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[89.24.57.65]) by smtp.gmail.com with ESMTPSA id a640c23a62f3a-c249685c573sm1528423166b.51.2026.08.24.04.38.20 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 24 Aug 2026 04:38:21 -0700 (PDT) Message-ID: Date: Mon, 24 Aug 2026 13:38:20 +0200 Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH net v3 0/3] net: don't strip zerocopy frag markers from a forwarded skb To: Willem de Bruijn , Ilya Maximets , Jakub Kicinski Cc: Norbert Szetei , netdev@vger.kernel.org, "David S. Miller" , Eric Dumazet , Paolo Abeni , Simon Horman , Aaron Conole , Eelco Chaudron , Steffen Klassert , Kuan-Ting Chen , "Michael S. Tsirkin" , linux-kernel@vger.kernel.org, dev@openvswitch.org, Jongmin Jang , Willem de Bruijn References: <34c64d08-0484-4f3b-b61a-ed68115d01a4@ovn.org> <20260821170950.3e278b1f@kernel.org> <86c61c39-901e-489e-aec9-fafe639276e7@ovn.org> Content-Language: en-US From: Ilya Maximets Autocrypt: addr=i.maximets@ovn.org; keydata= xsFNBF77bOMBEADVZQ4iajIECGfH3hpQMQjhIQlyKX4hIB3OccKl5XvB/JqVPJWuZQRuqNQG /B70MP6km95KnWLZ4H1/5YOJK2l7VN7nO+tyF+I+srcKq8Ai6S3vyiP9zPCrZkYvhqChNOCF pNqdWBEmTvLZeVPmfdrjmzCLXVLi5De9HpIZQFg/Ztgj1AZENNQjYjtDdObMHuJQNJ6ubPIW cvOOn4WBr8NsP4a2OuHSTdVyAJwcDhu+WrS/Bj3KlQXIdPv3Zm5x9u/56NmCn1tSkLrEgi0i /nJNeH5QhPdYGtNzPixKgPmCKz54/LDxU61AmBvyRve+U80ukS+5vWk8zvnCGvL0ms7kx5sA tETpbKEV3d7CB3sQEym8B8gl0Ux9KzGp5lbhxxO995KWzZWWokVUcevGBKsAx4a/C0wTVOpP FbQsq6xEpTKBZwlCpxyJi3/PbZQJ95T8Uw6tlJkPmNx8CasiqNy2872gD1nN/WOP8m+cIQNu o6NOiz6VzNcowhEihE8Nkw9V+zfCxC8SzSBuYCiVX6FpgKzY/Tx+v2uO4f/8FoZj2trzXdLk BaIiyqnE0mtmTQE8jRa29qdh+s5DNArYAchJdeKuLQYnxy+9U1SMMzJoNUX5uRy6/3KrMoC/ 7zhn44x77gSoe7XVM6mr/mK+ViVB7v9JfqlZuiHDkJnS3yxKPwARAQABzSJJbHlhIE1heGlt ZXRzIDxpLm1heGltZXRzQG92bi5vcmc+wsGUBBMBCAA+AhsDBQsJCAcCBhUKCQgLAgQWAgMB Ah4BAheAFiEEh+ma1RKWrHCY821auffsd8gpv5YFAmfB9JAFCQyI7q0ACgkQuffsd8gpv5YQ og/8DXt1UOznvjdXRHVydbU6Ws+1iUrxlwnFH4WckoFgH4jAabt25yTa1Z4YX8Vz0mbRhTPX M/j1uORyObLem3of4YCd4ymh7nSu++KdKnNsZVHxMcoiic9ILPIaWYa8kTvyIDT2AEVfn9M+ vskM0yDbKa6TAHgr/0jCxbS+mvN0ZzDuR/LHTgy3e58097SWJohj0h3Dpu+XfuNiZCLCZ1/G AbBCPMw+r7baH/0evkX33RCBZwvh6tKu+rCatVGk72qRYNLCwF0YcGuNBsJiN9Aa/7ipkrA7 Xp7YvY3Y1OrKnQfdjp3mSXmknqPtwqnWzXvdfkWkZKShu0xSk+AjdFWCV3NOzQaH3CJ67NXm aPjJCIykoTOoQ7eEP6+m3WcgpRVkn9bGK9ng03MLSymTPmdINhC5pjOqBP7hLqYi89GN0MIT Ly2zD4m/8T8wPV9yo7GRk4kkwD0yN05PV2IzJECdOXSSStsf5JWObTwzhKyXJxQE+Kb67Wwa