linux-kernel - Re: [PATCH v2 1/8] vsock/virtio: limit the memory used per-socket

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Date:   Mon, 20 May 2019 09:57:31 +0100
From:   Stefan Hajnoczi <stefanha@...hat.com>
To:     Stefano Garzarella <sgarzare@...hat.com>
Cc:     netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Jason Wang <jasowang@...hat.com>
Subject: Re: [PATCH v2 1/8] vsock/virtio: limit the memory used per-socket

On Fri, May 17, 2019 at 10:25:05AM +0200, Stefano Garzarella wrote:
> On Thu, May 16, 2019 at 04:25:33PM +0100, Stefan Hajnoczi wrote:
> > On Fri, May 10, 2019 at 02:58:36PM +0200, Stefano Garzarella wrote:
> > > +static struct virtio_vsock_buf *
> > > +virtio_transport_alloc_buf(struct virtio_vsock_pkt *pkt, bool zero_copy)
> > > +{
> > > +	struct virtio_vsock_buf *buf;
> > > +
> > > +	if (pkt->len == 0)
> > > +		return NULL;
> > > +
> > > +	buf = kzalloc(sizeof(*buf), GFP_KERNEL);
> > > +	if (!buf)
> > > +		return NULL;
> > > +
> > > +	/* If the buffer in the virtio_vsock_pkt is full, we can move it to
> > > +	 * the new virtio_vsock_buf avoiding the copy, because we are sure that
> > > +	 * we are not use more memory than that counted by the credit mechanism.
> > > +	 */
> > > +	if (zero_copy && pkt->len == pkt->buf_len) {
> > > +		buf->addr = pkt->buf;
> > > +		pkt->buf = NULL;
> > > +	} else {
> > > +		buf->addr = kmalloc(pkt->len, GFP_KERNEL);
> > 
> > buf and buf->addr could be allocated in a single call, though I'm not
> > sure how big an optimization this is.
> > 
> 
> IIUC, in the case of zero-copy I should allocate only the buf,
> otherwise I should allocate both buf and buf->addr in a single call
> when I'm doing a full-copy.
> 
> Is it correct?

Yes, but it's your choice whether optimization is worthwhile.  If it
increases the complexity of the code and doesn't result in a measurable
improvement, then it's not worth it.

Stefan

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