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[82.27.106.168]) by smtp.gmail.com with ESMTPSA id a4-20020a05600c348400b0038cd743830esm4681557wmq.29.2022.03.30.05.41.41 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 30 Mar 2022 05:41:42 -0700 (PDT) Date: Wed, 30 Mar 2022 13:41:17 +0100 From: Jean-Philippe Brucker To: Alexander H Duyck Cc: ilias.apalodimas@linaro.org, hawk@kernel.org, alexanderduyck@fb.com, linyunsheng@huawei.com, davem@davemloft.net, kuba@kernel.org, pabeni@redhat.com, netdev@vger.kernel.org Subject: Re: [PATCH net v2] skbuff: disable coalescing for page_pool fragment recycling Message-ID: References: <20220328132258.78307-1-jean-philippe@linaro.org> <2de8c5818582bd9dfe0406541e3326c2bed0b6f2.camel@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <2de8c5818582bd9dfe0406541e3326c2bed0b6f2.camel@gmail.com> Precedence: bulk List-ID: X-Mailing-List: netdev@vger.kernel.org On Mon, Mar 28, 2022 at 08:03:46AM -0700, Alexander H Duyck wrote: > > (3b) Now while handling TCP, coalesce SKB3 with SKB1: > > > > tcp_v4_rcv(SKB3) > > tcp_try_coalesce(to=SKB1, from=SKB3) // succeeds > > kfree_skb_partial(SKB3) > > skb_release_data(SKB3) // drops one dataref > > > > SKB1 _____ PAGE1 > > \____ > > SKB2 _____ PAGE2 > > / > > RX_BD3 _________/ > > > > In tcp_try_coalesce(), __skb_frag_ref() takes a page reference to > > PAGE2, where it should instead have increased the page_pool frag > > reference, pp_frag_count. Without coalescing, when releasing both > > SKB2 and SKB3, a single reference to PAGE2 would be dropped. Now > > when releasing SKB1 and SKB2, two references to PAGE2 will be > > dropped, resulting in underflow. > > > > (3c) Drop SKB2: > > > > af_packet_rcv(SKB2) > > consume_skb(SKB2) > > skb_release_data(SKB2) // drops second dataref > > page_pool_return_skb_page(PAGE2) // drops one pp_frag_count > > > > SKB1 _____ PAGE1 > > \____ > > PAGE2 > > / > > RX_BD3 _________/ > > > > (4) Userspace calls recvmsg() > > Copies SKB1 and releases it. Since SKB3 was coalesced with SKB1, we > > release the SKB3 page as well: > > > > tcp_eat_recv_skb(SKB1) > > skb_release_data(SKB1) > > page_pool_return_skb_page(PAGE1) > > page_pool_return_skb_page(PAGE2) // drops second pp_frag_count > > > > (5) PAGE2 is freed, but the third RX descriptor was still using it! > > In our case this causes IOMMU faults, but it would silently corrupt > > memory if the IOMMU was disabled. > > > > Prevent coalescing the SKB if it may hold shared page_pool fragment > > references. > > > > Fixes: 53e0961da1c7 ("page_pool: add frag page recycling support in page pool") > > Signed-off-by: Jean-Philippe Brucker > > --- > > net/core/skbuff.c | 7 +++++++ > > 1 file changed, 7 insertions(+) > > > > diff --git a/net/core/skbuff.c b/net/core/skbuff.c > > index 10bde7c6db44..56b45b9f0b4d 100644 > > --- a/net/core/skbuff.c > > +++ b/net/core/skbuff.c > > @@ -5276,6 +5276,13 @@ bool skb_try_coalesce(struct sk_buff *to, struct sk_buff *from, > > if (skb_cloned(to)) > > return false; > > > > + /* We don't support taking page_pool frag references at the moment. > > + * If the SKB is cloned and could have page_pool frag references, don't > > + * coalesce it. > > + */ > > + if (skb_cloned(from) && from->pp_recycle) > > + return false; > > + > > /* The page pool signature of struct page will eventually figure out > > * which pages can be recycled or not but for now let's prohibit slab > > * allocated and page_pool allocated SKBs from being coalesced. > > > This is close but not quite. Actually now that I think about it we can > probably alter the block below rather than adding a new one. > > The issue is we want only reference counted pages in standard skbs, and > pp_frag_count pages in pp_recycle skbs. So we already had logic along > the lines of: > if (to->pp_recycle != from->pp_recycle) > return false; > > I would say we need to change that because from->pp_recycle is the > piece that is probably too simplistic. Basically we will get a page > pool page if from->pp_recycle && !skb_cloned(from). So we can probably > just tweak the check below to be something along the lines of: > if (to->pp_recycle != (from->pp_recycle && !skb_cloned(from))) > return false; Just to confirm this is fine: the behavior now changes for to->pp_recycle == 0, from->pp_recycle == 1 and skb_cloned(from) == 1 In this case we now coalesce and take a page ref. So the page has two refs and two pp_frag_count. (3c) drops one pp_frag_count. If there wasn't another RX desc holding a pp_frag_count (ie. no step (5)), that would also drop a page ref, but since 'to' SKB is holding a second page ref the page is not recycled. That reference gets dropped at (4) and the page is freed there. With step (5), the page would get recycled into page_pool, but without (5) the page is discarded. I guess it works, just want to make sure that it's OK to mix page_pool pp_frag_count and normal reference counting at the same time. Thanks, Jean > > That should actually increase the number of cases where we can > correctly coalesce frames while also addressing the issue you > discovered as it will only merge cloned skbs into standard skbs instead > of page pool ones. >