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)" [off-list ref] writes:

quoted

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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