LYJgltFjpByF5GEO4Xe7iYTjwEoSSOfaR0kokUVM9pxIkZlzG1mwiytPadBt+VcmPQWcO5pi WxUI7biRYt4aLriuKeRpk94ai9+52KAk7Lz3KUWoyRwdZINqkI/aDZL6meWmcrOJWCUMW73e 4cMqK5XFnGqolhK4RQu+8IHkSXtmWui7LUeEvO/OwU0EXvts4wEQANCXyDOic0j2QKeyj/ga OD1oKl44JQfOgcyLVDZGYyEnyl6b/tV1mNb57y/YQYr33fwMS1hMj9eqY6tlMTNz+ciGZZWV YkPNHA+aFuPTzCLrapLiz829M5LctB2448bsgxFq0TPrr5KYx6AkuWzOVq/X5wYEM6djbWLc VWgJ3o0QBOI4/uB89xTf7mgcIcbwEf6yb/86Cs+jaHcUtJcLsVuzW5RVMVf9F+Sf/b98Lzrr 2/mIB7clOXZJSgtV79Alxym4H0cEZabwiXnigjjsLsp4ojhGgakgCwftLkhAnQT3oBLH/6ix 87ahawG3qlyIB8ZZKHsvTxbWte6c6xE5dmmLIDN44SajAdmjt1i7SbAwFIFjuFJGpsnfdQv1 OiIVzJ44kdRJG8kQWPPua/k+AtwJt/gjCxv5p8sKVXTNtIP/sd3EMs2xwbF8McebLE9JCDQ1 RXVHceAmPWVCq3WrFuX9dSlgf3RWTqNiWZC0a8Hn6fNDp26TzLbdo9mnxbU4I/3BbcAJZI9p 9ELaE9rw3LU8esKqRIfaZqPtrdm1C+e5gZa2gkmEzG+WEsS0MKtJyOFnuglGl1ZBxR1uFvbU VXhewCNoviXxkkPk/DanIgYB1nUtkPC+BHkJJYCyf9Kfl33s/bai34aaxkGXqpKv+CInARg3 fCikcHzYYWKaXS6HABEBAAHCwXwEGAEIACYCGwwWIQSH6ZrVEpascJjzbVq59+x3yCm/lgUC Z8H0qQUJDIjuxgAKCRC59+x3yCm/loAdD/wJCOhPp9711J18B9c4f+eNAk5vrC9Cj3RyOusH Hebb9HtSFm155Zz3xiizw70MSyOVikjbTocFAJo5VhkyuN0QJIP678SWzriwym+EG0B5P97h FSLBlRsTi4KD8f1Ll3OT03lD3o/5Qt37zFgD4mCD6OxAShPxhI3gkVHBuA0GxF01MadJEjMu jWgZoj75rCLG9sC6L4r28GEGqUFlTKjseYehLw0s3iR53LxS7HfJVHcFBX3rUcKFJBhuO6Ha /GggRvTbn3PXxR5UIgiBMjUlqxzYH4fe7pYR7z1m4nQcaFWW+JhY/BYHJyMGLfnqTn1FsIwP dbhEjYbFnJE9Vzvf+RJcRQVyLDn/TfWbETf0bLGHeF2GUPvNXYEu7oKddvnUvJK5U/BuwQXy TRFbae4Ie96QMcPBL9ZLX8M2K4XUydZBeHw+9lP1J6NJrQiX7MzexpkKNy4ukDzPrRE/ruui yWOKeCw9bCZX4a/uFw77TZMEq3upjeq21oi6NMTwvvWWMYuEKNi0340yZRrBdcDhbXkl9x/o skB2IbnvSB8iikbPng1ihCTXpA2yxioUQ96Akb+WEGopPWzlxTTK+T03G2ljOtspjZXKuywV Wu/eHyqHMyTu8UVcMRR44ki8wam0LMs+fH4dRxw5ck69AkV+JsYQVfI7tdOu7+r465LUfg== In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 8/22/26 10:55 PM, Willem de Bruijn wrote: > Ilya Maximets wrote: >> On 8/22/26 2:09 AM, Jakub Kicinski wrote: >>> On Fri, 21 Aug 2026 23:45:41 +0200 Ilya Maximets wrote: >>>> Unfortunately, this needs a rebase now that a conflicting change >>>> for skb_zerocopy() was merged: >>> >>> Ugh, I was supposed to merge this first, wasn't I? Sorry. >> >> Not a huge deal, I guess, the conflict is mechanical and the patches >> are simple. I can take care of manual backports once we get the >> 'failed to apply' emails. Just a bit of busy work. >> >>> I was hoping for Willem to TAL since skb_tx_error() is a tx ZC >>> thing, now I realized that he wasn't CCed :S (please do so on v4) >> >> FWIW, I