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

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Date:	Tue, 6 Mar 2007 22:51:01 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Nick Piggin <npiggin@...e.de>
Cc:	Linux Memory Management <linux-mm@...ck.org>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [patch 4/6] mm: merge populate and nopage into fault (fixes
 nonlinear)

On Wed, 21 Feb 2007 05:50:17 +0100 (CET) Nick Piggin <npiggin@...e.de> wrote:

> Nonlinear mappings are (AFAIKS) simply a virtual memory concept that
> encodes the virtual address -> file offset differently from linear
> mappings.
> 
> I can't see why the filesystem/pagecache code should need to know anything
> about it, except for the fact that the ->nopage handler didn't quite pass
> down enough information (ie. pgoff). But it is more logical to pass pgoff
> rather than have the ->nopage function calculate it itself anyway. And
> having the nopage handler install the pte itself is sort of nasty.
> 
> This patch introduces a new fault handler that replaces ->nopage and
> ->populate and (later) ->nopfn. Most of the old mechanism is still in place
> so there is a lot of duplication and nice cleanups that can be removed if
> everyone switches over.
> 
> The rationale for doing this in the first place is that nonlinear mappings
> are subject to the pagefault vs invalidate/truncate race too, and it seemed
> stupid to duplicate the synchronisation logic rather than just consolidate
> the two.
> 

It's awkward to layer a largely do-nothing patch like this on top of a
significant functional change.  Makes it harder to isolate the source of
regressions, harder to revert the do-something patch.

> After this patch, MAP_NONBLOCK no longer sets up ptes for pages present in
> pagecache. Seems like a fringe functionality anyway.

Does Ingo agree?

> NOPAGE_REFAULT is removed. This should be implemented with ->fault, and
> no users have hit mainline yet.

Did benh agree with that?


The patch unchangeloggedly adds a basic new structure to core mm
(fault_data).  Would be nice to document its fields, especially `flags'.


Please add less pointless blank lines.


How well has this been tested?  The ocfs2 changes?  gfs2?  We should at
least give those guys a heads-up.


Does anybody really pass a NULL `type' arg into filemap_nopage()?


This patch seems to churn things around an awful lot for minimal benefit.

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