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Wed, 11 Nov 2026 15:21:03 -0800 (PST) Received: from localhost ([2a03:2880:9ff:66::]) by smtp.gmail.com with ESMTPSA id 5a478bee46e88-3751fe2ff5dsm19921294eec.5.2026.11.11.15.21.02 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 11 Nov 2026 15:21:02 -0800 (PST) Date: Wed, 11 Nov 2026 15:05:25 -0800 From: Stanislav Fomichev To: =?utf-8?B?QmrDtnJuIFTDtnBlbA==?= Cc: Magnus Karlsson , Maciej Fijalkowski , Stanislav Fomichev , "David S. Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Simon Horman , Jonathan Corbet , Shuah Khan , Randy Dunlap , Alexander Duyck , kernel-team@meta.com, Andrew Lunn , Jesper Dangaard Brouer , Ilias Apalodimas , Alexei Starovoitov , Daniel Borkmann , John Fastabend , Pavel Begunkov , Jens Axboe , Andrii Nakryiko , Eduard Zingerman , Kumar Kartikeya Dwivedi , Martin KaFai Lau , Song Liu , Yonghong Song , Jiri Olsa , Emil Tsalapatis , Ihor Solodrai , netdev@vger.kernel.org, bpf@vger.kernel.org, io-uring@vger.kernel.org, Mina Almasry , "Mike Marciniszyn (Meta)" , Weiming Shi , Nikolay Aleksandrov , David Wei , Alexander Lobakin , linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, Stuart Clark Subject: Re: [PATCH net-next v1 03/11] xdp: Copy buffers with restricted lifetime Message-ID: References: <20261110102039.1002605-1-bjorn@kernel.org> <20261110102039.1002605-4-bjorn@kernel.org> <87eccra46g.fsf@all.your.base.are.belong.to.us> X-Mailing-List: bpf@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 Content-Transfer-Encoding: 8bit In-Reply-To: <87eccra46g.fsf@all.your.base.are.belong.to.us> On 11/11, Björn Töpel wrote: > Stanislav Fomichev writes: > > > On 11/10, Björn Töpel wrote: > > >> diff --git a/include/net/xdp.h b/include/net/xdp.h > >> index 07231adfb5f8..1105bb9e3292 100644 > >> --- a/include/net/xdp.h > >> +++ b/include/net/xdp.h > >> @@ -81,6 +81,16 @@ enum xdp_buff_flags { > >> * XDP program is not attached. > >> */ > >> XDP_FLAGS_FRAGS_UNREADABLE = BIT(2), > >> + /* Internal: the head netmem and the shared info are in &xdp_buff.ext. > >> + * Not copied to xdp_frames or skbs. > >> + */ > >> + XDP_FLAGS_EXT = BIT(3), > >> +}; > >> + > >> +/* For a head buffer that cannot hold kernel data, see xdp_init_buff_netmem() */ > >> +struct xdp_buff_ext { > >> + netmem_ref netmem; > > > > nit: netmem -> head ? and the same for xdp_init_buff_netmem netmem arg? > > Yup, will fix. > > >> @@ -138,7 +153,7 @@ static __always_inline void xdp_buff_set_frag_unreadable(struct xdp_buff *xdp) > >> > >> static __always_inline u32 xdp_buff_get_skb_flags(const struct xdp_buff *xdp) > >> { > >> - return xdp->flags; > >> + return xdp->flags & ~XDP_FLAGS_EXT; > > > > The only consumer is xdp_update_skb_frags_info, so probably not worth > > filtering out XDP_FLAGS_EXT here? Or is it for > > xdp_update_frame_from_buff? Can we explicitly clear it at the caller? > > It was for xdp_update_frame_from_buff(), but looking closer EXT never > gets there... > > I'll drop the mask. Thanks! > > >> diff --git a/net/core/devmem.c b/net/core/devmem.c > >> index 8f190b574beb..dea4a4a17e86 100644 > >> --- a/net/core/devmem.c > >> +++ b/net/core/devmem.c > >> @@ -698,5 +698,5 @@ static const struct memory_provider_ops dmabuf_devmem_ops = { > >> .release_netmem = mp_dmabuf_devmem_release_page, > >> .nl_fill = mp_dmabuf_devmem_nl_fill, > >> .uninstall = mp_dmabuf_devmem_uninstall, > >> - .caps = MP_CAP_FRAG, > >> + .caps = MP_CAP_FRAG | MP_CAP_RETAIN, > > > > Not sure I understand this part, can you explain? > > These are MP_CAP_RETAIN because they are unreadable? > > No, RETAIN is about lifetime: the stack may hold page pool references > after the RX poll, e.g. skbs on a TCP receive queue. That's how devmem > and zcrx work; without it the stack would try to copy them, and they'd > trip the DEBUG_NET check in skb_add_rx_frag_netmem(). > > A provider without RETAIN gets copied, so it must be readable. I'll try > to make that more clear in the kdoc, and probably refuse a provider that > is neither. Ah, yes, thanks, I did miss that warn in skb_add_rx_frag_netme, makes sense!