CCed a few people on v1 to have a conversation about a proper >> fix, but that wasn't fruitful. So, if I were Norbert, I wouldn't >> include them for the new versions either as doing so always feels like >> me being annoying. :) > > Having a look now. Thanks! > >> For now, the plan is to get v4 of these targeted fixes into net and >> stable and then remove skb_tx_error() entirely once net-next is open, >> as it seems to have lost all of its prior meaning. > > The original use case in tun_net_xmit introduced in commit > 149d36f7187c ("tun: report orphan frags errors to zero copy callback") > still exists. Not sure you can remove the function entirely. The skb_tx_error() prescribes to call kfree_skb() right after it and all the callers more or less do that (with the fixes applied). skb_tx_error() does two things: 1. skb_zcopy_downgrade_managed() that takes extra references on frags. 2. Calls skb_zcopy_clear(skb, true); The kfree_skb() called right after does: __kfree_skb skb_release_all skb_release_data if (skb_zcopy) bool skip_unref = shinfo->flags & SKBFL_MANAGED_FRAG_REFS; skb_zcopy_clear(skb, true); if (skip_unref) So, unless I'm missing something, the kfree_skb() already does everything that skb_tx_error() does. The fact that skb_tx_error() calls skb_zcopy_clear() with 'true' though feels weird. I would understand the need for the function, if it was actually signalling the error and not success. But you switched false to true in commit 1f8b977ab32d ("sock: enable MSG_ZEROCOPY") nine years ago and it seems like nobody complained so far... > > The bug is hit when this function is called with a cloned or shared > skb. The original zerocopy path through tun_net_xmit was probably > expected to not have this problem. > > Commit 0110d6f22f39 ("tun: orphan an skb on tx") explains why this > orphan in tun_net_xmit was needed: a (vhost) zerocopy skb injected in > tap1 arriving at tap2 and not being read there indefinitely. Such > loops are not allowed for zerocopy. > > Commit 868eefeb17d4 ("tun: orphan frags on xmit") then added the > frags orphan. Since an skb can be cloned on this tap to tap path, > e.g., with a packet socket, I think the bug goes back to the original > commit that introduced the first caller of skb_tx_error, commit > 149d36f7187c ("tun: report orphan frags errors to zero copy callback") > > If respinning it may be worthile to link to this thread.