To: Terence Ripperda <tripperda@nvidia.com>
cc: linux-kernel@vger.kernel.org
Subject: Re: 2.6.9-rc2-mm1
References: <2F1dX-2r9-5@gated-at.bofh.it> <2F7VJ-7o7-7@gated-at.bofh.it>
	<2F8fj-7yw-39@gated-at.bofh.it> <2F8oL-7DK-1@gated-at.bofh.it>
	<2F8oR-7DK-17@gated-at.bofh.it> <2FwKr-7VO-7@gated-at.bofh.it>
	<2GAsE-21G-9@gated-at.bofh.it>
From: Andi Kleen <ak@muc.de>
Date: Mon, 20 Sep 2004 21:30:07 +0200
In-Reply-To: <2GAsE-21G-9@gated-at.bofh.it> (Terence Ripperda's message of
 "Mon, 20 Sep 2004 19:50:08 +0200")
Message-ID: <m3brg0yc00.fsf@averell.firstfloor.org>
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Terence Ripperda <tripperda@nvidia.com> writes:

> this is some ugly code. we're doing a lookup on a physical address to
> see if this is memory we previously allocated and returning a kernel
> pointer to the page.
>
> the particular snippet in question (that uses MAXMEM) is an ugly attempt
> to verify the address is a real physical address, before using __va()
> on something like an i/o region. A better approach than comparing
> MAXMEM would probably be to convert the address to a mapnr and compare
> to max_mapnr.

pfn_valid() is intended for that. However it cannot work 
when you have more than 4GB memory and IO memory holes below 4GB.
Testing the reserved bit of the struct page * may work in
that case, although it can give false positives when you try
this with random memory (some people set reserved on real memory
for other reason) 

-Andi

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