Messages in this thread | | | Date | Mon, 5 May 2008 14:23:16 +0200 | From | Nick Piggin <> | Subject | Re: MAP_POPULATE behavior change? |
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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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