From: Jens Axboe <axboe@kernel.dk>
To: Alan Jenkins <alan.christopher.jenkins@gmail.com>,
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
Subject: Re: [PATCH 13/18] io_uring: add file set registration
Date: Tue, 12 Feb 2019 14:10:23 -0700 [thread overview]
Message-ID: <af6c2501-9efb-4177-74be-cd82b2ce7d6b@kernel.dk> (raw)
In-Reply-To: <b8555602-b2e7-c73b-b9bb-3b5f5569cfc7@gmail.com>
On 2/12/19 1:23 PM, Alan Jenkins wrote:
> On 12/02/2019 17:33, Jens Axboe wrote:
>> On 2/12/19 10:21 AM, Alan Jenkins wrote:
>>> 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.
>> How's this:
>>
>> http://git.kernel.dk/cgit/liburing/commit/?id=37e48698a09aa1e37690f8fa6dfd8da69a48ee60
>
> +.B EMFILE
> +.BR IORING_REGISTER_FILES
> +was specified and adding
> +.I nr_args
> +file references would exceed the maximum allowed number of files the process
> +is allowed to have according to the
> +.B
> +RLIMIT_NOFILE
> +resource limit and the caller does not have
> +.B CAP_SYS_RESOURCE
> +capability.
> +.TP
>
> I was struggling with this. The POSIX part of RLIMIT_NOFILE is applied
> per-process. But the part we're talking about here, the Linux-specific
> "unix_inflight" resource, is actually accounted per-user. It's like
> RLIMIT_NPROC. The value of RLIMIT_NPROC is per-process, but the
> resource it limits is counted in user->processes.
>
> This subtlety of the NOFILE limit is not made clear in the text above,
> nor in unix(7), nor in getrlimit(2). I would interpret all these docs
> as saying this limit is a per-process thing - I think they are misleading.
Fair point, I'll add an update to clearly state it's a per process
limit.
> IORING_MAX_FIXED_FILES is being raised to 1024, which is the same as the
> (soft limit) value for RLIMIT_NOFILE which the kernel sets for the init
> process. I have an unjustifiable nervousness, that there will be some
> `fio` command, or a test written that maxes out IORING_REGISTER_FILES.
> When you do that, it will provoke unexpected failures e.g. in GUI apps.
> If we can't rule that out, the next best thing is a friendly man page.
If we apply the limit to sendmsg and friends, it should be applied here
as well.
--
Jens Axboe
next prev parent reply other threads:[~2019-02-12 21:10 UTC|newest]
Thread overview: 73+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-02-07 19:55 [PATCHSET v12] io_uring IO interface Jens Axboe
2019-02-07 19:55 ` [PATCH 01/18] fs: add an iopoll method to struct file_operations Jens Axboe
2019-02-07 19:55 ` [PATCH 02/18] block: wire up block device iopoll method Jens Axboe
2019-02-07 19:55 ` [PATCH 03/18] block: add bio_set_polled() helper Jens Axboe
2019-02-07 19:55 ` [PATCH 04/18] iomap: wire up the iopoll method Jens Axboe
2019-02-07 19:55 ` [PATCH 05/18] Add io_uring IO interface Jens Axboe
2019-02-07 20:15 ` Keith Busch
2019-02-07 20:16 ` Jens Axboe
2019-02-07 19:55 ` [PATCH 06/18] io_uring: add fsync support Jens Axboe
2019-02-07 19:55 ` [PATCH 07/18] io_uring: support for IO polling Jens Axboe
2019-02-07 19:55 ` [PATCH 08/18] fs: add fget_many() and fput_many() Jens Axboe
