Re: do not disable ext4 discards on first discard failure? [was: Re: dm snapshot: ignore discards issued to the snapshot-origin target]

[Mike Snitzer <snitzer@redhat.com>]

On Wed, May 04 2011 at 11:10am -0400,
Martin K. Petersen <martin.petersen@oracle.com> wrote:

> >>>>> "Lukas" == Lukas Czerner <lczerner@redhat.com> writes:
> 
> Lukas> Nevertheless there is something weird going on, because even when
> Lukas> I create striped volume I get this:
> 
> Could you please try the following patch? It has a bunch of small tweaks
> to the discard stack in it. I'll split it up before posting for real but
> I'd like to know if it fixes your issue...
> 
> 
> block/libata/scsi: Various logical block provisioning fixes
> 
>  - Add sysfs documentation for the discard topology parameters
> 
>  - Fix discard stacking problem
> 
>  - Switch our libata SAT over to using the WRITE SAME limits
> 
>  - UNMAP alignment needs to be converted to bytes
> 
>  - Only report alignment and zeroes_data if the device supports discard
> 
> Reported-by: Lukas Czerner <lczerner@redhat.com>
> Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>
> 
> diff --git a/block/blk-settings.c b/block/blk-settings.c
> index 1fa7692..42d3bf5 100644
> --- a/block/blk-settings.c
> +++ b/block/blk-settings.c
> @@ -120,7 +120,7 @@ void blk_set_default_limits(struct queue_limits *lim)
>  	lim->discard_granularity = 0;
>  	lim->discard_alignment = 0;
>  	lim->discard_misaligned = 0;
> -	lim->discard_zeroes_data = -1;
> +	lim->discard_zeroes_data = 1;
>  	lim->logical_block_size = lim->physical_block_size = lim->io_min = 512;
>  	lim->bounce_pfn = (unsigned long)(BLK_BOUNCE_ANY >> PAGE_SHIFT);
>  	lim->alignment_offset = 0;

lim->discard_zeroes_data = -1; was suspect to me too.
But why default to 1 here?

> @@ -166,6 +166,7 @@ void blk_queue_make_request(struct request_queue *q, make_request_fn *mfn)
>  
>  	blk_set_default_limits(&q->limits);
>  	blk_queue_max_hw_sectors(q, BLK_SAFE_MAX_SECTORS);
> +	q->limits.discard_zeroes_data = 0;
>  
>  	/*
>  	 * by default assume old behaviour and bounce for any highmem page

Only to then reset to 0 here?  Shouldn't we default to 0 and only set to
1 where applicable (e.g. sd_config_discard)?