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

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Message-ID: <20140404140732.GG10526@twins.programming.kicks-ass.net>
Date:	Fri, 4 Apr 2014 16:07:32 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
Cc:	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>,
	Greg Troxel <gdt@...bbn.com>, bug-readline@....org
Subject: Re: [PATCH] mm: msync: require either MS_ASYNC or MS_SYNC [resend]

On Fri, Apr 04, 2014 at 09:12:58AM +0200, Michael Kerrisk (man-pages) wrote:
> >   * Clearer intentions.  Looking at the existing code and the code
> >     history, the fact that flags=0 behaves like flags=MS_ASYNC appears
> >     to be a coincidence, not the result of an intentional choice.
> 
> Maybe. You earlier asserted that the semantics when flags==0 may have
> been different, prior to Peter Zijstra's patch,
> https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=204ec841fbea3e5138168edbc3a76d46747cc987
> .
> It's not clear to me that that is the case. But, it would be wise to
> CC the developer, in case he has an insight.

Right; so before that patch there appears to have been a difference.
The code looked like:

  if (flags & MS_ASYNC) {
  	balance_dirty_pages_ratelimited();
  } else if (flags & MS_SYNC) {
  	do_fsync()
  } else {
  	/* do nothing */
  }

Which would give the following semantics:

  msync(.flags = 0) -- scan PTEs and update dirty page accounting
  msync(.flags = MS_ASYNC) -- scan PTEs and dirty throttle
  msync(.flags = MS_SYNC) -- scan PTEs and flush dirty pages

However with the introduction of accurate dirty page accounting in
.19 we always had an accurate dirty page count and both .flags=0 and
.flags=MS_ASYNC turn into the same NO-OP.

Yielding todays state, where 0 and MS_ASYNC don't do anything much and
MS_SYNC issues the fsync() -- although I understand Willy recently
posted a patch to do a data-range-sync instead of the full fsync.


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