From: Mel Gorman <mel@csn.ul.ie>
To: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>,
Linux-MM <linux-mm@kvack.org>,
LKML <linux-kernel@vger.kernel.org>,
Minchan Kim <minchan.kim@gmail.com>,
Christoph Lameter <cl@linux.com>, Rik van Riel <riel@redhat.com>,
Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [PATCH 3/3] mm,migration: Remove straggling migration PTEs when page tables are being moved after the VMA has already moved
Date: Wed, 28 Apr 2010 11:48:13 +0100 [thread overview]
Message-ID: <20100428104813.GD15815@csn.ul.ie> (raw)
In-Reply-To: <20100428162838.c762fcda.kamezawa.hiroyu@jp.fujitsu.com>
Thanks to you both for looking into this. I far prefer this general approach
than cleaning up the migration PTEs as the page tables get copied. While it
might "work", it's sloppy in the same way as having migration_entry_wait()
do the cleanup was sloppy. It's far preferable to make the VMA move and
page table copy atomic with anon_vma->lock.
On Wed, Apr 28, 2010 at 04:28:38PM +0900, KAMEZAWA Hiroyuki wrote:
> On Wed, 28 Apr 2010 11:49:44 +0900
> KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> wrote:
>
> > On Wed, 28 Apr 2010 04:42:27 +0200
> > Andrea Arcangeli <aarcange@redhat.com> wrote:
>
> > > migrate.c requires rmap to be able to find all ptes mapping a page at
> > > all times, otherwise the migration entry can be instantiated, but it
> > > can't be removed if the second rmap_walk fails to find the page.
> > >
> > > So shift_arg_pages must run atomically with respect of rmap_walk, and
> > > it's enough to run it under the anon_vma lock to make it atomic.
> > >
> > > And split_huge_page() will have the same requirements as migrate.c
> > > already has.
> > >
> > > Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
> >
> > Seems good.
> > Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> >
> > I'll test this and report if I see trouble again.
> >
> > Unfortunately, I'll have a week of holidays (in Japan) in 4/29-5/05,
> > my office is nearly closed. So, please consider no-mail-from-me is
> > good information.
> >
> Here is bad news. When move_page_tables() fails, "some ptes" are moved
> but others are not and....there is no rollback routine.
>
The biggest problem is that the reverse mapping is temporarily out of
sync until do_exit gets rid of the mess, but how serious is that really?
If there is a migration entry in there, the mapcount should already be zero and
migration holds a reference to the page to prevent it going away. rmap_walk()
may then miss the migration_pte so it gets left behind. Ordinarily this
would be bad but in exec(), we cannot be faulting this page so we won't
trigger the bug in swapops. Instead, do_exit ultimately will skip over the
migration PTE doing nothing with the page but as the mapcount is still zero,
the page won't leak.
> I bet the best way to fix this mess up is
> - disable overlap moving of arg pages
> - use do_mremap().
>
> But maybe you guys want to fix this directly.
> Here is a temporal fix from me. But don't trust me..
I see the point of your patch but I'm not yet seeing why it is
necessary to back out if move_page_tables fails.
That said, both patches have a greater problem. Both of them hold a spinlock
(anon_vma->lock) while calling into the page allocator with GFP_KERNEL (to
allocate the page tables). We don't want to change that to GFP_ATOMIC so
either we need to allocate the pages in advance or special case rmap_walk()
to not walk processes that are in exec.
