netdev - Re: [PATCH v4] net: add Faraday FTMAC100 10/100 Ethernet driver

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Date:	Tue, 25 Jan 2011 10:46:29 +0800
From:	Po-Yu Chuang <ratbert.chuang@...il.com>
To:	Michał Mirosław <mirqus@...il.com>
Cc:	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	bhutchings@...arflare.com, eric.dumazet@...il.com, joe@...ches.com,
	dilinger@...ued.net, Po-Yu Chuang <ratbert@...aday-tech.com>
Subject: Re: [PATCH v4] net: add Faraday FTMAC100 10/100 Ethernet driver

Dear Michał,

2011/1/25 Michał Mirosław <mirqus@...il.com>:
> W dniu 24 stycznia 2011 09:26 użytkownik Po-Yu Chuang
> <ratbert.chuang@...il.com> napisał:
>> 2011/1/21 Michał Mirosław <mirqus@...il.com>:
>>> Did you test this? This looks like it will result in double free after
>>> packet RX, as you are giving the same page (referenced once) to two
>>> distinct RX descriptors, that may be assigned different packets.
>>
>> Yes, this is tested.
>>
>>> Since your not implementing any RX offloads, you might just allocate
>>> fresh skb's with alloc_skb() and store skb pointer in rxdes3. Since
>>
>> rxdes3 does not store virtual address of an skb.
>> It stores the address of the buffer allocated while open() and freed
>> only when stop().
>> The data in that buffer will be memcpy()ed to an skb allocated in
>> ftmac100_rx_packet().
>> No double free happens.
>
> Ah, I blindly assumed that you're just appending the buffers to the
> skb (using skb_fill_page_desc() and friends). Since you have to mark
> descriptors for the device anyway, it might be faster to allocate new
> skbs and map those as rx buffers (changing the descriptor's buffer
> address after every RX) instead of keeping static buffer and copying
> every time. (For small packets it wastes lot of memory, though - so
> the right choice depends on the expected workload.)

The reason I chose to use memcpy rx buffer to skb is that I didn't know
how to deal with multi-segment packets if I preallocated skb for each
rx descriptor. This function seems to be what I need.
Let me rework this. Thanks.

best regards,
Po-Yu Chuang
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