Re: MAP_POPULATE behavior change?
From: Nick Piggin
Date: Mon May 05 2008 - 08:23:27 EST
On Mon, Apr 28, 2008 at 02:07:17PM +0200, Michael Kerrisk wrote:
> Hey Nick,
> -------- Original Message --------
> Subject: MAP_POPULATE behavior change?
> Date: Tue, 22 Apr 2008 15:16:19 +0200
> From: Michael Kerrisk <mtk.manpages@xxxxxxxxx>
> To: Nick Piggin <npiggin@xxxxxxx>
> CC: lkml <linux-kernel@xxxxxxxxxxxxxxx>, mk <michael.kerrisk@xxxxxxxxx>
> 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
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.
> commit 54cb8821de07f2ffcd28c380ce9b93d5784b40d7
> Author: Nick Piggin <npiggin@xxxxxxx>
> 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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