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

From: Christian Schoenebeck <[email-masked]>
Date: Fri, 17 Sep 2021 14:02:04 +0200

On Donnerstag, 16. September 2021 20:25:16 CEST Christian Schoenebeck wrote:
> The virtio transport supports by default a 9p 'msize' of up to
> approximately 500 kB. This patches adds support for larger 'msize'
> values by resizing the amount of scatter/gather lists if required.
> 
> Signed-off-by: Christian Schoenebeck <[email-masked]>
> ---

s/patches/patch/

>  net/9p/trans_virtio.c | 52 +++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 52 insertions(+)
> 
> diff --git a/net/9p/trans_virtio.c b/net/9p/trans_virtio.c
> index 1a45e0df2336..1f9a0283d7b8 100644
> --- a/net/9p/trans_virtio.c
> +++ b/net/9p/trans_virtio.c
> @@ -195,6 +195,30 @@ static struct virtqueue_sg *vq_sg_alloc(unsigned int
> nsgl) return vq_sg;
>  }
> 
> +/**
> + * Resize passed virtqueue scatter/gather lists to the passed amount of
> + * list blocks.
> + * @_vq_sg: scatter/gather lists to be resized
> + * @nsgl: new amount of scatter/gather list blocks
> + */
> +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;
> +

Missing

    kfree(*_vq_sg);

here, and probably also a prior memcpy() as in patch 7, i.e.:

    /* copy over old scatter gather lists */
    sz = sizeof(struct virtqueue_sg) +
        (*_vq_sg)->nsgl * sizeof(struct scatterlist *);
    memcpy(vq_sg, *_vq_sg, sz);

> +	*_vq_sg = vq_sg;
> +	return 0;
> +}
> +
>  /**
>   * p9_virtio_close - reclaim resources of a channel
>   * @client: client instance
> @@ -766,6 +790,10 @@ 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;
> @@ -788,6 +816,30 @@ 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) {
> +#if !defined(CONFIG_ARCH_NO_SG_CHAIN)
> +		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);
> +			}
> +		}
> +#endif /* !defined(CONFIG_ARCH_NO_SG_CHAIN) */
> +	}

Probably an error/info message for architectures not supporting SG chains 
would be useful here, to let users on those system know why they cannot get 
beyond 500k msize.

> +
>  	client->trans = (void *)chan;
>  	client->status = Connected;
>  	chan->client = client;