Date: Thu, 4 Jan 2024 16:48:23 +0800
From: Yunsheng Lin <linyunsheng@...wei.com>
To: Mina Almasry <almasrymina@...gle.com>
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Subject: Re: [RFC PATCH net-next v1 4/4] net: page_pool: use netmem_t instead
 of struct page in API

On 2024/1/4 2:38, Mina Almasry wrote:
> On Wed, Jan 3, 2024 at 1:47 AM Yunsheng Lin <linyunsheng@...wei.com> wrote:
>>
>> On 2024/1/3 0:14, Mina Almasry wrote:
>>>
>>> The idea being that skb_frag_page() can return NULL if the frag is not
>>> paged, and the relevant callers are modified to handle that.
>>
>> There are many existing drivers which are not expecting NULL returning for
>> skb_frag_page() as those drivers are not supporting devmem, adding additionl
>> checking overhead in skb_frag_page() for those drivers does not make much
>> sense, IMHO, it may make more sense to introduce a new helper for the driver
>> supporting devmem or networking core that needing dealing with both normal
>> page and devmem.
>>
>> And we are also able to keep the old non-NULL returning semantic for
>> skb_frag_page().
> 
> I think I'm seeing agreement that the direction we're heading into
> here is that most net stack & drivers should use the abstract netmem

As far as I see, at least for the drivers, I don't think we have a clear
agreement if we should have a unified driver facing struct or API for both
normal page and devmem yet.

> type, and only specific code that needs a page or devmem (like
> tcp_receive_zerocopy or tcp_recvmsg_dmabuf) will be the ones that
> unpack the netmem and get the underlying page or devmem, using
> skb_frag_page() or something like skb_frag_dmabuf(), etc.
> 
> As Jason says repeatedly, I'm not allowed to blindly cast a netmem to
> a page and assume netmem==page. Netmem can only be cast to a page
> after checking the low bits and verifying the netmem is actually a

I thought it would be best to avoid casting a netmem or devmem to a
page in the driver, I think the main argument is that it is hard
to audit very single driver doing a checking before doing the casting
in the future? and we can do better auditting if the casting is limited
to a few core functions in the networking core.

> page. I think any suggestions that blindly cast a netmem to page
> without the checks will get nacked by Jason & Christian, so the
> checking in the specific cases where the code needs to know the
> underlying memory type seems necessary.
> 
> IMO I'm not sure the checking is expensive. With likely/unlikely &
> static branches the checks should be very minimal or a straight no-op.
> For example in RFC v2 where we were doing a lot of checks for devmem
> (we don't do that anymore for RFCv5), I had run the page_pool perf
> tests and proved there is little to no perf regression:

For MAX_SKB_FRAGS being 17, it means we may have 17 additional checking
overhead for the drivers not supporting devmem, not to mention we may
have bigger value for MAX_SKB_FRAGS if BIG TCP is enable.

Even there is no notiable performance degradation for a specific case,
we should avoid the overhead as much as possible for the existing use
case when supporting a new use case.

> 
> https://lore.kernel.org/netdev/CAHS8izM4w2UETAwfnV7w+ZzTMxLkz+FKO+xTgRdtYKzV8RzqXw@mail.gmail.com/

The above test case does not even seems to be testing a code path calling
skb_frag_page() as my understanding.

> 

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