Re: [RFC PATCH net-next v1 4/4] net: page_pool: use netmem_t instead of struct page in API

[Mina Almasry]

On Wed, Jan 3, 2024 at 1:47 AM Yunsheng Lin <linyunsheng@xxxxxxxxxx> 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
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
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:

https://lore.kernel.org/netdev/CAHS8izM4w2UETAwfnV7w+ZzTMxLkz+FKO+xTgRdtYKzV8RzqXw@xxxxxxxxxxxxxx/

-- 
Thanks,
Mina