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    SubjectRe: [patch 4/6] mm: merge populate and nopage into fault (fixes nonlinear)
    On Tue, Mar 06, 2007 at 10:51:01PM -0800, Andrew Morton wrote:
    > 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?

    I cc'ed him when first posting it. He didn't disagree.

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

    Yes.

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

    OK. This is actually something that I would like more people to review.
    Do we need any different fields? Should it be passed as arguments instead
    of a structure?

    > 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.

    Yes we should. Not all those filesystem changes have been tested.

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

    Dunno, it's exported. I remove that completely in a subsequent patch
    anyway.

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

    Well it fixes the whole design of the nonlinear fault path.
    -
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