linux-kernel - Re: [PATCH 2/2] IPC: message queue stealing feature introduced

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Date:	Thu, 5 Apr 2012 11:59:11 +1200
From:	Michael Kerrisk <mtk.manpages@...il.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Stanislav Kinsbursky <skinsbursky@...allels.com>,
	serge.hallyn@...onical.com, criu@...nvz.org,
	lucas.demarchi@...fusion.mobi, linux-kernel@...r.kernel.org,
	cmetcalf@...era.com, dhowells@...hat.com,
	Arnd Bergmann <arnd@...db.de>,
	Linux API <linux-api@...r.kernel.org>
Subject: Re: [PATCH 2/2] IPC: message queue stealing feature introduced

Stanislav,

I reread your mail, and see I missed a point.

On Thu, Apr 5, 2012 at 11:50 AM, Michael Kerrisk (man-pages)
<mtk.manpages@...il.com> wrote:
> On Thu, Apr 5, 2012 at 11:12 AM, Andrew Morton
> <akpm@...ux-foundation.org> wrote:
>> On Wed, 15 Feb 2012 20:54:39 +0400
>> Stanislav Kinsbursky <skinsbursky@...allels.com> wrote:
>
> Stanislav,
>
> Luckily, Andrew added me to CC. However, as noted in
> Documentation/SubmitChecklist, all patches that change the ABI/API
> should CC linux-api@...r.kernel.org. Please do that for the future.
> (CC added for this thread.)
>
>>> v2:
>>> 1) compat functions added.
>>> 2) message slot size in array is now aligned by struct msgbuf_a.
>>> 3) check for enough free space in buffer before message copying added.
>>> 4) if MSG_STEAL flag is set, then do_msgrcv() returns number of bytes written
>>> to buffer.
>
> By the way, why the name "MSG_STEAL"? At first glance, it sounds like
> that means we're taking messages that belong to someone else. But I
> gather you are in fact just flushing all messages from a queue. So I
> more intuitive name seems to be in order (MSG_READ_ALL,
> MSG_FLUSH_ALL?).

Looking at what you say below, perhaps a better name for this flag is
MSG_PEEK_ALL (by analogy with recvmsg(2)'s MSG_PEEK_ALL)?

>>> 5) flag MSG_NOERROR is ignored if MSG_STEAL flag is set.
>
> Would it not be better to return an error if MSG_NOERROR is specified?
>
>>> This patch is required for checkpoint/restore in userspace.
>>> IOW, c/r requires some way to get all pending IPC messages without deleting
>>> them for the queue (checkpoint can fail and in this case tasks will be resumed,
>>> so queue have to be valid).
>>> To achive this, new operation flag MSG_STEAL for sys_msgrcv() system call
>>> introduced.
>>> If this flag is set, then passed struct msgbuf pointer will be used for storing
>>> array of structures:
>>>
>>> struct msgbuf_a {
>>>       long mtype;         /* type of message */
>>>       int msize;          /* size of message */
>>>       char mtext[0];      /* message text */
>>> };
>
> So, I'm not clear on how things look here. We get back array like this:
>
> [0]                     [1]                     [2]
> | mtype | msize | mtext | mtype | msize | mtext | ...
>
> How do I calculate the offset of element 1 of the array? I assume it
> can't be just
> (&item[0].mtext + msize), since there'll be alignment issues to deal
> with, so that there are padding bytes after each mtext. In that case,
> msize on its own seems insufficient (since there is no null-terminator
> in msize).
>
>>> each of which will be followed by corresponding message data.
>>>
>>
>> I'd be a bit more comfortable if there was some sign that other c/r
>> developers have reviewed and tested this and have successfully used it
>> in c/r operation testing?
>
> Indeed, do you have a userspace test program that you could post?
>
>> We've been trying to isolate the c/r-specific functions inside #ifdef
>> CONFIG_CHECKPOINT_RESTORE, but this patch doesn't do that.  I have been
>> encouraging this isolation so that people who aren't using c/r don't
>> have to carry the overhead it adds and so that we can more easily hunt
>> down and remove everything if the entire c/r project doesn't work out
>> successfully.
>>
>> This patch modifies the sys_msgrcv() API and so we should update the
>> manpage for that syscall.  Please work with Michael on this.
>
> Yes please.
>
> Cheers,
>
> Michael

Thanks,

Michael

-- 
Michael Kerrisk Linux man-pages maintainer;
http://www.kernel.org/doc/man-pages/
Author of "The Linux Programming Interface", http://blog.man7.org/
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