linux-kernel - Re: [kvm-devel] [patch 1/6] mmu

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Date:	Thu, 31 Jan 2008 01:12:58 +0100
From:	Andrea Arcangeli <andrea@...ranet.com>
To:	Christoph Lameter <clameter@....com>
Cc:	Nick Piggin <npiggin@...e.de>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>, linux-mm@...ck.org,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Jack Steiner <steiner@....com>, linux-kernel@...r.kernel.org,
	Avi Kivity <avi@...ranet.com>, kvm-devel@...ts.sourceforge.net,
	daniel.blueman@...drics.com, Robin Holt <holt@....com>,
	Hugh Dickins <hugh@...itas.com>
Subject: Re: [kvm-devel] [patch 1/6] mmu_notifier: Core code

On Wed, Jan 30, 2008 at 03:55:37PM -0800, Christoph Lameter wrote:
> On Thu, 31 Jan 2008, Andrea Arcangeli wrote:
> 
> > > I think Andrea's original concept of the lock in the mmu_notifier_head
> > > structure was the best.  I agree with him that it should be a spinlock
> > > instead of the rw_lock.
> > 
> > BTW, I don't see the scalability concern with huge number of tasks:
> > the lock is still in the mm, down_write(mm->mmap_sem); oneinstruction;
> > up_write(mm->mmap_sem) is always going to scale worse than
> > spin_lock(mm->somethingelse); oneinstruction;
> > spin_unlock(mm->somethinglese).
> 
> If we put it elsewhere in the mm then we increase the size of the memory 
> used in the mm_struct.

Yes, and it will increase of the same amount of RAM that you pretend
everyone to pay even if MMU_NOTIFIER=n after your patch is applied (vs
mine that generated 0 ram utilization increase when
MMU_NOTIFIER=n). And the additional ram will provide not just
self-contained locking but higher scalability too.

I think it's much more important to generate zero ram and CPU overhead
for the embedded (this is something I was very careful to enforce in
all my patches), than to reduce scalability and not having a self
contained locking on full configurations with MMU_NOTIFIER=y.

> Hmmmm.. exit_mmap is only called when the last reference is removed 
> against the mm right? So no tasks are running anymore. No pages are left. 
> Do we need to serialize at all for mmu_notifier_release?

KVM sure doesn't need any locking there.  I thought somebody had to
possibly take a pin on the "mm_count" and pretend to call
mmu_notifier_register at will until mmdrop was finally called, in a
out of order fashion given mmu_notifier_release was implemented like
if the list could change from under it. Note mmdrop != mmput. mmput
and in turn mm_users is the serialization point if you prefer to drop
all locking from _release. Nobody must ever attempt a mmu_notifier_*
after calling mmput for that mm. That should be enough to be
safe. I'm fine either ways...
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