Date: Fri, 5 Oct 2001 13:55:05 +0100 (BST)
From: Hugh Dickins <hugh@veritas.com>
To: Bernd Harries <mlbha@gmx.de>
cc: linux-kernel@vger.kernel.org, mingo@elte.hu
Subject: Re: __get_free_pages(): is the MEM really mine?
In-Reply-To: <26148.1001936004@www42.gmx.net>
Message-ID: <Pine.LNX.4.21.0110051327180.997-100000@localhost.localdomain>
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Sender: linux-kernel-owner@vger.kernel.org

On Mon, 1 Oct 2001, Bernd Harries wrote:
> 
> I wonder why only I see problems so far. Maybe it's because I also mmap()
> that RAM to user space?

Probably.

munmap() will handle each order-0-page of your order-9
allocation separately.  __get_free_pages gave you count 1 on the
first of those order-0-pages, leaving count 0 on the rest.  I don't
know whether you're following the mmap-makes-all-pages-present
model (using remap_page_range), or the fault-page-by-page model
(supplying your own nopage function).  But either way it sounds like
you bump each page count by 1 when you map it in, and then when it's
unmapped the count goes down to 0 on all the later order-0-pages,
so they get freed before you're done with them.

Either you should force page count 1 on each of the order-0-pages
before you mmap them in (and raise count to 2); or you should set
the Reserved bit on each them, and clear it before freeing (see use
of mem_map_reserve and mem_map_unreserve in various drivers/sound
sources using remap_page_range; there's also a couple of examples
of the nopage method down there too).

Hugh

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