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Thu, 05 Nov 2026 09:55:54 -0800 (PST) Received: from localhost ([2a03:2880:9ff:65::]) by smtp.gmail.com with ESMTPSA id a92af1059eb24-1b245b86fbesm18947609c88.1.2026.11.05.09.55.54 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 05 Nov 2026 09:55:54 -0800 (PST) Date: Thu, 5 Nov 2026 09:53:41 -0800 From: Stanislav Fomichev To: Mina Almasry Cc: netdev@vger.kernel.org, davem@davemloft.net, edumazet@google.com, kuba@kernel.org, pabeni@redhat.com, andrew+netdev@lunn.ch, edumazet@kernel.org, horms@kernel.org, corbet@lwn.net, skhan@linuxfoundation.org, rdunlap@infradead.org, michael.chan@broadcom.com, pavan.chebbi@broadcom.com, sdf@fomichev.me, bobbyeshleman@meta.com, asml.silence@gmail.com, razor@blackwall.org, daniel@iogearbox.net, leitao@debian.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org Subject: Re: [PATCH net-next v2 4/7] net: devmem: add a per-device netmem TX frag size limit Message-ID: References: <20261103235152.1904727-1-sdf@fomichev.me> <20261103235152.1904727-5-sdf@fomichev.me> <18e4e1daa184cadc.3137322e32382e36372e3935.ceab9.0000sendgmr@google.com> X-Mailing-List: netdev@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: <18e4e1daa184cadc.3137322e32382e36372e3935.ceab9.0000sendgmr@google.com> On 11/04, Mina Almasry wrote: > On Tue, Nov 3, 2026 at 3:51 PM Stanislav Fomichev wrote: > > > > Devmem TX frags are bounded only by the net_iov size, which is about > > to grow up to 2MB, more than some devices can describe in a single TX > > descriptor. > > > > Add netmem_tx_max_frag_size, U32_MAX by default, and cap devmem TX > > frags to the limit of the device the binding is attached to. > > > > Signed-off-by: Stanislav Fomichev > > --- > > Documentation/networking/net_cachelines/net_device.rst | 1 + > > Documentation/networking/netmem.rst | 6 ++++++ > > include/linux/netdevice.h | 2 ++ > > net/core/datagram.c | 1 + > > net/core/dev.c | 1 + > > net/core/devmem.c | 1 + > > net/core/devmem.h | 1 + > > 7 files changed, 13 insertions(+) > > > ... > > diff --git a/net/core/dev.c b/net/core/dev.c > > index 3ac700d37e59..ba2f887dcd39 100644 > > --- a/net/core/dev.c > > +++ b/net/core/dev.c > > @@ -12350,6 +12350,7 @@ struct net_device *alloc_netdev_mqs(int sizeof_priv, const char *name, > > dev->gro_ipv4_max_size = GRO_LEGACY_MAX_SIZE; > > dev->tso_max_size = TSO_LEGACY_MAX_SIZE; > > dev->tso_max_segs = TSO_MAX_SEGS; > > + dev->netmem_tx_max_frag_size = U32_MAX; > > I wonder if netmem_tx_max_frag_size should default to PAGE_SIZE instead > of U32_MAX, and have drivers opt in by raising it after verification > (e.g. SZ_32K for bnxt and U32_MAX for mlx5 in this series)? > > With U32_MAX as the default, large frags are enabled on all NETMEM_TX > drivers unless they opt out. For example, fbnic also has a 16-bit TX > descriptor length (FBNIC_TWD_LEN_MASK) and only avoids hitting > dma_error here because its tso_max_size happens to be 64KB today, and > gve hasn't been tested with >PAGE_SIZE devmem TX frags yet. Defaulting > to PAGE_SIZE (similar to tso_max_size = TSO_LEGACY_MAX_SIZE above) seems > safer for existing and future drivers. Hmm, I did not see fbnic :-/ That sounds fair, let me switch to an opt in, should be safer, agreed... Will only option mlx/bnxt and most likely fbnic.