linux-kernel - Re: sync_file_range(SYNC_FILE_RANGE

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Message-Id: <200806021318.26404.rjw@sisk.pl>
Date:	Mon, 2 Jun 2008 13:18:25 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Hugh Dickins <hugh@...itas.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Pavel Machek <pavel@...e.cz>, mtk.manpages@...il.com,
	kernel list <linux-kernel@...r.kernel.org>
Subject: Re: sync_file_range(SYNC_FILE_RANGE_WRITE) blocks?

On Monday, 2 of June 2008, Hugh Dickins wrote:
> On Sun, 1 Jun 2008, Andrew Morton wrote:
> > On Mon, 2 Jun 2008 01:00:40 +0200 Pavel Machek <pavel@...e.cz> wrote:
> > > > How about this:
> > > > 
> > > > - Add a new SYNC_FILE_RANGE_NON_BLOCKING
> > > > 
> > > > - If userspace set that flag, turn on writeback_control.nonblocking
> > > >   in __filemap_fdatawrite_range().
> > > > 
> > > > - test it a lot.
> > > 
> > > Works for me. Is the expectation that I code this? I can certainly
> > > provide testing ;-).
> > 
> > Something like this:
> 
> Though this fits very easily into the current kernel implementation,
> I don't think it's the right interface for userspace.
> 
> If we do go this kind of a way, then I'd say SYNC_FILE_RANGE_NON_BLOCKING
> needs to tell the caller how far it got before giving up, rather than just
> success or failure.  Why? um, um, because it feels right; and would help
> the caller help the kernel by not overloading it with needlessly repeated
> loop ranges - any stronger reasons?  But sync_file_range() was defined
> to return int rather than ssize_t, so that becomes awkward.
> 
> Never mind, I don't think it is the right way anyway.  We don't need
> additions to the existing sync_file_range() interface, we just need it
> to behave as naive people like Pavel and I expected it to behave in the
> first place: SYNC_FILE_RANGE_WRITE should be nonblocking (with respect
> to queue congestion, and maybe page locking also).

Well, frankly, I'm not sure if we need anything better than we already have.
In fact my numbers show that SYNC_FILE_RANGE_WRITE works quite well -
please see
http://sourceforge.net/mailarchive/message.php?msg_name=200806020122.36193.rjw%40sisk.pl
"early writeout" means that SYNC_FILE_RANGE_WRITE is used and the file with
the results is attached for convenience.

My interpretation of the results is here:
http://sourceforge.net/mailarchive/forum.php?thread_name=200806021238.17100.rjw%40sisk.pl&forum_name=suspend-devel

Thanks,
Rafael

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