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Mon, 26 Feb 2024 06:52:47 -0800 (PST) Received: from [192.168.1.116] ([96.43.243.2]) by smtp.gmail.com with ESMTPSA id m8-20020a056638270800b00474420a484esm1292694jav.98.2024.02.26.06.52.47 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 26 Feb 2024 06:52:47 -0800 (PST) Message-ID: Date: Mon, 26 Feb 2024 07:52:46 -0700 Precedence: bulk X-Mailing-List: io-uring@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 8/8] io_uring/net: set MSG_MORE if we're doing multishot send and have more Content-Language: en-US To: Pavel Begunkov , Dylan Yudaken Cc: io-uring@vger.kernel.org References: <20240225003941.129030-1-axboe@kernel.dk> <20240225003941.129030-9-axboe@kernel.dk> From: Jens Axboe In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 2/26/24 7:24 AM, Pavel Begunkov wrote: > On 2/26/24 13:42, Jens Axboe wrote: >> On 2/26/24 3:59 AM, Dylan Yudaken wrote: >>> On Sun, Feb 25, 2024 at 12:46?AM Jens Axboe wrote: >>>> >>>> If we have more data pending, we know we're going to do one more loop. >>>> If that's the case, then set MSG_MORE to inform the networking stack >>>> that there's more data coming shortly for this socket. >>>> >>>> Signed-off-by: Jens Axboe >>>> --- >>>> io_uring/net.c | 10 +++++++--- >>>> 1 file changed, 7 insertions(+), 3 deletions(-) >>>> >>>> diff --git a/io_uring/net.c b/io_uring/net.c >>>> index 240b8eff1a78..07307dd5a077 100644 >>>> --- a/io_uring/net.c >>>> +++ b/io_uring/net.c >>>> @@ -519,6 +519,10 @@ int io_sendmsg(struct io_kiocb *req, unsigned int issue_flags) >>>> if (!io_check_multishot(req, issue_flags)) >>>> return io_setup_async_msg(req, kmsg, issue_flags); >>>> >>>> + flags = sr->msg_flags; >>>> + if (issue_flags & IO_URING_F_NONBLOCK) >>>> + flags |= MSG_DONTWAIT; >>>> + >>>> retry_multishot: >>>> if (io_do_buffer_select(req)) { >>>> void __user *buf; >>>> @@ -528,12 +532,12 @@ int io_sendmsg(struct io_kiocb *req, unsigned int issue_flags) >>>> if (!buf) >>>> return -ENOBUFS; >>>> >>>> + if ((req->flags & (REQ_F_BL_EMPTY|REQ_F_APOLL_MULTISHOT)) == >>>> + REQ_F_APOLL_MULTISHOT) >>>> + flags |= MSG_MORE; >>>> iov_iter_ubuf(&kmsg->msg.msg_iter, ITER_SOURCE, buf, len); >>>> } >>> >>> This feels racy. I don't have an exact sequence in mind, but I believe >>> there are cases where between >>> the two calls to __sys_sendmsg_sock, another submission could be >>> issued and drain the buffer list. >>> I guess the result would be that the packet is never sent out, but I >>> have not followed the codepaths of MSG_MORE. >> >> This is true, but that race always exists depending on how gets to go >> first (the adding of the buffer, or the send itself). The way I see it, >> when the send is issued we're making the guarantee that we're going to >> at least deplete the queue as it looks when entered. If more is added >> while it's being processed, we _may_ see it. >> >> Outside of that, we don't want it to potentially run in perpetuity. It >> may actually be a good idea to make the rule of "just issue what was >> there when first seen/issued" a hard one, though I don't think it's >> really worth doing. But making any guarantees on buffers added in >> parallel will be impossible. If you do that, then you have to deal with >> figuring out what's left in the queue once you get a completion withou >> CQE_F_MORE. >> >>> The obvious other way to trigger this codepath is if the user messes >>> with the ring by decrementing >>> the buffer counter. I do not believe there are any nefarious outcomes >>> - but just to point out that >>> REQ_F_BL_EMPTY is essentially user controlled. >> >> The user may certainly shoot himself in the foot. As long as that >> doesn't lead to a nefarious outcome, then that's not a concern. For this >> case, the head is kernel local, user can only write to the tail. So we >> could have a case of user fiddling with the tail and when we grab the >> next buffer (and the previous one did not have REQ_F_BL_EMPTY set), the >> ring will indeed appear to be empty. At that point you get an -ENOBUFS >> without CQE_F_MORE set. > > A side note, don't forget that there are other protocols apart > from TCP. AFAIK UDP corking will pack it into a single datagram, > which is not the same as two separate sends. Yeah, should really have labeled this one as a test/rfc kind of patch. I wasn't even convinced we want to do this uncondtionally for TCP. I'll just leave it at the end for now, it's a separate kind of discussion imho and this is why it was left as a separate patch rather than being bundled with the multishot send in general. -- Jens Axboe