On 2021/5/1 0:24, Ilias Apalodimas wrote: > [...] >>>> >>>> 1. skb frag page recycling do not need "struct xdp_rxq_info" or >>>> "struct xdp_mem_info" to bond the relation between "struct page" and >>>> "struct page_pool", which seems uncessary at this point if bonding >>>> a "struct page_pool" pointer directly in "struct page" does not cause >>>> space increasing. >>> >>> We can't do that. The reason we need those structs is that we rely on the >>> existing XDP code, which already recycles it's buffers, to enable >>> recycling. Since we allocate a page per packet when using page_pool for a >>> driver , the same ideas apply to an SKB and XDP frame. We just recycle the >> >> I am not really familar with XDP here, but a packet from hw is either a >> "struct xdp_frame/xdp_buff" for XDP or a "struct sk_buff" for TCP/IP stack, >> a packet can not be both "struct xdp_frame/xdp_buff" and "struct sk_buff" at >> the same time, right? >> > > Yes, but the payload is irrelevant in both cases and that's what we use > page_pool for. You can't use this patchset unless your driver usues > build_skb(). So in both cases you just allocate memory for the payload and I am not sure I understood why build_skb() matters here. If the head data of a skb is a page frag and is from page pool, then it's page->signature should be PP_SIGNATURE, otherwise it's page->signature is zero, so a recyclable skb does not require it's head data being from a page pool, right? > decide what the wrap the buffer with (XDP or SKB) later. [...] >> >> I am not sure I understand what you meant by "free the skb", does it mean >> that kfree_skb() is called to free the skb. > > Yes > >> >> As my understanding, if the skb completely own the page(which means page_count() >> == 1) when kfree_skb() is called, __page_pool_put_page() is called, otherwise >> page_ref_dec() is called, which is exactly what page_pool_atomic_sub_if_positive() >> try to handle it atomically. >> > > Not really, the opposite is happening here. If the pp_recycle bit is set we > will always call page_pool_return_skb_page(). If the page signature matches > the 'magic' set by page pool we will always call xdp_return_skb_frame() will > end up calling __page_pool_put_page(). If the refcnt is 1 we'll try > to recycle the page. If it's not we'll release it from page_pool (releasing > some internal references we keep) unmap the buffer and decrement the refcnt. Yes, I understood the above is what the page pool do now. But the question is who is still holding an extral reference to the page when kfree_skb()? Perhaps a cloned and pskb_expand_head()'ed skb is holding an extral reference to the same page? So why not just do a page_ref_dec() if the orginal skb is freed first, and call __page_pool_put_page() when the cloned skb is freed later? So that we can always reuse the recyclable page from a recyclable skb. This may make the page_pool_destroy() process delays longer than before, I am supposed the page_pool_destroy() delaying for cloned skb case does not really matters here. If the above works, I think the samiliar handling can be added to RX zerocopy if the RX zerocopy also hold extral references to the recyclable page from a recyclable skb too? > > [1] https://lore.kernel.org/netdev/154413868810.21735.572808840657728172.stgit@firesoul/ > > Cheers > /Ilias > > . >
