue_discard_zeroes_data_show(struct request_queue *q, char *page)
>  {
> -	return queue_var_show(queue_discard_zeroes_data(q), page);
> +	return 0;
>  }

Hello Christoph,

It seems to me like the documentation in Documentation/ABI/testing/sysfs-block
and the above code are not in sync. I think the above code will cause reading
from the discard_zeroes_data attribute to return an empty string ("") instead
of "0\n".

BTW, my personal preference is to remove this attribute entirely because keeping
it will cause confusion, no matter how well we document the behavior of this
attribute.

Thanks,

Bart.