linux-kernel - Re: [patch 5/6] mmu_notifier: Support for drivers with revers maps (f.e. for XPmem)

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Message-ID: <Pine.LNX.4.64.0802271440530.13186@schroedinger.engr.sgi.com>
Date:	Wed, 27 Feb 2008 14:43:41 -0800 (PST)
From:	Christoph Lameter <clameter@....com>
To:	Nick Piggin <nickpiggin@...oo.com.au>
cc:	akpm@...ux-foundation.org, Andrea Arcangeli <andrea@...ranet.com>,
	Robin Holt <holt@....com>, Avi Kivity <avi@...ranet.com>,
	Izik Eidus <izike@...ranet.com>,
	kvm-devel@...ts.sourceforge.net,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	general@...ts.openfabrics.org,
	Steve Wise <swise@...ngridcomputing.com>,
	Roland Dreier <rdreier@...co.com>,
	Kanoj Sarcar <kanojsarcar@...oo.com>, steiner@....com,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	daniel.blueman@...drics.com
Subject: Re: [patch 5/6] mmu_notifier: Support for drivers with revers maps
 (f.e. for XPmem)

On Wed, 20 Feb 2008, Nick Piggin wrote:

> I don't know how this is supposed to solve anything. The sleeping
> problem happens I guess mostly in truncate. And all you are doing
> is putting these rmap callbacks in page_mkclean and try_to_unmap.

truncate is handled by the range invalidates. This is special code to deal 
with the unnap/clean of an individual page.

> That doesn't seem right. To start with, the new callbacks aren't
> even called in the places where invalidate_page isn't allowed to
> sleep.
> 
> The problem is unmap_mapping_range, right? And unmap_mapping_range
> must walk the rmaps with the mmap lock held, which is why it can't
> sleep. And it can't hold any mmap_sem so it cannot prevent address

Nope. unmap_mapping_range is already handled by the range callbacks.

> So in the meantime, you could have eg. a fault come in and set up a
> new page for one of the processes, and that page might even get
> exported via the same external driver. And now you have a totally
> inconsistent view.

The situation that you are imagining has already been dealt with by the 
earlier patches. This is only to allow sleeping while unmapping individual 
pages.
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