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

[Mina Almasry]

On Wed, Nov 15, 2023 at 1:29 AM Yunsheng Lin <linyunsheng@xxxxxxxxxx> 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@xxxxxxxx/
> >
> > 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.


>     +------------------+
>     |       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