On 2023/3/8 2:14, Alexander Lobakin wrote: > 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. That may depends on what is a PP pages and what is a non-PP pages, it seems hard to answer now. For a skb with ->pp_recycle being true and its frag page with page->pp_magic being PP_SIGNATURE, when calling skb_clone()/pskb_expand_head() or skb_try_coalesce(), we may call __skb_frag_ref() for the frag page, which mean a page with page->pp_magic being PP_SIGNATURE can be both PP page and non-PP page at the same time. So it is important to set the ->pp_recycle correctly, and it seems hard to get that right from past experience，that's why a per page marker is suggested. > 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 > > . >
