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[89.243.172.104]) by smtp.gmail.com with ESMTPSA id q8sm25955467wrr.9.2019.02.12.09.21.28 (version=TLS1_3 cipher=AEAD-AES128-GCM-SHA256 bits=128/128); Tue, 12 Feb 2019 09:21:28 -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> <1f1dc8b8-ad8f-16d1-c688-0ab1ce2df378@kernel.dk> From: Alan Jenkins Message-ID: <6c7ea7da-ab56-f1b8-2399-c0579b4eceec@gmail.com> Date: Tue, 12 Feb 2019 17:21:27 +0000 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.5.0 MIME-Version: 1.0 In-Reply-To: <1f1dc8b8-ad8f-16d1-c688-0ab1ce2df378@kernel.dk> 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 12/02/2019 15:17, Jens Axboe wrote: > On 2/12/19 5:29 AM, Alan Jenkins wrote: >> 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. > How about we just do something like the below? I think that's the saner > approach, rather than bypass user->unix_inflight. It's literally the > same thing. > > > diff --git a/fs/io_uring.c b/fs/io_uring.c > index a4973af1c272..5196b3aa935e 100644 > --- a/fs/io_uring.c > +++ b/fs/io_uring.c > @@ -2041,6 +2041,13 @@ static int __io_sqe_files_scm(struct io_ring_ctx *ctx, int nr, int offset) > struct sk_buff *skb; > int i; > > + if (!capable(CAP_SYS_RESOURCE) && !capable(CAP_SYS_ADMIN)) { > + struct user_struct *user = ctx->user; > + > + if (user->unix_inflight > task_rlimit(current, RLIMIT_NOFILE)) > + return -EMFILE; > + } > + > fpl = kzalloc(sizeof(*fpl), GFP_KERNEL); > if (!fpl) > return -ENOMEM; > > Welp, you gave me exactly what I asked for.  So now I'd better be positive about it :-D. I hope this will be documented accurately, at least where the EMFILE result is explained for this syscall. Because EMFILE is different from the errno in af_unix.c, I will add a wish for the existing documentation of ETOOMANYREFS in unix(7) to reference this. I'll stop bikeshedding there.  EMFILE sounds ok.  strerror() calls ETOOMANYREFS "Too many references: cannot splice"; it doesn't seem to be particularly helpful or well-known. Thanks Alan