netdev - Re: [PATCH] vhost-net: fall back to vmalloc if high-order allocation fails

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Date:	Fri, 25 Jan 2013 13:15:51 +0800
From:	Jason Wang <jasowang@...hat.com>
To:	Cong Wang <xiyou.wangcong@...il.com>
Cc:	kvm@...r.kernel.org, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] vhost-net: fall back to vmalloc if high-order allocation fails

On Friday, January 25, 2013 03:03:13 AM Cong Wang wrote:
> ["Followup-To:" header set to gmane.linux.network.]
> 
> On Wed, 23 Jan 2013 at 20:46 GMT, Romain Francoise <romain@...bokech.com> 
wrote:
> > Creating a vhost-net device allocates an object large enough (34320 bytes
> > on x86-64) to trigger an order-4 allocation, which may fail if memory if
> > 
> > fragmented:
> >  libvirtd: page allocation failure: order:4, mode:0x2000d0
> >  ...
> >  SLAB: Unable to allocate memory on node 0 (gfp=0xd0)
> >  
> >    cache: size-65536, object size: 65536, order: 4
> >    node 0: slabs: 8/8, objs: 8/8, free: 0
> > 
> > In that situation, rather than forcing the caller to use regular
> > virtio-net, try to allocate the descriptor with vmalloc().
> 
> The real problem is vhost_net struct is really big, it
> should be reduced rather than workarounded like this.
> 

Looks like iov in vhost_virtqueue is a little big:

    struct iovec iov[UIO_MAXIOV];

Maybe we can use pointer and allocate it like indirect in 
vhost_dev_alloc_iovecs().

> > +static void vhost_net_kvfree(void *addr)
> > +{
> > +	if (is_vmalloc_addr(addr))
> > +		vfree(addr);
> > +	else
> > +		kfree(addr);
> > +}
> > +
> 
> This kind of stuff should really go to mm, not netdev.
> 
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