linux-kernel - Re: [PATCH net 2/2] macvtap: limit head length of skb allocated

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Date:	Wed, 13 Nov 2013 13:58:01 +0800
From:	Jason Wang <jasowang@...hat.com>
To:	Greg Rose <gregory.v.rose@...el.com>
CC:	davem@...emloft.net, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, mst@...hat.com, stefanha@...hat.com
Subject: Re: [PATCH net 2/2] macvtap: limit head length of skb allocated

On 11/13/2013 01:45 AM, Greg Rose wrote:
> On Tue, 12 Nov 2013 18:02:57 +0800
> Jason Wang <jasowang@...hat.com> wrote:
>
>> We currently use hdr_len as a hint of head length which is advertised
>> by guest. But when guest advertise a very big value, it can lead to
>> an 64K+ allocating of kmalloc() which has a very high possibility of
>> failure when host memory is fragmented or under heavy stress. The
>> huge hdr_len also reduce the effect of zerocopy or even disable if a
>> gso skb is linearized in guest.
>>
>> To solves those issues, this patch introduces an upper limit
>> (PAGE_SIZE) of the head, which guarantees an order 0 allocation each
>> time.
>>
>> Cc: Stefan Hajnoczi <stefanha@...hat.com>
>> Cc: Michael S. Tsirkin <mst@...hat.com>
>> Signed-off-by: Jason Wang <jasowang@...hat.com>
>> ---
>> The patch was needed for stable.
>> ---
>>  drivers/net/macvtap.c | 5 +++++
>>  1 file changed, 5 insertions(+)
>>
>> diff --git a/drivers/net/macvtap.c b/drivers/net/macvtap.c
>> index 9dccb1e..7ee6f9d 100644
>> --- a/drivers/net/macvtap.c
>> +++ b/drivers/net/macvtap.c
>> @@ -523,6 +523,11 @@ static inline struct sk_buff
>> *macvtap_alloc_skb(struct sock *sk, size_t prepad, int noblock, int
>> *err) {
>>  	struct sk_buff *skb;
>> +	int good_linear = SKB_MAX_HEAD(prepad);
>> +
>> +	/* Don't use huge linear part */
>> +	if (linear > good_linear)
>> +		linear = good_linear;
>>  
>>  	/* Under a page?  Don't bother with paged skb. */
>>  	if (prepad + len < PAGE_SIZE || !linear)
> I see no problem with this or the tuntap patch except that in both
> cases kernel coding style would prefer that you align the local
> variable declarations in a reverse pyramid, longest at the beginning,
> shortest at the end.
>
> - Greg

Sure, will do it in V2.
Thanks
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