Date: Fri, 5 Nov 2004 05:03:09 +0100
From: Andrea Arcangeli <andrea@novell.com>
To: Dave Hansen <haveblue@us.ibm.com>
Cc: linux-mm <linux-mm@kvack.org>,
       Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
       Andi Kleen <ak@suse.de>, Andrew Morton <akpm@osdl.org>
Subject: Re: fix iounmap and a pageattr memleak (x86 and x86-64)
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On Thu, Nov 04, 2004 at 06:23:11PM -0800, Dave Hansen wrote:
> I'm not quite sure if this has any other weird effects, so I'll hold on
> to it for a week or so and see if anything turns up.  

this fixed the problem for me too.

However I'm not convinced this is correct, nothing in the kernel should
ever free a bootmem piece of memory after the machine has booted.

If this helps, it also means we found an existing pte (not pmd) with
page_count 0 during the first unmap event (bootmem allocated). The
transition from mapped to unmapped works fine, but the transition from
unmapped to mapped will thorw the pte away and we'll regenerate a 2M pmd
where there was a pte instead. I wonder why there are 4k pages there in
the first place.

Anyways I understand what's going on now thanks to your debugging, and I
believe the only real fix is to use PageReserved to catch if we're
working on a newly allocated page or not, I don't like to depend on the
page_count being 0 for the bootmem pages like the previous code was
doing. I believe my code would now fall apart even if you were using it
with PSE disabled (nopentium or something). So I've to fix that bit at
least and I will use PageReserved for that.

The page_count of bootmem pages really doesn't matter since they must
never be freed. It really should remain 0 so we catch if anybody
executes a put_page on it.

I'll fix it up...
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