[uml-devel] Why does mem_init call map_cb twice?

[uml-devel] Why does mem_init call map_cb twice?

[John Reiser]

In arch/um/kernel/mem.c, mem_init calls map_cb twice:
once directly and once indirectly via initial_thread_cb.  Why?

-----
void mem_init(void)
{
        /* clear the zero-page */
        memset((void *) empty_zero_page, 0, PAGE_SIZE);

        /* Map in the area just after the brk now that kmalloc is about
         * to be turned on.
         */
        brk_end = (unsigned long) UML_ROUND_UP(sbrk(0));
        map_cb(NULL);
        initial_thread_cb(map_cb, NULL);
-----

-- 
John Reiser, jreiser@BitWagon.com

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Re: [uml-devel] Why does mem_init call map_cb twice?

[Jeff Dike]

On Fri, Dec 14, 2007 at 09:31:27AM -0800, John Reiser wrote:
> In arch/um/kernel/mem.c, mem_init calls map_cb twice:
> once directly and once indirectly via initial_thread_cb.  Why?
> 
> -----
> void mem_init(void)
> {
>         /* clear the zero-page */
>         memset((void *) empty_zero_page, 0, PAGE_SIZE);
> 
>         /* Map in the area just after the brk now that kmalloc is about
>          * to be turned on.
>          */
>         brk_end = (unsigned long) UML_ROUND_UP(sbrk(0));
>         map_cb(NULL);
>         initial_thread_cb(map_cb, NULL);
> -----

It's a bug.  That's left over from the tt mode days, when the kernel
ran in the same address spaces as its processes and there was a
separate thread ptracing everything.  The tracing thread address space
needed to be kept in sync with the others, and this was one part of
that.

				Jeff

-- 
Work email - jdike at linux dot intel dot com

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