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
