linux-kernel - Re: [PATCH 01 of 12] Core of mmu notifiers

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Date:	Sun, 27 Apr 2008 14:27:27 +0200
From:	Andrea Arcangeli <andrea@...ranet.com>
To:	Robin Holt <holt@....com>
Cc:	Jack Steiner <steiner@....com>,
	Christoph Lameter <clameter@....com>,
	Nick Piggin <npiggin@...e.de>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	kvm-devel@...ts.sourceforge.net,
	Kanoj Sarcar <kanojsarcar@...oo.com>,
	Roland Dreier <rdreier@...co.com>,
	Steve Wise <swise@...ngridcomputing.com>,
	linux-kernel@...r.kernel.org, Avi Kivity <avi@...ranet.com>,
	linux-mm@...ck.org, general@...ts.openfabrics.org,
	Hugh Dickins <hugh@...itas.com>, akpm@...ux-foundation.org,
	Rusty Russell <rusty@...tcorp.com.au>
Subject: Re: [PATCH 01 of 12] Core of mmu notifiers

On Sat, Apr 26, 2008 at 08:17:34AM -0500, Robin Holt wrote:
> the first four sets.  The fifth is the oversubscription test which trips
> my xpmem bug.  This is as good as the v12 runs from before.

Now that mmu-notifier-core #v14 seems finished and hopefully will
appear in 2.6.26 ;), I started exercising more the kvm-mmu-notifier
code with the full patchset applied and not only with
mmu-notifier-core. I soon found the full patchset has a swap deadlock
bug. Then I tried without using kvm (so with mmu notifier disarmed)
and I could still reproduce the crashes. After grabbing a few stack
traces I tracked it down to a bug in the i_mmap_lock->i_mmap_sem
conversion. If you oversubscription means swapping, you should retest
with this applied on #v14 i_mmap_sem patch as it would eventually
deadlock with all tasks allocating memory in D state without this. Now
the full patchset is as rock solid as with only mmu-notifier-core
applied. It's swapping 2G memhog on top of a 3G VM with 2G of ram for
the last hours without a problem. Everything is working great with KVM
at least.

Talking about post 2.6.26: the refcount with rcu in the anon-vma
conversion seems unnecessary and may explain part of the AIM slowdown
too. The rest looks ok and probably we should switch the code to a
compile-time decision between rwlock and rwsem (so obsoleting the
current spinlock).

diff --git a/mm/rmap.c b/mm/rmap.c
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1008,7 +1008,7 @@ static int try_to_unmap_file(struct page
 	list_for_each_entry(vma, &mapping->i_mmap_nonlinear, shared.vm_set.list)
 		vma->vm_private_data = NULL;
 out:
-	up_write(&mapping->i_mmap_sem);
+	up_read(&mapping->i_mmap_sem);
 	return ret;
 }

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