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

[Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>]

On Thu, 15 Feb 2007 18:25:01 -0800 Jeremy Fitzhardinge <jeremy@xxxxxxxx> 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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