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    SubjectRe: MAP_POPULATE behavior change?
    On Mon, Apr 28, 2008 at 02:07:17PM +0200, Michael Kerrisk wrote:
    > Hey Nick,
    >
    > Ping!
    >
    > Cheers,
    >
    > Michael
    >
    >
    > -------- Original Message --------
    > Subject: MAP_POPULATE behavior change?
    > Date: Tue, 22 Apr 2008 15:16:19 +0200
    > From: Michael Kerrisk <mtk.manpages@gmail.com>
    > To: Nick Piggin <npiggin@suse.de>
    > CC: lkml <linux-kernel@vger.kernel.org>, mk <michael.kerrisk@gmail.com>
    >
    > Hi Nick,
    >
    > In 2.6.23, you had the commit below, which seems to have changed the
    > kernel-userland API.
    >
    > Am I right to understand that with this patch MAP_POPULATE now also works with
    > MAP_PRIVATE mappings? (Formerly it only worked with MAP_SHARED mappings, but
    > some very quick testing suggests that MAP_POPULATE now also has an affect for
    > MAP_PRIVATE.)

    Yeah it should also set up anonymous mappings. Very observant :)


    > Also, can you summarize the status of MAP_NONBLOCK? Is it now a no-op?

    Yes, it is. The problem is you need to walk the page tables, and it
    is fairly annoying to have to make a special case for that to be
    non blocking (after just getting rid of the ->populate special case).

    It is possible of course to implement. It might even be easier because
    you could pass in a NONBLOCK flag to ->fault from get_user_pages... but
    for now it is a noop.

    Thanks,
    Nick

    >
    > Cheers,
    >
    > Michael
    >
    > ==
    >
    > commit 54cb8821de07f2ffcd28c380ce9b93d5784b40d7
    > Author: Nick Piggin <npiggin@suse.de>
    > Date: Thu Jul 19 01:46:59 2007 -0700
    >
    > mm: merge populate and nopage into fault (fixes nonlinear)
    >
    > Nonlinear mappings are (AFAIKS) simply a virtual memory concept that encodes
    > the virtual address -> file offset differently from linear mappings.
    >
    > ->populate is a layering violation because the filesystem/pagecache code
    > should need to know anything about the virtual memory mapping. The hitch here
    > is that the ->nopage handler didn't pass down enough information (ie. pgoff).
    > But it is more logical to pass pgoff rather than have the ->nopage function
    > calculate it itself anyway (because that's a similar layering violation).
    >
    > Having the populate handler install the pte itself is likewise a nasty thing
    > to be doing.
    >
    > 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.
    >
    > After this patch, MAP_NONBLOCK no longer sets up ptes for pages present in
    > pagecache. Seems like a fringe functionality anyway.
    >
    > NOPAGE_REFAULT is removed. This should be implemented with ->fault, and no
    > users have hit mainline yet.
    >
    >
    >
    >
    > --
    > Michael Kerrisk
    > Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
    > Found a bug? http://www.kernel.org/doc/man-pages/reporting_bugs.html
    >


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