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

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Message-Id: <E1HWGtr-0004ZN-00@dorka.pomaz.szeredi.hu>
Date:	Tue, 27 Mar 2007 21:00:07 +0200
From:	Miklos Szeredi <miklos@...redi.hu>
To:	linux@...izon.com
CC:	akpm@...ux-foundation.org, linux@...izon.com,
	linux-kernel@...r.kernel.org, linux-mm@...r.kernel.org
Subject: Re: [patch resend v4] update ctime and mtime for mmaped write

> > I agree that msync(MS_ASYNC) has no semantics if time is ignored.
> > But it's a useful way to tell the OS that the page is not going
> > to be dirtied again.
> 
> Just to clarify, here's the header comment for sys_msync():
> 
> /*
>  * MS_SYNC syncs the entire file - including mappings.
>  *
>  * MS_ASYNC does not start I/O (it used to, up to 2.5.67).
>  * Nor does it marks the relevant pages dirty (it used to up to 2.6.17).
>  * Now it doesn't do anything, since dirty pages are properly tracked.
>  *
>  * The application may now run fsync() to
>  * write out the dirty pages and wait on the writeout and check the result.
>  * Or the application may run fadvise(FADV_DONTNEED) against the fd to start
>  * async writeout immediately.
>  * So by _not_ starting I/O in MS_ASYNC we provide complete flexibility to
>  * applications.
>  */
> 
> It's actually wrong about FADV_DONTNEED, which I think doesn't start
> writeout either.  So there you have it ;)

Sorry, FADV_DONTNEED _does_ seem to start writeout, but it does it
indiscriminately, not just on the specified range.  That should be
easy to fix though.

Miklos
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