From: Yunsheng Lin <linyunsheng@xxxxxxxxxx> Date: Tue, 7 Mar 2023 10:50:34 +0800 > On 2023/3/6 19:58, Alexander Lobakin wrote: >> From: Yunsheng Lin <linyunsheng@xxxxxxxxxx> >> Date: Mon, 6 Mar 2023 09:09:31 +0800 [...] >> Ah, from that perspective. Yes, you're probably right, but would need to >> be tested anyway. I don't see any open problems with the PP recycling >> right now on the lists, but someone may try to change it one day. >> Anyway, this flag is only to do a quick test. We do have >> sk_buff::pfmemalloc, but this flag doesn't mean every page from this skb >> was pfmemalloced. > > The point seems to be that sk_buff::pfmemalloc allow false positive, which > means skb->pfmemalloc can be set to true while every page from this skb is > not pfmemalloced as you mentioned. > > While skb->pp_recycle can't allow false positive, if that happens, reference > counting of the page will not be handled properly if pp and non-pp skb shares > the page as the wireless adapter does. You mean false-positives in both directions? Because if ->pp_recycle is set, the stack can still free non-PP pages. In the opposite case, I mean when ->pp_recycle is false and an skb page belongs to a page_pool, yes, there'll be issues. But I think the deal is to propagate the flag when you want to attach a PP-backed page to the skb? I mean, if someone decides to mix pages with different memory models, it's his responsibility to make sure everything is fine, because it's not a common/intended way. Isn't it? > >> >>> >>>> >>>>> >>>>> Anyway, I am not sure checking ::pp_magic is correct when a >>>>> page will be passing between different subsystem and back to >>>>> the network stack eventually, checking ::pp_magic may not be >>>>> correct if this happens. >>>>> >>>>> Another way is to use the bottom two bits in bv_page, see: >>>>> https://www.spinics.net/lists/netdev/msg874099.html This one is interesting actually. We'd only need one bit -- which is 100% free and available in case of page pointers. >>>>> >>>>>> >>>>>>> >>>>>>>> >>>>>>>> /* Allow SKB to reuse area used by xdp_frame */ >>>>>>>> xdp_scrub_frame(xdpf); [...] Thanks, Olek