2019-02-07 19:55 ` [PATCH 09/18] io_uring: use fget/fput_many() for file references Jens Axboe
2019-02-07 19:55 ` [PATCH 10/18] io_uring: batch io_kiocb allocation Jens Axboe
2019-02-07 19:55 ` [PATCH 11/18] block: implement bio helper to add iter bvec pages to bio Jens Axboe
2019-02-07 19:55 ` [PATCH 12/18] io_uring: add support for pre-mapped user IO buffers Jens Axboe
2019-02-07 20:57 ` Jeff Moyer
2019-02-07 21:02 ` Jens Axboe
2019-02-07 22:38 ` Jeff Moyer
2019-02-07 22:47 ` Jens Axboe
2019-02-07 19:55 ` [PATCH 13/18] io_uring: add file set registration Jens Axboe
2019-02-08 12:17 ` Alan Jenkins
2019-02-08 12:57 ` Jens Axboe
2019-02-08 14:02 ` Alan Jenkins
2019-02-08 15:13 ` Jens Axboe
2019-02-12 12:29 ` Alan Jenkins
2019-02-12 15:17 ` Jens Axboe
2019-02-12 17:21 ` Alan Jenkins
2019-02-12 17:33 ` Jens Axboe
2019-02-12 20:23 ` Alan Jenkins
2019-02-12 21:10 ` Jens Axboe [this message]
2019-02-07 19:55 ` [PATCH 14/18] io_uring: add submission polling Jens Axboe
2019-02-07 19:55 ` [PATCH 15/18] io_uring: add io_kiocb ref count Jens Axboe
2019-02-07 19:55 ` [PATCH 16/18] io_uring: add support for IORING_OP_POLL Jens Axboe
2019-02-07 22:12 ` Jeff Moyer
2019-02-07 22:18 ` Jens Axboe
2019-02-07 19:55 ` [PATCH 17/18] io_uring: allow workqueue item to handle multiple buffered requests Jens Axboe
2019-02-07 19:55 ` [PATCH 18/18] io_uring: add io_uring_event cache hit information Jens Axboe
-- strict thread matches above, loose matches on Subject: below --
2019-02-01 15:23 [PATCHSET v11] io_uring IO interface Jens Axboe
2019-02-01 15:24 ` [PATCH 13/18] io_uring: add file set registration Jens Axboe
2019-01-30 21:55 [PATCHSET v10] io_uring IO interface Jens Axboe
2019-01-30 21:55 ` [PATCH 13/18] io_uring: add file set registration Jens Axboe
2019-01-29 19:26 [PATCHSET v9] io_uring IO interface Jens Axboe
2019-01-29 19:26 ` [PATCH 13/18] io_uring: add file set registration Jens Axboe
2019-01-30 1:29 ` Jann Horn
2019-01-30 15:35 ` Jens Axboe
2019-02-04 2:56 ` Al Viro
2019-02-05 2:19 ` Jens Axboe
2019-02-05 17:57 ` Jens Axboe
2019-02-05 19:08 ` Jens Axboe
2019-02-06 0:27 ` Jens Axboe
2019-02-06 1:01 ` Al Viro
2019-02-06 17:56 ` Jens Axboe
2019-02-07 4:05 ` Al Viro
2019-02-07 16:14 ` Jens Axboe
2019-02-07 16:30 ` Al Viro
2019-02-07 16:35 ` Jens Axboe
2019-02-07 16:51 ` Al Viro
2019-02-06 0:56 ` Al Viro
2019-02-06 13:41 ` Jens Axboe
2019-02-07 4:00 ` Al Viro
2019-02-07 9:22 ` Miklos Szeredi
2019-02-07 13:31 ` Al Viro
2019-02-07 14:20 ` Miklos Szeredi
2019-02-07 15:20 ` Al Viro
2019-02-07 15:27 ` Miklos Szeredi
2019-02-07 16:26 ` Al Viro
2019-02-07 19:08 ` Miklos Szeredi
2019-02-07 18:45 ` Jens Axboe
2019-02-07 18:58 ` Jens Axboe
2019-02-11 15:55 ` Jonathan Corbet
2019-02-11 17:35 ` Al Viro
2019-02-11 20:33 ` Jonathan Corbet
2019-01-28 21:35 [PATCHSET v8] io_uring IO interface Jens Axboe
2019-01-28 21:35 ` [PATCH 13/18] io_uring: add file set registration Jens Axboe
2019-01-29 16:36 ` Jann Horn
2019-01-29 18:13 ` Jens Axboe
2019-01-23 15:35 [PATCHSET v7] io_uring IO interface Jens Axboe
2019-01-23 15:35 ` [PATCH 13/18] io_uring: add file set registration Jens Axboe
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