linux-kernel - Re: [patch resend v4] update ctime and mtime for mmaped write

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Date:	Tue, 27 Mar 2007 12:34:22 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	linux@...izon.com
Cc:	miklos@...redi.hu, linux-kernel@...r.kernel.org,
	linux-mm@...r.kernel.org
Subject: Re: [patch resend v4] update ctime and mtime for mmaped write

On 27 Mar 2007 15:24:52 -0400
linux@...izon.com wrote:

> > * MS_ASYNC does not start I/O (it used to, up to 2.5.67).
> 
> Yes, I noticed.  See
> http://www.ussg.iu.edu/hypermail/linux/kernel/0602.1/0450.html
> for a bug report on the subject February 2006.

Suggest you use msync(MS_ASYNC), then
sync_file_range(SYNC_FILE_RANGE_WAIT_BEFORE|SYNC_FILE_RANGE_WRITE).

The new (post 2.6.17) MAP_SHARED dirty-page semantics mean that the msync()
isn't actually needed.

> That's why this application is still running on 2.4.
> 
> As I mentioned at the time, the SUS says:
> (http://opengroup.org/onlinepubs/007908799/xsh/msync.html)
> "When MS_ASYNC is specified, msync() returns immediately once all the
> write operations are initiated or queued for servicing."
> 
> You can argue that putting it on the dirty list constitutes "queued for
> servicing", but the intent seems pretty clear to me: MS_ASYNC is supposed
> to start the I/O.  Although strict standards-ese parsing says that
> either branch of an or is acceptable, it is a common English language
> convention that the first alternative is preferred and the second
> is a fallback.
> 
> It makes sense in this case: start the write or, if that's not possible
> (the disk is already busy), queue it for service as soon as the disk
> is available.
> 
> They perhaps didn't mandate it this strictly, but that's clearly the
> intent.

We can fix your application, and we'll break someone else's.

I don't think it's solveable, really - the range of applications is so
broad, and the "standard" is so vague as to be useless.  This is why we've
been extending these things with linux-specific goodies which permit
applications to actually tell the kernel what they want to be done in a
more finely-grained fashion.
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