linux-kernel - Re: [patch 2/6] mmu_notifier: Callbacks to invalidate address ranges

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Date:	Tue, 29 Jan 2008 16:34:16 -0800 (PST)
From:	Christoph Lameter <clameter@....com>
To:	Andrea Arcangeli <andrea@...ranet.com>
cc:	Robin Holt <holt@....com>, Avi Kivity <avi@...ranet.com>,
	Izik Eidus <izike@...ranet.com>, Nick Piggin <npiggin@...e.de>,
	kvm-devel@...ts.sourceforge.net,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>, steiner@....com,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	daniel.blueman@...drics.com, Hugh Dickins <hugh@...itas.com>
Subject: Re: [patch 2/6] mmu_notifier: Callbacks to invalidate address ranges

On Wed, 30 Jan 2008, Andrea Arcangeli wrote:

> > A user space spinlock plays into this??? That is irrelevant to the kernel. 
> > And we are discussing "your" placement of the invalidate_range not mine.
> 
> With "my" code, invalidate_range wasn't placed there at all, my
> modification to ptep_clear_flush already covered it in a automatic
> way, grep from the word fremap in my latest patch you won't find it,
> like you won't find any change to do_wp_page. Not sure why you keep
> thinking I added those invalidate_range when infact you did.

Well you moved the code at minimum. Hmmm... according 
http://marc.info/?l=linux-kernel&m=120114755620891&w=2 it was Robin.

> The user space spinlock plays also in declaring rdtscp unworkable to
> provide a monotone vgettimeofday w/o kernel locking.

No idea what you are talking about.

> My patch by calling invalidate_page inside ptep_clear_flush guaranteed
> that both the thread writing through sptes and the thread writing
> through linux ptes, couldn't possibly simultaneously write to two
> different physical pages.

But then the ptep_clear_flush will issue invalidate_page() for ranges 
that were already covered by invalidate_range(). There are multiple calls 
to clear the same spte.
>
> Your patch allows the thread writing through linux-pte to write to a
> new populated page while the old thread writing through sptes still
> writes to the old page. Is that safe? I don't know for sure. The fact
> the physical page backing the virtual address could change back and
> forth, perhaps invalidates the theory that somebody could possibly do
> some useful locking out of it relaying on all threads seeing the same
> physical page at the same time.

This is referrring to the remap issue not do_wp_page right?

> Actually above I was describing remap_file_pages not do_wp_page.

Ok.

The serialization of remap_file_pages does not seem that critical since we 
only take a read lock on mmap_sem here. There may already be concurrent 
access to pages from other processors while the ptes are remapped. So 
there is already some overlap.

We could take mmap_sem there writably and keep it writably for the case 
that we have an mmu notifier in the mm.


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