On 2024/1/5 2:24, Mina Almasry wrote: > On Thu, Jan 4, 2024 at 12:48 AM Yunsheng Lin <linyunsheng@xxxxxxxxxx> wrote: >> >> On 2024/1/4 2:38, Mina Almasry wrote: >>> 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 >> >> 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. >> > > To be honest I definitely read that we have agreement that we should > have a unified driver facing struct from the responses in this thread > like this one: > > https://lore.kernel.org/netdev/20231215190126.1040fa12@xxxxxxxxxx/ Which specific comment made you thinking as above? I think it definitely need clarifying here, as I read it differently as you did. > > But I'll let folks correct me if I'm wrong. > >>> 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. >> > > Correct, the drivers should never cast directly, but helpers like > skb_frag_page() must check that the netmem is a page before doing a > cast. > >>> 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. >> > > With static branch the checks should be complete no-ops unless the > user's set up enabled devmem. What if the user does set up enabled devmem and still want to enable page_pool for normal page in the same system? Is there a reason I don't know, which stops you from keeping the old helper and introducing a new helper if it is needed for the new netmem thing? > >> 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@xxxxxxxxxxxxxx/ >> >> The above test case does not even seems to be testing a code path calling >> skb_frag_page() as my understanding. >> >>> > > >
