Re: [PATCH RFC 3/8] memory-provider: dmabuf devmem memory provider

[Mina Almasry <almasrymina@google.com>]

On Wed, Nov 15, 2023 at 10:07 AM Mina Almasry <almasrymina@google.com> wrote:
>
> On Wed, Nov 15, 2023 at 1:29 AM Yunsheng Lin <linyunsheng@huawei.com> wrote:
> >
> > On 2023/11/14 23:41, Willem de Bruijn wrote:
> > >>
> > >> I am not sure dma-buf maintainer's concern is still there with this patchset.
> > >>
> > >> Whatever name you calling it for the struct, however you arrange each field
> > >> in the struct, some metadata is always needed for dmabuf to intergrate into
> > >> page pool.
> > >>
> > >> If the above is true, why not utilize the 'struct page' to have more unified
> > >> handling?
> > >
> > > My understanding is that there is a general preference to simplify struct
> > > page, and at the least not move in the other direction by overloading the
> > > struct in new ways.
> >
> > As my understanding, the new struct is just mirroring the struct page pool
> > is already using, see:
> > https://elixir.free-electrons.com/linux/v6.7-rc1/source/include/linux/mm_types.h#L119
> >
> > If there is simplifying to the struct page_pool is using, I think the new
> > stuct the devmem memory provider is using can adjust accordingly.
> >
> > As a matter of fact, I think the way 'struct page' for devmem is decoupled
> > from mm subsystem may provide a way to simplify or decoupled the already
> > existing 'struct page' used in netstack from mm subsystem, before this
> > patchset, it seems we have the below types of 'struct page':
> > 1. page allocated in the netstack using page pool.
> > 2. page allocated in the netstack using buddy allocator.
> > 3. page allocated in other subsystem and passed to the netstack, such as
> >    zcopy or spliced page?
> >
> > If we can decouple 'struct page' for devmem from mm subsystem, we may be able
> > to decouple the above 'struct page' from mm subsystem one by one.
> >
> > >
> > > If using struct page for something that is not memory, there is ZONE_DEVICE.
> > > But using that correctly is non-trivial:
> > >
> > > https://lore.kernel.org/all/ZKyZBbKEpmkFkpWV@ziepe.ca/
> > >
> > > Since all we need is a handle that does not leave the network stack,
> > > a network specific struct like page_pool_iov entirely avoids this issue.
> >
> > Yes, I am agree about the network specific struct.
> > I am wondering if we can make the struct more generic if we want to
> > intergrate it into page_pool and use it in net stack.
> >
> > > RFC v3 seems like a good simplification over RFC v1 in that regard to me.
> > > I was also pleasantly surprised how minimal the change to the users of
> > > skb_frag_t actually proved to be.
> >
> > Yes, I am agreed about that too. Maybe we can make it simpler by using
> > a more abstract struct as page_pool, and utilize some features of
> > page_pool too.
> >
> > For example, from the page_pool doc, page_pool have fast cache and
> > ptr-ring cache as below, but if napi_frag_unref() call
> > page_pool_page_put_many() and return the dmabuf chunk directly to
> > gen_pool in the memory provider, then it seems we are bypassing the
> > below caches in the page_pool.
> >
>
> I think you're just misunderstanding the code. The page recycling
> works with my patchset. napi_frag_unref() calls napi_pp_put_page() if
> recycle == true, and that works the same with devmem as with regular
> pages.
>
> If recycle == false, we call page_pool_page_put_many() which will call
> put_page() for regular pages and page_pool_iov_put_many() for devmem
> pages. So, the memory recycling works exactly the same as before with
> devmem as with regular pages. In my tests I do see the devmem being
> recycled correctly. We are not bypassing any caches.
>
>

Ah, taking a closer look here, the devmem recycling works for me but I
think that's a side effect to the fact that the page_pool support I
implemented with GVE is unusual. I currently allocate pages from the
page_pool but do not set skb_mark_for_recycle(). The page recycling
still happens when GVE is done with the page and calls
page_pool_put_full_pgae(), as that eventually checks the refcount on
the devmem and recycles it.

I will fix up the GVE to call skb_mark_for_recycle() and ensure the
napi_pp_put_page() path recycles the devmem or page correctly in the
next version.

> >     +------------------+
> >     |       Driver     |
> >     +------------------+
> >             ^
> >             |
> >             |
> >             |
> >             v
> >     +--------------------------------------------+
> >     |                request memory              |
> >     +--------------------------------------------+
> >         ^                                  ^
> >         |                                  |
> >         | Pool empty                       | Pool has entries
> >         |                                  |
> >         v                                  v
> >     +-----------------------+     +------------------------+
> >     | alloc (and map) pages |     |  get page from cache   |
> >     +-----------------------+     +------------------------+
> >                                     ^                    ^
> >                                     |                    |
> >                                     | cache available    | No entries, refill
> >                                     |                    | from ptr-ring
> >                                     |                    |
> >                                     v                    v
> >                           +-----------------+     +------------------+
> >                           |   Fast cache    |     |  ptr-ring cache  |
> >                           +-----------------+     +------------------+
> >
> >
> > >
> > > .
> > >
>
>
>
> --
> Thanks,
> Mina



-- 
Thanks,
Mina