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Subject: Re: [PATCH] net: don't use kvzalloc for DMA memory
To:     Mikulas Patocka <mpatocka@redhat.com>,
        "David S. Miller" <davem@davemloft.net>,
        Eric Dumazet <edumazet@google.com>
Cc:     Joby Poriyath <joby.poriyath@citrix.com>,
        Ben Hutchings <bhutchings@solarflare.com>,
        netdev@vger.kernel.org, linux-kernel@vger.kernel.org
References: <alpine.LRH.2.02.1804181029270.19294@file01.intranet.prod.int.rdu2.redhat.com>
From:   Eric Dumazet <eric.dumazet@gmail.com>
Message-ID: <3e65977e-53cd-bf09-bc4b-0ce40e9091fe@gmail.com>
Date:   Wed, 18 Apr 2018 09:05:54 -0700
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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.