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

Andrew Morton
2007-03-07T06:51:14Z

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