--
Mel Gorman
Part-time Phd Student Linux Technology Center
University of Limerick IBM Dublin Software Lab
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next prev parent reply other threads:[~2010-04-28 10:48 UTC|newest]
Thread overview: 66+ messages / expand[flat|nested] mbox.gz Atom feed top
2010-04-27 21:30 [PATCH 0/3] Fix migration races in rmap_walk() V2 Mel Gorman
2010-04-27 21:30 ` [PATCH 1/3] mm,migration: During fork(), wait for migration to end if migration PTE is encountered Mel Gorman
2010-04-27 22:22 ` Andrea Arcangeli
2010-04-27 23:52 ` KAMEZAWA Hiroyuki
2010-04-28 0:18 ` Andrea Arcangeli
2010-04-28 0:19 ` Andrea Arcangeli
2010-04-28 0:28 ` KAMEZAWA Hiroyuki
2010-04-28 0:59 ` Andrea Arcangeli
2010-04-28 8:24 ` Mel Gorman
2010-04-27 21:30 ` [PATCH 2/3] mm,migration: Prevent rmap_walk_[anon|ksm] seeing the wrong VMA information Mel Gorman
2010-04-27 23:10 ` Andrea Arcangeli
2010-04-28 9:15 ` Mel Gorman
2010-04-28 15:35 ` Andrea Arcangeli
2010-04-28 15:39 ` Andrea Arcangeli
2010-04-28 15:55 ` Mel Gorman
2010-04-28 16:23 ` Andrea Arcangeli
2010-04-28 17:34 ` Mel Gorman
2010-04-28 17:58 ` Andrea Arcangeli
2010-04-28 17:47 ` [RFC PATCH] take all anon_vma locks in anon_vma_lock Rik van Riel
2010-04-28 18:03 ` Andrea Arcangeli
2010-04-28 18:09 ` Rik van Riel
2010-04-28 18:25 ` [RFC PATCH -v2] " Rik van Riel
2010-04-28 19:07 ` Mel Gorman
2010-04-28 20:17 ` [RFC PATCH -v3] " Rik van Riel
2010-04-28 20:57 ` Rik van Riel
2010-04-29 0:28 ` Minchan Kim
2010-04-29 2:10 ` Rik van Riel
2010-04-29 2:55 ` Minchan Kim
2010-04-29 6:42 ` Minchan Kim
2010-04-29 15:39 ` Rik van Riel
2010-04-29 7:37 ` Mel Gorman
2010-04-29 8:15 ` Mel Gorman
2010-04-29 8:32 ` Minchan Kim
2010-04-29 8:44 ` Mel Gorman
2010-04-27 21:30 ` [PATCH 3/3] mm,migration: Remove straggling migration PTEs when page tables are being moved after the VMA has already moved Mel Gorman
2010-04-27 22:30 ` Andrea Arcangeli
2010-04-27 22:58 ` Andrea Arcangeli
2010-04-28 0:39 ` KAMEZAWA Hiroyuki
2010-04-28 1:05 ` Andrea Arcangeli
2010-04-28 1:09 ` Andrea Arcangeli
2010-04-28 1:18 ` KAMEZAWA Hiroyuki
2010-04-28 1:36 ` Andrea Arcangeli
2010-04-28 1:29 ` KAMEZAWA Hiroyuki
2010-04-28 1:44 ` Andrea Arcangeli
2010-04-28 2:12 ` KAMEZAWA Hiroyuki
2010-04-28 2:42 ` Andrea Arcangeli
2010-04-28 2:49 ` KAMEZAWA Hiroyuki
2010-04-28 7:28 ` KAMEZAWA Hiroyuki
2010-04-28 10:48 ` Mel Gorman [this message]
2010-04-28 0:03 ` KAMEZAWA Hiroyuki
2010-04-28 0:08 ` Andrea Arcangeli
2010-04-28 0:36 ` KAMEZAWA Hiroyuki
2010-04-28 8:30 ` KAMEZAWA Hiroyuki
2010-04-28 14:46 ` Andrea Arcangeli
2010-04-27 22:27 ` [PATCH 0/3] Fix migration races in rmap_walk() V2 Christoph Lameter
2010-04-27 22:32 ` Andrea Arcangeli
2010-04-28 0:13 ` KAMEZAWA Hiroyuki
2010-04-28 0:20 ` Andrea Arcangeli
2010-04-28 14:23 ` Mel Gorman
2010-04-28 14:57 ` Mel Gorman
2010-04-28 15:16 ` Andrea Arcangeli
2010-04-28 15:23 ` Mel Gorman
2010-04-28 15:45 ` Andrea Arcangeli
2010-04-28 20:40 ` Andrea Arcangeli
2010-04-28 21:05 ` Andrea Arcangeli
2010-04-28 9:17 ` Mel Gorman
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