linux-kernel - RE: [PATCH 1/3] virtio_console:Merge struct buffer_token into struct port

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Date:	Wed, 26 Sep 2012 09:48:12 +0200
From:	Sjur BRENDELAND <sjur.brandeland@...ricsson.com>
To:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
Cc:	Amit Shah <amit.shah@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"virtualization@...ts.linux-foundation.org" 
	<virtualization@...ts.linux-foundation.org>,
	"sjurbren@...il.com" <sjurbren@...il.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	"Michael S. Tsirkin" <mst@...hat.com>,
	Linus Walleij <linus.walleij@...aro.org>,
	"yrl.pp-manager.tt@...achi.com" <yrl.pp-manager.tt@...achi.com>
Subject: RE: [PATCH 1/3] virtio_console:Merge struct buffer_token into
 struct port_buffer

> > This merge reduces code size by unifying the approach for
> > sending scatter-lists and regular buffers. Any type of
> > write operation (splice, write, put_chars) will now allocate
> > a port_buffer and send_buf() and free_buf() can always be used.
> 
> Thanks!
> This looks much nicer and simpler. I just have some comments below.

OK, good to hear that you agree to this kind of change. I'll do a respin
of this patch fixing the issues you have pointed out.

> >  static void free_buf(struct port_buffer *buf)
> >  {
> > +	int i;
> > +
> >  	kfree(buf->buf);
> 
> this should be done only when !buf->sgpages, or (see below)

Agree, I'll put this statement in an else branch to the if-statement below.

> 
> > +
> > +	if (buf->sgpages)
> > +		for (i = 0; i < buf->sgpages; i++) {
> > +			struct page *page = sg_page(&buf->sg[i]);
> > +			if (!page)
> > +				break;
> > +			put_page(page);
> > +		}
> > +
> >  	kfree(buf);
> >  }
> >
> > -static struct port_buffer *alloc_buf(size_t buf_size)
> > +static struct port_buffer *alloc_buf(struct virtqueue *vq, size_t buf_size,
> > +				     int nrbufs)
> >  {
> >  	struct port_buffer *buf;
> > +	size_t alloc_size;
> >
> > -	buf = kmalloc(sizeof(*buf), GFP_KERNEL);
> > +	/* Allocate buffer and the scatter list */
> > +	alloc_size = sizeof(*buf) + sizeof(struct scatterlist) * nrbufs;
> 
> This allocates one redundant sg entry when nrbuf > 0,
> but I think it is OK. (just a comment)

I did this on purpose for the sake of simplicity, but I can
change this to something like:
	 alloc_size = sizeof(*buf) + sizeof(buf->sg) * max(nrbufs - 1, 1);


> > +	buf = kmalloc(alloc_size, GFP_ATOMIC);
> 
> This should be kzalloc(), or buf->buf and others are not initialized,
> which will cause unexpected kfree bug at kfree(buf->buf) in free_buf.

Agree, kzalloc() is better in this case. 

> >  	if (!buf)
> >  		goto fail;
> > -	buf->buf = kzalloc(buf_size, GFP_KERNEL);
> > +
> > +	buf->sgpages = nrbufs;
> > +	if (nrbufs > 0)
> > +		return buf;
> > +
> > +	buf->buf = kmalloc(buf_size, GFP_ATOMIC);
> 
> You can also use kzalloc here as previous code does.
> But if the reason why using kzalloc comes from the security,
> I think kmalloc is enough here, since the host can access
> all the guest pages anyway.

With this new patch alloc_buf() is used both for both RX and TX.
The out_vq did previously use malloc(). But I have preserved
the legacy behavior for the in_vq by calling memset() in function
fill_queue().

Thanks,
Sjur