Subject: Re: Remapping pages mapped to userspace (was: [PATCH 10 of 20]
	ipath - support for userspace apps using core driver)
From: "Bryan O'Sullivan" <bos@pathscale.com>
To: Hugh Dickins <hugh@veritas.com>
Cc: Roland Dreier <rdreier@cisco.com>, Andrew Morton <akpm@osdl.org>,
       Alan Cox <alan@lxorguk.ukuu.org.uk>, torvalds@osdl.org,
       hch@infradead.org, linux-kernel@vger.kernel.org
In-Reply-To: <Pine.LNX.4.61.0603171631240.32660@goblin.wat.veritas.com>
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Organization: PathScale, Inc.
Date: Fri, 17 Mar 2006 09:17:28 -0800
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On Fri, 2006-03-17 at 17:13 +0000, Hugh Dickins wrote:

> You seem to be asking to "revoke" an mmap:

Yes.  We'd like this ability, too.

> Though it looks like subsequent userspace accesses will then go to
> do_anonymous_page, giving ZERO_PAGE to read faults or a fresh anon
> page to write faults: I think I'd prefer it if pte_none accesses in
> a VM_PFNMAP area gave SIGBUS, but unsure if we can change that now.

It would be unfortunate if userspace were spinning on a chip register,
waiting for the register to transition from zero to non-zero, and we
replaced that mapping with an anonymous page.  In that case, userspace
could potentially spin forever, having no way to detect the demise of
the device.

	<b

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