linux-kernel - Re: [PATCH] - Fix unmap_vma() bug related to mmu

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Message-ID: <20100125211033.GA24272@sgi.com>
Date:	Mon, 25 Jan 2010 15:10:33 -0600
From:	Jack Steiner <steiner@....com>
To:	Andrea Arcangeli <aarcange@...hat.com>, Robin Holt <holt@....com>,
	cl@...ux-foundation.org
Cc:	mingo@...e.hu, tglx@...utronix.de, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] - Fix unmap_vma() bug related to mmu_notifiers

On Mon, Jan 25, 2010 at 08:00:52PM +0100, Andrea Arcangeli wrote:
> On Mon, Jan 25, 2010 at 11:45:56AM -0600, Jack Steiner wrote:
> > unmap_vmas() can fail to correctly flush the TLB if a
> > callout to mmu_notifier_invalidate_range_start() sleeps.
> 
> Not sure I understand: the callbacks invoked by
> mmu_notifier_invalidate_range_start can't sleep, or rcu locking inside
> mmu notifier will break too (first thing that should be replaced with
> srcu if they were allowed to sleep).
> 
> In short there's no schedule that could be added because of those
> callbacks so if this code isn't ok and schedules and screw on the
> mmu_gather tlb it's probably not mmu notifier related.

Hmmmm. I was under the impression that the range callbacks _were_ allowed to
sleep. That was certainly in the original patches posted by christoph. I was
not aware that it had changed.

I'll let Robin provide details but the way mmu_notifiers are used by
xpmem, I believe sleeping is essential. The 'start" callout sends messages to
another OS instances to teardown cross-SSI mappings. That is difficult
to do w/o a sleep.

We'll dig deeper tomorrow & see what we can do....

--- jack
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