Date: Thu, 4 Jan 2024 10:24:29 -0800
From: Mina Almasry <almasrymina@...gle.com>
To: Yunsheng Lin <linyunsheng@...wei.com>
Cc: Shakeel Butt <shakeelb@...gle.com>, Jakub Kicinski <kuba@...nel.org>, linux-kernel@...r.kernel.org, 
	netdev@...r.kernel.org, bpf@...r.kernel.org, 
	Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org, 
	"H. Peter Anvin" <hpa@...or.com>, Greg Kroah-Hartman <gregkh@...uxfoundation.org>, 
	"Rafael J. Wysocki" <rafael@...nel.org>, Sumit Semwal <sumit.semwal@...aro.org>, 
	Christian König <christian.koenig@....com>, 
	Michael Chan <michael.chan@...adcom.com>, "David S. Miller" <davem@...emloft.net>, 
	Eric Dumazet <edumazet@...gle.com>, Paolo Abeni <pabeni@...hat.com>, 
	Alexei Starovoitov <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>, 
	Jesper Dangaard Brouer <hawk@...nel.org>, John Fastabend <john.fastabend@...il.com>, Wei Fang <wei.fang@....com>, 
	Shenwei Wang <shenwei.wang@....com>, Clark Wang <xiaoning.wang@....com>, 
	NXP Linux Team <linux-imx@....com>, Jeroen de Borst <jeroendb@...gle.com>, 
	Praveen Kaligineedi <pkaligineedi@...gle.com>, Shailend Chand <shailend@...gle.com>, 
	Yisen Zhuang <yisen.zhuang@...wei.com>, Salil Mehta <salil.mehta@...wei.com>, 
	Jesse Brandeburg <jesse.brandeburg@...el.com>, Tony Nguyen <anthony.l.nguyen@...el.com>, 
	Thomas Petazzoni <thomas.petazzoni@...tlin.com>, Marcin Wojtas <mw@...ihalf.com>, 
	Russell King <linux@...linux.org.uk>, Sunil Goutham <sgoutham@...vell.com>, 
	Geetha sowjanya <gakula@...vell.com>, Subbaraya Sundeep <sbhatta@...vell.com>, 
	hariprasad <hkelam@...vell.com>, Felix Fietkau <nbd@....name>, John Crispin <john@...ozen.org>, 
	Sean Wang <sean.wang@...iatek.com>, Mark Lee <Mark-MC.Lee@...iatek.com>, 
	Lorenzo Bianconi <lorenzo@...nel.org>, Matthias Brugger <matthias.bgg@...il.com>, 
	AngeloGioacchino Del Regno <angelogioacchino.delregno@...labora.com>, 
	Saeed Mahameed <saeedm@...dia.com>, Leon Romanovsky <leon@...nel.org>, 
	Horatiu Vultur <horatiu.vultur@...rochip.com>, UNGLinuxDriver@...rochip.com, 
	"K. Y. Srinivasan" <kys@...rosoft.com>, Haiyang Zhang <haiyangz@...rosoft.com>, Wei Liu <wei.liu@...nel.org>, 
	Dexuan Cui <decui@...rosoft.com>, Jassi Brar <jaswinder.singh@...aro.org>, 
	Ilias Apalodimas <ilias.apalodimas@...aro.org>, 
	Alexandre Torgue <alexandre.torgue@...s.st.com>, Jose Abreu <joabreu@...opsys.com>, 
	Maxime Coquelin <mcoquelin.stm32@...il.com>, Siddharth Vadapalli <s-vadapalli@...com>, 
	Ravi Gunasekaran <r-gunasekaran@...com>, Roger Quadros <rogerq@...nel.org>, 
	Jiawen Wu <jiawenwu@...stnetic.com>, Mengyuan Lou <mengyuanlou@...-swift.com>, 
	Ronak Doshi <doshir@...are.com>, VMware PV-Drivers Reviewers <pv-drivers@...are.com>, 
	Ryder Lee <ryder.lee@...iatek.com>, Shayne Chen <shayne.chen@...iatek.com>, 
	Kalle Valo <kvalo@...nel.org>, Juergen Gross <jgross@...e.com>, 
	Stefano Stabellini <sstabellini@...nel.org>, 
	Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>, Andrii Nakryiko <andrii@...nel.org>, 
	Martin KaFai Lau <martin.lau@...ux.dev>, Song Liu <song@...nel.org>, 
	Yonghong Song <yonghong.song@...ux.dev>, KP Singh <kpsingh@...nel.org>, 
	Stanislav Fomichev <sdf@...gle.com>, Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>, 
	Stefan Hajnoczi <stefanha@...hat.com>, Stefano Garzarella <sgarzare@...hat.com>, Shuah Khan <shuah@...nel.org>, 
	Mickaël Salaün <mic@...ikod.net>, 
	Nathan Chancellor <nathan@...nel.org>, Nick Desaulniers <ndesaulniers@...gle.com>, 
	Bill Wendling <morbo@...gle.com>, Justin Stitt <justinstitt@...gle.com>, 
	Jason Gunthorpe <jgg@...dia.com>, Willem de Bruijn <willemdebruijn.kernel@...il.com>
Subject: Re: [RFC PATCH net-next v1 4/4] net: page_pool: use netmem_t instead
 of struct page in API

On Thu, Jan 4, 2024 at 12:48 AM Yunsheng Lin <linyunsheng@...wei.com> wrote:
>
> 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.
>

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@kernel.org/

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.

> 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.
>
> >



-- 
Thanks,
Mina

Powered by blists - more mailing lists