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Mon, 02 Nov 2026 10:25:09 -0800 (PST) Received: from localhost ([2a03:2880:9ff:42::]) by smtp.gmail.com with ESMTPSA id a92af1059eb24-1ac1854da78sm46557054c88.6.2026.11.02.10.25.08 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 02 Nov 2026 10:25:08 -0800 (PST) Date: Mon, 2 Nov 2026 10:25:04 -0800 From: Stanislav Fomichev To: Rongzheng Wang Cc: netdev@vger.kernel.org, Maciej Fijalkowski , Magnus Karlsson , Jason Xing , Jesper Dangaard Brouer , stable@vger.kernel.org, bpf@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH net v4 2/2] selftests/bpf: cover AF_XDP multi-buffer TX software checksum Message-ID: References: <20261030023248.78802-1-testwrzwrz@gmail.com> <20261030023248.78802-3-testwrzwrz@gmail.com> <20261031013143.42509-1-testwrzwrz@gmail.com> X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20261031013143.42509-1-testwrzwrz@gmail.com> On 10/31, Rongzheng Wang wrote: > > Any reason you're not reading the real value from > > /proc/sys/net/core/max_skb_frags ? > > The test wants to exercise the maximum supported number of frames, but also > needs to run on every build. CONFIG_MAX_SKB_FRAGS ranges from 17 to 45, so > using the minimum (17, i.e. 18 frames) gives the largest bound that works > everywhere. af_xdp.rst documents the same limit: > > "For copy mode, the maximum supported number of frames in a packet is > equal to CONFIG_MAX_SKB_FRAGS + 1." > > xsk checks MAX_SKB_FRAGS directly (net/xdp/xsk.c), not > net.core.max_skb_frags. I also set net.core.max_skb_frags=4 and ran > xdp_metadata; it still passes, which confirms xsk does not consume that > sysctl. Reading it in this test would not tell us the xsk limit, and would > only reduce coverage if it is lowered. > > CONFIG_MAX_SKB_FRAGS is compile-time and has no userspace interface, so 18 > frames is the portable upper bound. That is why I used 17. Sure, people can set max_skb_frags to < CONFIG_MAX_SKB_FRAGS, but I don't think we are gonna do it on our runners. So for the purpose of this test we can essentially assume CONFIG_MAX_SKB_FRAGS == max_skb_frags. I'm just trying to understand whether on the builds that have CONFIG_MAX_SKB_FRAGS > 17 we want to exercise all CONFIG_MAX_SKB_FRAGS or we are ok wit just 17.. But we probably aren't going to bump CONFIG_MAX_SKB_FRAGS on upstream kernels anytime soon.. Acked-by: Stanislav Fomichev