Re: [Qemu-devel] [RFC PATCH 2/6] virtio/console: Add a failback for unstealable pipe buffer

[Amit Shah <amit.shah@redhat.com>]

On (Tue) 24 Jul 2012 [11:37:18], Yoshihiro YUNOMAE wrote:
> From: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
> 
> Add a failback memcpy path for unstealable pipe buffer.
> If buf->ops->steal() fails, virtio-serial tries to
> copy the page contents to an allocated page, instead
> of just failing splice().
> 
> Signed-off-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
> Cc: Amit Shah <amit.shah@redhat.com>
> Cc: Arnd Bergmann <arnd@arndb.de>
> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> ---
> 
>  drivers/char/virtio_console.c |   28 +++++++++++++++++++++++++---
>  1 files changed, 25 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/char/virtio_console.c b/drivers/char/virtio_console.c
> index fe31b2f..911cb3e 100644
> --- a/drivers/char/virtio_console.c
> +++ b/drivers/char/virtio_console.c
> @@ -794,7 +794,7 @@ static int pipe_to_sg(struct pipe_inode_info *pipe, struct pipe_buffer *buf,
>  			struct splice_desc *sd)
>  {
>  	struct sg_list *sgl = sd->u.data;
> -	unsigned int len = 0;
> +	unsigned int offset, len;
>  
>  	if (sgl->n == MAX_SPLICE_PAGES)
>  		return 0;
> @@ -807,9 +807,31 @@ static int pipe_to_sg(struct pipe_inode_info *pipe, struct pipe_buffer *buf,
>  
>  		len = min(buf->len, sd->len);
>  		sg_set_page(&(sgl->sg[sgl->n]), buf->page, len, buf->offset);
> -		sgl->n++;
> -		sgl->len += len;
> +	} else {
> +		/* Failback to copying a page */
> +		struct page *page = alloc_page(GFP_KERNEL);

I prefer zeroing out the page.  If there's not enough data to be
filled in the page, the remaining data can be leaked to the host.

		Amit