linux-kernel - Re: [patch 12/21] Xen-paravirt: Allocate and free vmalloc areas

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Date:	Thu, 15 Feb 2007 22:43:22 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	Andi Kleen <ak@....de>, linux-kernel@...r.kernel.org,
	virtualization@...ts.osdl.org, xen-devel@...ts.xensource.com,
	Chris Wright <chrisw@...s-sol.org>,
	Zachary Amsden <zach@...are.com>,
	Ian Pratt <ian.pratt@...source.com>,
	Christian Limpach <Christian.Limpach@...cam.ac.uk>,
	"Jan Beulich" <JBeulich@...ell.com>
Subject: Re: [patch 12/21] Xen-paravirt: Allocate and free vmalloc areas

On Thu, 15 Feb 2007 18:25:01 -0800 Jeremy Fitzhardinge <jeremy@...p.org> wrote:

> +void lock_vm_area(struct vm_struct *area)
> +{
> +	unsigned long i;
> +	char c;
> +
> +	/*
> +	 * Prevent context switch to a lazy mm that doesn't have this area
> +	 * mapped into its page tables.
> +	 */
> +	preempt_disable();
> +
> +	/*
> +	 * Ensure that the page tables are mapped into the current mm. The
> +	 * page-fault path will copy the page directory pointers from init_mm.
> +	 */
> +	for (i = 0; i < area->size; i += PAGE_SIZE)
> +		(void)__get_user(c, (char __user *)area->addr + i);
> +}
> +EXPORT_SYMBOL_GPL(lock_vm_area);

This won't work when CONFIG_PREEMPT=y.  The pagefault handler will see
in_atomic() and will scram.

(pet-peeve-from-someone-who-remembers-fortran: the reader expects the
variable `i' to be signed.  signed int really)
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