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[89.243.172.104]) by smtp.gmail.com with ESMTPSA id a20sm1831141wmj.39.2019.02.12.04.29.30 (version=TLS1_3 cipher=AEAD-AES128-GCM-SHA256 bits=128/128); Tue, 12 Feb 2019 04:29:31 -0800 (PST) Subject: Re: [PATCH 13/18] io_uring: add file set registration To: Jens Axboe , linux-aio@kvack.org, linux-block@vger.kernel.org, linux-api@vger.kernel.org Cc: hch@lst.de, jmoyer@redhat.com, avi@scylladb.com, jannh@google.com, viro@ZenIV.linux.org.uk References: <20190207195552.22770-1-axboe@kernel.dk> <20190207195552.22770-14-axboe@kernel.dk> <2ac73020-6ab0-e351-3a1a-180d0f1f801b@kernel.dk> <02e71636-5b63-41e6-0ffd-646f305011c9@gmail.com> From: Alan Jenkins Message-ID: Date: Tue, 12 Feb 2019 12:29:29 +0000 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.4.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Content-Language: en-GB Sender: linux-block-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org On 08/02/2019 15:13, Jens Axboe wrote: > On 2/8/19 7:02 AM, Alan Jenkins wrote: >> On 08/02/2019 12:57, Jens Axboe wrote: >>> On 2/8/19 5:17 AM, Alan Jenkins wrote: >>>>> +static int io_sqe_files_scm(struct io_ring_ctx *ctx) >>>>> +{ >>>>> +#if defined(CONFIG_NET) >>>>> + struct scm_fp_list *fpl = ctx->user_files; >>>>> + struct sk_buff *skb; >>>>> + int i; >>>>> + >>>>> + skb = __alloc_skb(0, GFP_KERNEL, 0, NUMA_NO_NODE); >>>>> + if (!skb) >>>>> + return -ENOMEM; >>>>> + >>>>> + skb->sk = ctx->ring_sock->sk; >>>>> + skb->destructor = unix_destruct_scm; >>>>> + >>>>> + fpl->user = get_uid(ctx->user); >>>>> + for (i = 0; i < fpl->count; i++) { >>>>> + get_file(fpl->fp[i]); >>>>> + unix_inflight(fpl->user, fpl->fp[i]); >>>>> + fput(fpl->fp[i]); >>>>> + } >>>>> + >>>>> + UNIXCB(skb).fp = fpl; >>>>> + skb_queue_head(&ctx->ring_sock->sk->sk_receive_queue, skb); >>>> This code sounds elegant if you know about the existence of unix_gc(), >>>> but quite mysterious if you don't. (E.g. why "inflight"?) Could we >>>> have a brief comment, to comfort mortal readers on their journey? >>>> >>>> /* A message on a unix socket can hold a reference to a file. This can >>>> cause a reference cycle. So there is a garbage collector for unix >>>> sockets, which we hook into here. */ >>> Yes that's a good idea, I've added a comment as to why we go through the >>> trouble of doing this socket + skb dance. >> Great, thanks. >> >>>> I think this is bypassing too_many_unix_fds() though? I understood that >>>> was intended to bound kernel memory allocation, at least in principle. >>> As the code stands above, it'll cap it at 253. I'm just now reworking it >>> to NOT be limited to the SCM max fd count, but still impose a limit of >>> 1024 on the number of registered files. This is important to cap the >>> memory allocation attempt as well. >> I saw you were limiting to SCM_MAX_FD per io_uring. On the other hand, >> there's no specific limit on the number of io_urings you can open (only >> the standard limits on fds). So this would let you allocate hundreds of >> times more files than the previous limit RLIMIT_NOFILE... > But there is, the io_uring itself is under the memlock rlimit. > >> static inline bool too_many_unix_fds(struct task_struct *p) >> { >> struct user_struct *user = current_user(); >> >> if (unlikely(user->unix_inflight > task_rlimit(p, RLIMIT_NOFILE))) >> return !capable(CAP_SYS_RESOURCE) && !capable(CAP_SYS_ADMIN); >> return false; >> } >> >> RLIMIT_NOFILE is technically per-task, but here it is capping >> unix_inflight per-user. So the way I look at this, the number of file >> descriptors per user is bounded by NOFILE * NPROC. Then >> user->unix_inflight can have one additional process' worth (NOFILE) of >> "inflight" files. (Plus SCM_MAX_FD slop, because too_many_fds() is only >> called once per SCM_RIGHTS). >> >> Because io_uring doesn't check too_many_unix_fds(), I think it will let >> you have about 253 (or 1024) more process' worth of open files. That >> could be big proportionally when RLIMIT_NPROC is low. >> >> I don't know if it matters. It maybe reads like an oversight though. >> >> (If it does matter, it might be cleanest to change too_many_unix_fds() >> to get rid of the "slop". Since that may be different between af_unix >> and io_uring; 253 v.s. 1024 or whatever. E.g. add a parameter for the >> number of inflight files we want to add.) > I don't think it matters. The files in the fixed file set have already > been opened by the application, so it counts towards the number of open > files that is allowed to have. I don't think we should impose further > limits on top of that. A process can open one io_uring and 199 other files.  Register the 199 files in the io_uring, then close their file descriptors.  The main NOFILE limit only counts file descriptors.  So then you can open one io_uring, 198 other files, and repeat. You're right, I had forgotten the memlock limit on io_uring.  That makes it much less of a practical problem. But it raises a second point.  It's not just that it lets users allocate more files.  You might not want to be limited by user->unix_inflight.  But you are calling unix_inflight(), which increments it!  Then if unix->inflight exceeds the NOFILE limit, you will avoid seeing any errors with io_uring, but the user will not be able to send files over unix sockets. So I think this is confusing to read, and confusing to troubleshoot if the limit is ever hit. I would be happy if io_uring didn't increment user->unix_inflight.  I'm not sure what the best way is to arrange that. Regards Alan