Re: Document POSIX MQ /proc/sys/fs/mqueue files

[Michael Kerrisk (man-pages)]

Hi Doug,

On Mon, Sep 29, 2014 at 7:28 PM, Doug Ledford <dledford@xxxxxxxxxx> wrote:
> On Mon, 2014-09-29 at 11:10 +0200, Michael Kerrisk (man-pages) wrote:
>> Hello Doug, David,
>>
>> I think you two were the last ones to make significant
>> changes to the semantics of the files in /proc/sys/fs/mqueue,
>> so I wonder if you (or anyone else who is willing) might
>> take a look at the man page text below that I've written
>> (for the mq_overview(7) page) to describe past and current
>> reality, and let me know of improvements of corrections.
>>
>> By the way, Doug, your commit ce2d52cc1364 appears to have
>> changed/broken the semantics of the files in the /dev/mqueue
>> filesystem. Formerly, the QSIZE field in these files showed
>> the number of bytes of real user data in all of the queued
>> messages. After that commit, QSIZE now includes kernel
>> overhead bytes, which does not seem very useful for user
>> space. Was that change intentional? I see no mention of the
>> change in the commit message, so it sounds like it was not
>> intended.
>
> That change didn't come in that commit.  That commit modified it, but
> didn't introduce it.

(Which commit was it then? d6629859b36 ?)

> Now, was it intentional? Yes.  Is it valuable, useful?  That depends on
> your perspective.

Thanks for the detailed explanation below. However, I don't understand
why the (useful) work that you describe below necessitated a change in
the QSIZE value that was exposed to user space. Surely the necessary
changes could have been done internally while still leaving QSIZE to
expose the same value it ever did? As things stand now (and unless I
am missing something), QSIZE exposes an implementation-specific
internal value that has little meaning or value to user space. And,
it's unfortunate that the commit message made no mention of the fact
that there was an ABI change here.

[...]

> The man page below looks fine to me.

Thanks for checking it!

Cheers,

Michael


> It covers the various
> incarnations.  If I add some tweaks to the priorities value though, it
> will need updating again ;-)
>
> Although this section wasn't included below, I would update how the
> memory is calculated to match what I wrote above.  However, I would also
> put in a notation that the calculation can change when the kernel's
> internal implementation changes and resource usage therefore changes.
>
>> Cheers,
>>
>> Michael
>>
>> From mq_overview(7) draft:
>>
>>    /proc interfaces
>>        The following interfaces can be used to limit the amount of  kerâ
>>        nel  memory  consumed  by  POSIX  message  queues  and to set the
>>        default attributes for new message queues:
>>
>>        /proc/sys/fs/mqueue/msg_default (since Linux 3.5)
>>               This file  defines  the  value  used  for  a  new  queue's
>>               mq_maxmsg setting when the queue is created with a call to
>>               mq_open(3) where attr is specified as NULL.   The  default
>>               value for this file is 10.  The minimum and maximum are as
>>               for /proc/sys/fs/mqueue/msg_max.  If  msg_default  exceeds
>>               msg_max,  a  new queue's default mq_maxmsg value is capped
>>               to the msg_max limit.  Up until Linux 2.6.28, the  default
>>               mq_maxmsg  was  10;  from  Linux  2.6.28 to Linux 3.4, the
>>               default was the value defined for the msg_max limit.
>>
>>        /proc/sys/fs/mqueue/msg_max
>>               This file can be used to view and change the ceiling value
>>               for the maximum number of messages in a queue.  This value
>>               acts as a ceiling on the attr->mq_maxmsg argument given to
>>               mq_open(3).   The  default  value  for msg_max is 10.  The
>>               minimum value is 1 (10 in  kernels  before  2.6.28).   The
>>               upper  limit is HARD_MSGMAX.  The msg_max limit is ignored
>>               for  privileged  processes  (CAP_SYS_RESOURCE),  but   the
>>               HARD_MSGMAX ceiling is nevertheless imposed.
>>
>>               The  definition  of  HARD_MSGMAX has changed across kernel
>>               versions:
>>
>>               *  Up to Linux 2.6.32: 131072 / sizeof(void *)
>>
>>               *  Linux 2.6.33 to 3.4: (32768 * sizeof(void *) / 4)
>>
>>               *  Since Linux 3.5: 65,536
>>
>>        /proc/sys/fs/mqueue/msgsize_default (since Linux 3.5)
>>               This file defines the value used for a new queue's mq_msgâ
>>               size  setting  when  the  queue  is created with a call to
>>               mq_open(3) where attr is specified as NULL.   The  default
>>               value  for this file is 8192.  The minimum and maximum are
>>               as   for   /proc/sys/fs/mqueue/msgsize_max.     If    msgâ
>>               size_default  exceeds  msgsize_max,  a new queue's default
>>               mq_msgsize value is capped to the msgsize_max  limit.   Up
>>               until  Linux 2.6.28, the default mq_msgsize was 8192; from
>>               Linux 2.6.28 to Linux  3.4,  the  default  was  the  value
>>               defined for the msgsize_max limit.
>>
>>        /proc/sys/fs/mqueue/msgsize_max
>>               This  file  can  be used to view and change the ceiling on
>>               the maximum message size.  This value acts as a ceiling on
>>               the  attr->mq_msgsize  argument  given to mq_open(3).  The
>>               default value for msgsize_max is 8192 bytes.  The  minimum
>>               value  is  128 (8192 in kernels before 2.6.28).  The upper
>>               limit for msgsize_max has varied across kernel versions:
>>
>>               *  Before Linux 2.6.28, the upper limit is INT_MAX.
>>
>>               *  From Linux 2.6.28 to 3.4, the limit is 1,048,576.
>>
>>               *  Since Linux 3.5, the limit is 16,777,216 (HARD_MSGSIZEâ
>>                  MAX).
>>
>>               The  msgsize_max  limit  is ignored for privileged process
>>               (CAP_SYS_RESOURCE), but, since Linux  3.5,  the  HARD_MSGâ
>>               SIZEMAX ceiling is enforced for privileged processes.
>>
>>        /proc/sys/fs/mqueue/queues_max
>>               This  file  can be used to view and change the system-wide
>>               limit on the number of message queues that can be created.
>>               The default value for queues_max is 256.  The semantics of
>>               this limit have changed across kernel versions as follows:
>>
>>               *  Before Linux 3.5, this limit could be  changed  to  any
>>                  value  in  the  range 0 to INT_MAX, but privileged proâ
>>                  cesses (CAP_SYS_RESOURCE) can exceed the limit.
>>
>>               *  Since Linux 3.5, there is a ceiling for this  limit  of
>>                  1024     (HARD_QUEUESMAX).      Privileged    processes
>>                  (CAP_SYS_RESOURCE) can exceed the queues_max limit, but
>>                  the  HARD_QUEUESMAX  limit  is enforced even for priviâ
>>                  leged processes.
>>
>>               *  Starting with Linux 3.14, the HARD_QUEUESMAX ceiling is
>>                  removed: no ceiling is imposed on the queues_max limit,
>>                  and privileged processes (CAP_SYS_RESOURCE) can  exceed
>>                  the limit.
>>
>
>
> --
> Doug Ledford <dledford@xxxxxxxxxx>
>               GPG KeyID: 0E572FDD
>
>



-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
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