On 2021/5/7 15:06, Ilias Apalodimas wrote: > On Fri, May 07, 2021 at 11:23:28AM +0800, Yunsheng Lin wrote: >> On 2021/5/6 20:58, Ilias Apalodimas wrote: >>>>>> >>>>> >>>>> 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? >>>> >>> >>> Right, this sounds doable, but I'll have to go back code it and see if it >>> really makes sense. However I'd still prefer the support to go in as-is >>> (including the struct xdp_mem_info in struct page, instead of a page_pool >>> pointer). >>> >>> There's a couple of reasons for that. If we keep the struct xdp_mem_info we >>> can in the future recycle different kind of buffers using __xdp_return(). >>> And this is a non intrusive change if we choose to store the page pool address >>> directly in the future. It just affects the internal contract between the >>> page_pool code and struct page. So it won't affect any drivers that already >>> use the feature. >> >> This patchset has embeded a signature field in "struct page", and xdp_mem_info >> is stored in page_private(), which seems not considering the case for associating >> the page pool with "struct page" directly yet? > > Correct > >> Is the page pool also stored in >> page_private() and a different signature is used to indicate that? > > No only struct xdp_mem_info as you mentioned before > >> >> I am not saying we have to do it in this patchset, but we have to consider it >> while we are adding new signature field to "struct page", right? > > We won't need a new signature. The signature in both cases is there to > guarantee the page you are trying to recycle was indeed allocated by page_pool. > > Basically we got two design choices here: > - We store the page_pool ptr address directly in page->private and then, > we call into page_pool APIs directly to do the recycling. > That would eliminate the lookup through xdp_mem_info and the > XDP helpers to locate page pool pointer (through __xdp_return). > - You store the xdp_mem_info on page_private. In that case you need to go > through __xdp_return() to locate the page_pool pointer. Although we might > loose some performance that would allow us to recycle additional memory types > and not only MEM_TYPE_PAGE_POOL (in case we ever need it). So the signature field in "struct page" is used to only indicate a page is from a page pool, then how do we tell the content of page_private() if both of the above choices are needed, we might still need an extra indicator to tell page_private() is page_pool ptr or xdp_mem_info. It seems storing the page pool ptr in page_private() is clear for recyclable page from a recyclable skb use case, and the use case for storing xdp_mem_info in page_private() is unclear yet? As XDP seems to have the xdp_mem_info in the "struct xdp_frame", so it does not need the xdp_mem_info from page_private(). If the above is true, what does not really makes sense to me here is that: why do we first implement a unclear use case for storing xdp_mem_info in page_private(), why not implement the clear use case for storing page pool ptr in page_private() first? > > > I think both choices are sane. What I am trying to explain here, is > regardless of what we choose now, we can change it in the future without > affecting the API consumers at all. What will change internally is the way we > lookup the page pool pointer we are trying to recycle. It seems the below API need changing? +static inline void skb_mark_for_recycle(struct sk_buff *skb, struct page *page, + struct xdp_mem_info *mem) > > [...] > > > Cheers > /Ilias > > . >
