linux-kernel - Re: [PATCH] mm: msync: require either MS_ASYNC or MS

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Date:	Fri, 04 Apr 2014 09:11:19 +0200
From:	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
To:	Greg Troxel <gdt@...bbn.com>
CC:	mtk.manpages@...il.com, Richard Hansen <rhansen@....com>,
	Steven Whitehouse <swhiteho@...hat.com>,
	Christoph Hellwig <hch@...radead.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	lkml <linux-kernel@...r.kernel.org>,
	Linux API <linux-api@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH] mm: msync: require either MS_ASYNC or MS_SYNC

Hi Greg,

On 04/03/2014 02:57 PM, Greg Troxel wrote:
> 
> "Michael Kerrisk (man-pages)" <mtk.manpages@...il.com> writes:
> 
>> I think the only reasonable solution is to better document existing
>> behavior and what the programmer should do. With that in mind, I've
>> drafted the following text for the msync(2) man page:
>>
>>     NOTES
>>        According to POSIX, exactly one of MS_SYNC and MS_ASYNC  must  be
>>        specified  in  flags.   However,  Linux permits a call to msync()
>>        that specifies neither of these flags, with  semantics  that  are
>>        (currently)  equivalent  to  specifying  MS_ASYNC.   (Since Linux
>>        2.6.19, MS_ASYNC is in fact a no-op, since  the  kernel  properly
>>        tracks  dirty  pages  and  flushes them to storage as necessary.)
>>        Notwithstanding the Linux behavior, portable, future-proof appli‐
>>        cations  should  ensure  that they specify exactly one of MS_SYNC
>>        and MS_ASYNC in flags.
>>
>> Comments on this draft welcome.
> 
> I think it's a step backwards to document unspecified behavior.  If
> anything, the man page should make it clear that providing neither flag
> results in undefined behavior and will lead to failure on systems on
> than Linux.  While I can see the point of not changing the previous
> behavior to protect buggy code, there's no need to document it in the
> man page and further enshrine it.

The Linux behavior is what it is. For the reasons I mentioned already,
it's unlikely to change. And, when the man pages omit to explain what
Linux actually does, there will one day come a request to actually
document the behavior. So, I don't think it's quite enough to say the 
behavior is undefined. On the other hand, it does not hurt to further
expand the portability warning. I made the text now:

    NOTES
       According to POSIX, either MS_SYNC or MS_ASYNC must be  specified
       in  flags, and  indeed failure to include one of these flags will
       cause msync() to fail on some systems.  However, Linux permits  a
       call  to  msync()  that  specifies  neither  of these flags, with
       semantics that are (currently) equivalent to specifying MS_ASYNC.
       (Since  Linux 2.6.19, MS_ASYNC is in fact a no-op, since the ker‐
       nel properly tracks dirty pages and flushes them  to  storage  as
       necessary.)    Notwithstanding   the  Linux  behavior,  portable,
       future-proof applications should ensure that they specify  either
       MS_SYNC or MS_ASYNC in flags.




-- 
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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