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Subject: Re: [PATCH] net: don't use kvzalloc for DMA memory
To:     Mikulas Patocka <mpatocka@redhat.com>,
        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
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 <alpine.LRH.2.02.1804181218270.19136@file01.intranet.prod.int.rdu2.redhat.com>
From:   Eric Dumazet <eric.dumazet@gmail.com>
Message-ID: <5f4e1286-b79f-0b9f-9a30-47d7654f3889@gmail.com>
Date:   Wed, 18 Apr 2018 09:51:25 -0700
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On 04/18/2018 09:44 AM, Mikulas Patocka wrote:
> 
> 
> 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.

I get that, but how is the original xenvif problem will be solved ?

Your patch would add a bug in some other driver(s)

I suggest that virtio_net clearly identifies which part needs a specific allocation
and does its itself, instead of abusing the netdev_priv storage.

Ie use a pointer to a block of memory, allocated by virtio_net, for virtio_net.