netdev - Re: [PATCH 1/2] af_packet: use vmalloc_to_page() instead for the addresss returned by vmalloc()

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Date:	Tue, 30 Nov 2010 22:27:48 +0800
From:	Changli Gao <xiaosuo@...il.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	"David S. Miller" <davem@...emloft.net>,
	Jiri Pirko <jpirko@...hat.com>,
	Neil Horman <nhorman@...driver.com>, netdev@...r.kernel.org
Subject: Re: [PATCH 1/2] af_packet: use vmalloc_to_page() instead for the
 addresss returned by vmalloc()

On Tue, Nov 30, 2010 at 10:12 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> Le mardi 30 novembre 2010 à 21:56 +0800, Changli Gao a écrit :
>> The following commit causes the pgv->buffer may point to the memory
>> returned by vmalloc(). And we can't use virt_to_page() for the vmalloc
>> address.
>>
>> This patch introduces a new inline function pgv_to_page(), which calls
>> vmalloc_to_page() for the vmalloc address, and virt_to_page() for the
>> __get_free_pages address.
>>
>>     commit 0e3125c755445664f00ad036e4fc2cd32fd52877
>>     Author: Neil Horman <nhorman@...driver.com>
>>     Date:   Tue Nov 16 10:26:47 2010 -0800
>>
>>     packet: Enhance AF_PACKET implementation to not require high order contiguous memory allocation (v4)
>>
>
> nice catch.
>
>> Signed-off-by: Changli Gao <xiaosuo@...il.com>
>> ---
>>  net/packet/af_packet.c |   21 ++++++++++++++-------
>>  1 file changed, 14 insertions(+), 7 deletions(-)
>> diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
>> index 422705d..0171b20 100644
>> --- a/net/packet/af_packet.c
>> +++ b/net/packet/af_packet.c
>> @@ -224,6 +224,13 @@ struct packet_skb_cb {
>>
>>  #define PACKET_SKB_CB(__skb) ((struct packet_skb_cb *)((__skb)->cb))
>>
>> +static inline struct page *pgv_to_page(void *addr)
>> +{
>> +     if (is_vmalloc_addr(addr))
>> +             return vmalloc_to_page(addr);
>
> Hmm, I am wondering if calling vmalloc_to_page(addr) several times for
> each packet is not too expensive ? I believe it is.
>
> What about caching "struct page *" pointer somewhere ?
>
> Then later we have :
>
>> -             p_start = virt_to_page(h.raw);
>> -             p_end = virt_to_page(h_end);
>> +             p_start = pgv_to_page(h.raw);
>> +             p_end = pgv_to_page(h_end);
>>               while (p_start <= p_end) {
>>                       flush_dcache_page(p_start);
>>                       p_start++;
>
> This was OK before Neil patch... after vmalloc(), assumption that
> p_start can be incremented is completely wrong.
>
> To fix this, we need something else than your patch.
>

Yes, you are right, and tpacket_fill_packet() has the same issue. Thanks.

-- 
Regards,
Changli Gao(xiaosuo@...il.com)
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