linux-kernel - Re: [PATCH] virtio-blk: Assign discard

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Date:   Mon, 28 Feb 2022 10:51:56 +0000
From:   Stefan Hajnoczi <stefanha@...hat.com>
To:     Akihiko Odaki <akihiko.odaki@...il.com>
Cc:     linux-kernel@...r.kernel.org, linux-block@...r.kernel.org,
        virtualization@...ts.linux-foundation.org,
        Jens Axboe <axboe@...nel.dk>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Christoph Hellwig <hch@....de>
Subject: Re: [PATCH] virtio-blk: Assign discard_granularity

On Thu, Feb 24, 2022 at 06:38:02PM +0900, Akihiko Odaki wrote:
> Virtual I/O Device (VIRTIO) Version 1.1
> https://docs.oasis-open.org/virtio/virtio/v1.1/csprd01/virtio-v1.1-csprd01.html
> > discard_sector_alignment can be used by OS when splitting a request
> > based on alignment.
> 
> According to Documentation/ABI/stable/sysfs-block, the corresponding
> field in the kernel is, confusingly, discard_granularity, not
> discard_alignment.

Good catch, struct virtio_blk_config->discard_sector_alignment is Linux
q->limits.discard_granularity.

> 
> Signed-off-by: Akihiko Odaki <akihiko.odaki@...il.com>
> ---
>  drivers/block/virtio_blk.c | 4 +---
>  1 file changed, 1 insertion(+), 3 deletions(-)
> 
> diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
> index c443cd64fc9b..1fb3c89900e3 100644
> --- a/drivers/block/virtio_blk.c
> +++ b/drivers/block/virtio_blk.c
> @@ -913,11 +913,9 @@ static int virtblk_probe(struct virtio_device *vdev)
>  		blk_queue_io_opt(q, blk_size * opt_io_size);
>  
>  	if (virtio_has_feature(vdev, VIRTIO_BLK_F_DISCARD)) {
> -		q->limits.discard_granularity = blk_size;
> -
>  		virtio_cread(vdev, struct virtio_blk_config,
>  			     discard_sector_alignment, &v);
> -		q->limits.discard_alignment = v ? v << SECTOR_SHIFT : 0;

Should we use struct virtio_blk_config->topology.alignment_offset
("offset of first aligned logical block" and used for Linux
blk_queue_alignment_offset()) for q->limits.discard_alignment?

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