linux-kernel - Re: [PATCH] mm: Limit pgd range freeing to mm->task

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Date:	Wed, 13 Feb 2013 13:47:56 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Catalin Marinas <catalin.marinas@....com>
Cc:	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	linux-arch@...r.kernel.org, Andrea Arcangeli <aarcange@...hat.com>,
	Russell King <linux@....linux.org.uk>
Subject: Re: [PATCH] mm: Limit pgd range freeing to mm->task_size

On Wed, 13 Feb 2013 11:39:29 +0000
Catalin Marinas <catalin.marinas@....com> wrote:

> ARM processors with LPAE enabled use 3 levels of page tables, with an
> entry in the top level (pgd) covering 1GB of virtual space. Because of
> the branch relocation limitations on ARM, the loadable modules are
> mapped 16MB below PAGE_OFFSET, making the corresponding 1GB pgd shared
> between kernel modules and user space.
> 
> Since free_pgtables() is called with ceiling == 0, free_pgd_range() (and
> subsequently called functions) also frees the page table
> shared between user space and kernel modules (which is normally handled
> by the ARM-specific pgd_free() function).
> 
> This patch changes the ceiling argument to mm->task_size for the
> free_pgtables() and free_pgd_range() function calls. We cannot use
> TASK_SIZE since this macro may not be a run-time constant on 64-bit
> systems supporting compat applications.

I'm trying to work out why we're using 0 in there at all, rather than
->task_size.  But that's lost in the mists of time.

As you've discovered, handling of task_size and TASK_SIZE is somewhat
inconsistent across architectures and with compat tasks.  I guess we
toss it in there and see if anything breaks...
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