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Date:   Wed, 18 Apr 2018 12:44:25 -0400 (EDT)
From:   Mikulas Patocka <mpatocka@redhat.com>
To:     Eric Dumazet <eric.dumazet@gmail.com>
cc:     "David S. Miller" <davem@davemloft.net>,
        Eric Dumazet <edumazet@google.com>,
        Joby Poriyath <joby.poriyath@citrix.com>,
        Ben Hutchings <bhutchings@solarflare.com>,
        netdev@vger.kernel.org, linux-kernel@vger.kernel.org,
        "Michael S. Tsirkin" <mst@redhat.com>,
        Jason Wang <jasowang@redhat.com>,
        virtualization@lists.linux-foundation.org
Subject: Re: [PATCH] net: don't use kvzalloc for DMA memory
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On Wed, 18 Apr 2018, Eric Dumazet wrote:

> 
> 
> On 04/18/2018 07:34 AM, Mikulas Patocka wrote:
> > The patch 74d332c13b21 changes alloc_netdev_mqs to use vzalloc if kzalloc
> > fails (later patches change it to kvzalloc).
> > 
> > The problem with this is that if the vzalloc function is actually used, 
> > virtio_net doesn't work (because it expects that the extra memory should 
> > be accessible with DMA-API and memory allocated with vzalloc isn't).
> > 
> > This patch changes it back to kzalloc and adds a warning if the allocated
> > size is too large (the allocation is unreliable in this case).
> > 
> > Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
> > Fixes: 74d332c13b21 ("net: extend net_device allocation to vmalloc()")
> > 
> > ---
> >  net/core/dev.c |    3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> > 
> > Index: linux-2.6/net/core/dev.c
> > ===================================================================
> > --- linux-2.6.orig/net/core/dev.c	2018-04-16 21:08:36.000000000 +0200
> > +++ linux-2.6/net/core/dev.c	2018-04-18 16:24:43.000000000 +0200
> > @@ -8366,7 +8366,8 @@ struct net_device *alloc_netdev_mqs(int
> >  	/* ensure 32-byte alignment of whole construct */
> >  	alloc_size += NETDEV_ALIGN - 1;
> >  
> > -	p = kvzalloc(alloc_size, GFP_KERNEL | __GFP_RETRY_MAYFAIL);
> > +	WARN_ON(alloc_size > PAGE_SIZE << PAGE_ALLOC_COSTLY_ORDER);
> > +	p = kzalloc(alloc_size, GFP_KERNEL | __GFP_RETRY_MAYFAIL);
> >  	if (!p)
> >  		return NULL;
> >  
> > 
> 
> Since when a net_device needs to be in DMA zone ???
> 
> I would rather fix virtio_net, this looks very suspect to me.
> 
> Each virtio_net should probably allocate the exact amount of DMA-memory it wants,
> instead of expecting core networking stack to have a huge chunk of DMA-memory for everything.

The structure net_device is followed by arbitrary driver-specific data 
(accessible with the function netdev_priv). And for virtio-net, these 
driver-specific data must be in DMA memory.

Mikulas