Re: [PATCH v3] ipc: Modify message queue accounting to not take kernel data structures into account

[Davidlohr Bueso]

On Mon, 2015-07-06 at 17:49 +0200, Marcus Gelderie wrote:
> A while back, the message queue implementation in the kernel was
> improved to use btrees to speed up retrieval of messages (commit
> d6629859b36). The patch introducing the improved kernel handling of
> message queues (using btrees) has, as a by-product, changed the
> meaning of the QSIZE field in the pseudo-file created for the queue.
> Before, this field reflected the size of the user-data in the queue.
> Since, it also takes kernel data structures into account. For
> example, if 13 bytes of user data are in the queue, on my machine the
> file reports a size of 61 bytes.
> 
> There was some discussion on this topic before (for example
> https://lkml.org/lkml/2014/10/1/115). Commenting on a th lkml, Michael
> Kerrisk gave the following background (https://lkml.org/lkml/2015/6/16/74):
> 
>     The pseudofiles in the mqueue filesystem (usually mounted at
>     /dev/mqueue) expose fields with metadata describing a message
>     queue. One of these fields, QSIZE, as originally implemented,
>     showed the total number of bytes of user data in all messages in
>     the message queue, and this feature was documented from the
>     beginning in the mq_overview(7) page. In 3.5, some other (useful)
>     work happened to break the user-space API in a couple of places,
>     including the value exposed via QSIZE, which now includes a measure
>     of kernel overhead bytes for the queue, a figure that renders QSIZE
>     useless for its original purpose, since there's no way to deduce
>     the number of overhead bytes consumed by the implementation.
>     (The other user-space breakage was subsequently fixed.)

Michael, this breakage was never finally documented in the manpage,
right? I took a look and there is no mention, but it was a quick look.
It's just that if this patch goes in, I'd hate ending up with something
like this in the manpage:

as of 3.5
  <accounts for kernel overhead>

as of 4.3
  <behavior reverted back to not include kernel overhead... *sigh*>

If there are changes to be made to the manpage, it should probably be
posted with this patch, methinks.

> 
> This patch removes the accounting of kernel data structures in the
> queue. Reporting the size of these data-structures in the QSIZE field
> was a breaking change (see Michael's comment above). Without the QSIZE
> field reporting the total size of user-data in the queue, there is no
> way to deduce this number.
> 
> It should be noted that the resource limit RLIMIT_MSGQUEUE is counted
> against the worst-case size of the queue (in both the old and the new
> implementation). Therefore, the kernel overhead accounting in QSIZE is
> not necessary to help the user understand the limitations RLIMIT imposes
> on the processes.

Also, I would suggest adding some comment in struct mqueue_inode_info 
for future reference, ie:

-       unsigned long qsize; /* size of queue in memory (sum of all msgs) */
+       /*
+        * Size of queue in memory (sum of all msgs). Accounts for
+        * only userspace overhead; ignoring any in-kernel rbtree nodes.
+        */
+       unsigned long qsize;

But no big deal in any case.

I think this is the right approach, but would still like to know if Doug
has any concerns about it.

Thanks,
Davidlohr

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