Re: [PATCH 1/3] block: try one write zeroes request before going further

[Hannes Reinecke <hare@suse.de>]

On 12/6/20 6:53 AM, Tom Yan wrote:
> At least the SCSI disk driver is "benevolent" when it try to decide
> whether the device actually supports write zeroes, i.e. unless the
> device explicity report otherwise, it assumes it does at first.
> 
> Therefore before we pile up bios that would fail at the end, we try
> the command/request once, as not doing so could trigger quite a
> disaster in at least certain case. For example, the host controller
> can be messed up entirely when one does `blkdiscard -z` a UAS drive.
> 
> Signed-off-by: Tom Yan <tom.ty89@gmail.com>
> ---
>   block/blk-lib.c | 14 +++++++++++++-
>   1 file changed, 13 insertions(+), 1 deletion(-)
> 
> diff --git a/block/blk-lib.c b/block/blk-lib.c
> index e90614fd8d6a..c1e9388a8fb8 100644
> --- a/block/blk-lib.c
> +++ b/block/blk-lib.c
> @@ -250,6 +250,7 @@ static int __blkdev_issue_write_zeroes(struct block_device *bdev,
>   	struct bio *bio = *biop;
>   	unsigned int max_write_zeroes_sectors;
>   	struct request_queue *q = bdev_get_queue(bdev);
> +	int i = 0;
>   
>   	if (!q)
>   		return -ENXIO;
> @@ -264,7 +265,17 @@ static int __blkdev_issue_write_zeroes(struct block_device *bdev,
>   		return -EOPNOTSUPP;
>   
>   	while (nr_sects) {
> -		bio = blk_next_bio(bio, 0, gfp_mask);
> +		if (i != 1) {
> +			bio = blk_next_bio(bio, 0, gfp_mask);
> +		} else {
> +			submit_bio_wait(bio);
> +			bio_put(bio);
> +
> +			if (bdev_write_zeroes_sectors(bdev) == 0)
> +				return -EOPNOTSUPP;
> +			else
> +				bio = bio_alloc(gfp_mask, 0);
> +		}
>   		bio->bi_iter.bi_sector = sector;
>   		bio_set_dev(bio, bdev);
>   		bio->bi_opf = REQ_OP_WRITE_ZEROES;
> @@ -280,6 +291,7 @@ static int __blkdev_issue_write_zeroes(struct block_device *bdev,
>   			nr_sects = 0;
>   		}
>   		cond_resched();
> +		i++;
>   	}
>   
>   	*biop = bio;
> 
We do want to keep the chain of bios intact such that end_io processing 
will recurse back to the original end_io callback.
As such we need to call bio_chain on the first bio, submit that 
(possibly with submit_bio_wait()), and then decide whether we can / 
should continue.
With your patch we'll lose the information that indeed other bios might 
be linked to the original one.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke                Kernel Storage Architect
hare@suse.de                              +49 911 74053 688
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