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

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Date:	Thu, 9 Aug 2012 14:33:12 +0530
From:	Amit Shah <amit.shah@...hat.com>
To:	Yoshihiro YUNOMAE <yoshihiro.yunomae.ez@...achi.com>
Cc:	linux-kernel@...r.kernel.org,
	Anthony Liguori <anthony@...emonkey.ws>,
	Arnd Bergmann <arnd@...db.de>, Borislav Petkov <bp@...64.org>,
	"Franch Ch. Eigler" <fche@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Herbert Xu <herbert@...dor.hengli.com.au>,
	Ingo Molnar <mingo@...hat.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	virtualization@...ts.linux-foundation.org, qemu-devel@...gnu.org,
	yrl.pp-manager.tt@...achi.com,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
Subject: Re: [RFC PATCH 2/6] virtio/console: Add a failback for unstealable
 pipe buffer

On (Tue) 24 Jul 2012 [11:37:18], Yoshihiro YUNOMAE wrote:
> From: Masami Hiramatsu <masami.hiramatsu.pt@...achi.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@...achi.com>
> Cc: Amit Shah <amit.shah@...hat.com>
> Cc: Arnd Bergmann <arnd@...db.de>
> Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.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
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