Subject: Re: [V9fs-developer] [PATCH] net/9p: avoid request size exceed to the virtqueue number in the zero copy

From: piaojun <[email-masked]>
Date: Mon, 6 Aug 2018 16:32:59 +0800

Hi Yiwen,

On 2018/8/3 14:50, jiangyiwen wrote:
> Unfortunately, when the address(input and response headers) are not
> at page boundary, it will need two extra entry in the zero copy, or
> else it will cause sg array out of bounds.
> 
> To avoid the problem, we should subtract two pages for maxsize.
> 
> Signed-off-by: Yiwen Jiang <[email-masked]>
> ---
>  net/9p/trans_virtio.c | 9 +++++----
>  1 file changed, 5 insertions(+), 4 deletions(-)
> 
> diff --git a/net/9p/trans_virtio.c b/net/9p/trans_virtio.c
> index 6265d1d..63591b2 100644
> --- a/net/9p/trans_virtio.c
> +++ b/net/9p/trans_virtio.c
> @@ -754,11 +754,12 @@ static void p9_virtio_remove(struct virtio_device *vdev)
>  	.cancel = p9_virtio_cancel,
>  	/*
>  	 * We leave one entry for input and one entry for response
> -	 * headers. We also skip one more entry to accomodate, address
> -	 * that are not at page boundary, that can result in an extra
> -	 * page in zero copy.
> +	 * headers. We also skip three more entrys to accomodate
> +	 * (input + response headers + data pages), address
> +	 * that are not at page boundary, that can result in
> +	 * an extra page in zero copy.

Here should be two extra pages.

Thanks,
Jun

>  	 */
> -	.maxsize = PAGE_SIZE * (VIRTQUEUE_NUM - 3),
> +	.maxsize = PAGE_SIZE * (VIRTQUEUE_NUM - 5),
>  	.def = 1,
>  	.owner = THIS_MODULE,
>  };
>