linux-kernel - Re: [patch 4/6] mm: merge populate and nopage into fault (fixes nonlinear)

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Message-Id: <20070307012638.793d9a9f.akpm@linux-foundation.org>
Date:	Wed, 7 Mar 2007 01:26:38 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Nick Piggin <npiggin@...e.de>
Cc:	Miklos Szeredi <miklos@...redi.hu>, mingo@...e.hu,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	benh@...nel.crashing.org, Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [patch 4/6] mm: merge populate and nopage into fault (fixes
 nonlinear)

On Wed, 7 Mar 2007 10:18:23 +0100 Nick Piggin <npiggin@...e.de> wrote:

> On Wed, Mar 07, 2007 at 01:07:56AM -0800, Andrew Morton wrote:
> > On Wed, 07 Mar 2007 09:51:57 +0100 Miklos Szeredi <miklos@...redi.hu> wrote:
> > 
> > > > > Dirty page accounting doesn't work either on
> > > > > non-linear mappings
> > > > 
> > > > It doesn't?  Confused - these things don't have anything to do with each
> > > > other do they?
> > > 
> > > Look in page_mkclean().  Where does it handle non-linear mappings?
> > > 
> > 
> > OK, I'd forgotten about that.  It won't break dirty memory accounting,
> > but it'll potentially break dirty memory balancing.
> > 
> > If we have the wrong page (due to nonlinear), page_check_address() will
> > fail and we'll leave the pte dirty.  That puts us back to the pre-2.6.17
> > algorithms and I guess it'll break the msync guarantees.
> > 
> > Peter, I thought we went through the nonlinear problem ages ago and decided
> > it was OK?
> 
> msync breakage is bad, but otherwise I don't know that we care about
> dirty page writeout efficiency.

Well.  We made so many changes to support the synchronous
dirty-the-page-when-we-dirty-the-pte thing that I'm rather doubtful that
the old-style approach still works.  It might seem to, most of the time. 
But if it _is_ subtly broken, boy it's going to take a long time for us to
find out.

> But I think we discovered that those msync changes are bogus anyway
> becuase there is a small race window where pte could be dirtied without
> page being set dirty?

Dunno, I don't recall that.  We dirty the page before the pte...
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