Re: [net-next v1 09/16] page_pool: device memory support

[Mina Almasry]

On Sun, Dec 10, 2023 at 6:04 PM Yunsheng Lin <linyunsheng@xxxxxxxxxx> wrote:
>
> On 2023/12/9 0:05, Mina Almasry wrote:
> > On Fri, Dec 8, 2023 at 1:30 AM Yunsheng Lin <linyunsheng@xxxxxxxxxx> wrote:
> >>
> >>
> >> As mentioned before, it seems we need to have the above checking every
> >> time we need to do some per-page handling in page_pool core, is there
> >> a plan in your mind how to remove those kind of checking in the future?
> >>
> >
> > I see 2 ways to remove the checking, both infeasible:
> >
> > 1. Allocate a wrapper struct that pulls out all the fields the page pool needs:
> >
> > struct netmem {
> >         /* common fields */
> >         refcount_t refcount;
> >         bool is_pfmemalloc;
> >         int nid;
> >         ...
> >         union {
> >                 struct dmabuf_genpool_chunk_owner *owner;
> >                 struct page * page;
> >         };
> > };
> >
> > The page pool can then not care if the underlying memory is iov or
> > page. However this introduces significant memory bloat as this struct
> > needs to be allocated for each page or ppiov, which I imagine is not
> > acceptable for the upside of removing a few static_branch'd if
> > statements with no performance cost.
> >
> > 2. Create a unified struct for page and dmabuf memory, which the mm
> > folks have repeatedly nacked, and I imagine will repeatedly nack in
> > the future.
> >
> > So I imagine the special handling of ppiov in some form is critical
> > and the checking may not be removable.
>
> If the above is true, perhaps devmem is not really supposed to be intergated
> into page_pool.
>
> Adding a checking for every per-page handling in page_pool core is just too
> hacky to be really considerred a longterm solution.
>

The only other option is to implement another page_pool for ppiov and
have the driver create page_pool or ppiov_pool depending on the state
of the netdev_rx_queue (or some helper in the net stack to do that for
the driver). This introduces some code duplication. The ppiov_pool &
page_pool would look similar in implementation.

But this was all discussed in detail in RFC v2 and the last response I
heard from Jesper was in favor if this approach, if I understand
correctly:

https://lore.kernel.org/netdev/7aedc5d5-0daf-63be-21bc-3b724cc1cab9@xxxxxxxxxx/

Would love to have the maintainer weigh in here.

> It is somewhat ironical that devmem is using static_branch to alliviate the
> performance impact for normal memory at the possible cost of performance
> degradation for devmem, does it not defeat some purpose of intergating devmem
> to page_pool?
>

I don't see the issue. The static branch sets the non-ppiov path as
default if no memory providers are in use, and flips it when they are,
making the default branch prediction ideal in both cases.

> >
> >> Even though a static_branch check is added in page_is_page_pool_iov(), it
> >> does not make much sense that a core has tow different 'struct' for its
> >> most basic data.
> >>
> >> IMHO, the ppiov for dmabuf is forced fitting into page_pool without much
> >> design consideration at this point.
> >>
> > ...
> >>
> >> For now, the above may work for the the rx part as it seems that you are
> >> only enabling rx for dmabuf for now.
> >>
> >> What is the plan to enable tx for dmabuf? If it is also intergrated into
> >> page_pool? There was a attempt to enable page_pool for tx, Eric seemed to
> >> have some comment about this:
> >> https://lkml.kernel.org/netdev/2cf4b672-d7dc-db3d-ce90-15b4e91c4005@xxxxxxxxxx/T/#mb6ab62dc22f38ec621d516259c56dd66353e24a2
> >>
> >> If tx is not intergrated into page_pool, do we need to create a new layer for
> >> the tx dmabuf?
> >>
> >
> > I imagine the TX path will reuse page_pool_iov, page_pool_iov_*()
> > helpers, and page_pool_page_*() helpers, but will not need any core
> > page_pool changes. This is because the TX path will have to piggyback
>
> We may need another bit/flags checking to demux between page_pool owned
> devmem and non-page_pool owned devmem.
>

The way I'm imagining the support, I don't see the need for such
flags. We'd be re-using generic helpers like
page_pool_iov_get_dma_address() and what not that don't need that
checking.

> Also calling page_pool_*() on non-page_pool owned devmem is confusing
> enough that we may need a thin layer handling non-page_pool owned devmem
> in the end.
>

The page_pool_page* & page_pool_iov* functions can be renamed if
confusing. I would think that's no issue (note that the page_pool_*
functions need not be called for TX path).

> > on MSG_ZEROCOPY (devmem is not copyable), so no memory allocation from
> > the page_pool (or otherwise) is needed or possible. RFCv1 had a TX
> > implementation based on dmabuf pages without page_pool involvement, I
> > imagine I'll do something similar.
> It would be good to have a tx implementation for the next version, so
> that we can have a whole picture of devmem.
>
> >



-- 
Thanks,
Mina