Re: [PATCH v2 6/7] 9p/trans_virtio: support larger msize values

[Christian Schoenebeck <linux_oss@crudebyte.com>]

On Montag, 20. September 2021 18:44:13 CEST Christian Schoenebeck wrote:
> The virtio transport supports by default a 9p 'msize' of up to
> approximately 500 kB. This patch adds support for larger 'msize'
> values by resizing the amount of scatter/gather lists if required.
> 
> Signed-off-by: Christian Schoenebeck <linux_oss@crudebyte.com>
> ---
>  net/9p/trans_virtio.c | 57 +++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 57 insertions(+)
> 
> diff --git a/net/9p/trans_virtio.c b/net/9p/trans_virtio.c
> index 0db8de84bd51..4cb75f45aa15 100644
> --- a/net/9p/trans_virtio.c
> +++ b/net/9p/trans_virtio.c
> @@ -200,6 +200,31 @@ static struct virtqueue_sg *vq_sg_alloc(unsigned int
> nsgl) return vq_sg;
>  }
> 
> +/**
> + * vq_sg_resize - resize passed virtqueue scatter/gather lists to the
> passed + * amount of lists
> + * @_vq_sg: scatter/gather lists to be resized
> + * @nsgl: new amount of scatter/gather lists
> + */
> +static int vq_sg_resize(struct virtqueue_sg **_vq_sg, unsigned int nsgl)
> +{
> +	struct virtqueue_sg *vq_sg;
> +
> +	BUG_ON(!_vq_sg || !nsgl);
> +	vq_sg = *_vq_sg;
> +	if (vq_sg->nsgl == nsgl)
> +		return 0;
> +
> +	/* lazy resize implementation for now */
> +	vq_sg = vq_sg_alloc(nsgl);
> +	if (!vq_sg)
> +		return -ENOMEM;
> +
> +	kfree(*_vq_sg);
> +	*_vq_sg = vq_sg;
> +	return 0;
> +}
> +
>  /**
>   * p9_virtio_close - reclaim resources of a channel
>   * @client: client instance
> @@ -771,6 +796,10 @@ p9_virtio_create(struct p9_client *client, const char
> *devname, char *args) struct virtio_chan *chan;
>  	int ret = -ENOENT;
>  	int found = 0;
> +#if !defined(CONFIG_ARCH_NO_SG_CHAIN)
> +	size_t npages;
> +	size_t nsgl;
> +#endif
> 
>  	if (devname == NULL)
>  		return -EINVAL;
> @@ -793,6 +822,34 @@ p9_virtio_create(struct p9_client *client, const char
> *devname, char *args) return ret;
>  	}
> 
> +	/*
> +	 * if user supplied an 'msize' option that's larger than what this
> +	 * transport supports by default, then try to allocate more sg lists
> +	 */
> +	if (client->msize > client->trans_maxsize) {
> +#ifdef CONFIG_ARCH_NO_SG_CHAIN
> +		pr_info("limiting 'msize' to %d because architecture does not "
> +			"support chained scatter gather lists\n",
> +			client->trans_maxsize);
> +#else
> +		npages = DIV_ROUND_UP(client->msize, PAGE_SIZE);
> +		if (npages > chan->p9_max_pages)
> +			npages = chan->p9_max_pages;
> +		nsgl = DIV_ROUND_UP(npages, SG_USER_PAGES_PER_LIST);
> +		if (nsgl > chan->vq_sg->nsgl) {
> +			/*
> +			 * if resize fails, no big deal, then just
> +			 * continue with default msize instead
> +			 */
> +			if (!vq_sg_resize(&chan->vq_sg, nsgl)) {
> +				client->trans_maxsize =
> +					PAGE_SIZE *
> +					((nsgl * SG_USER_PAGES_PER_LIST) - 3);
> +			}
> +		}

Maybe an ...

			} else {
				pr_info(...);
			}

would be useful here to explain the user why transport refrained from 
increasing its max size even though user's msize option demanded it.

> +#endif /* CONFIG_ARCH_NO_SG_CHAIN */
> +	}
> +
>  	client->trans = (void *)chan;
>  	client->status = Connected;
>  	chan->client = client;