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

[Christoph Hellwig <hch@infradead.org>]

On Sun, Dec 06, 2020 at 01:53:30PM +0800, 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

This means you now massively slow down say nvme operations by adding
a wait.  If at all we need a maybe supports write zeroes flag and
only do that if the driver hasn't decided yet if write zeroes is
actually supported.