kmap_atomic and preemption

[Vineet Gupta]

Hi,

I was staring at some recent ARC highmem crashes and see that kmap_atomic()
disables preemption even when page is in lowmem and call returns right away.
This seems to be true for other arches as well.

arch/arc/mm/highmem.c:

void *kmap_atomic(struct page *page)
{
        int idx, cpu_idx;
        unsigned long vaddr;

        preempt_disable();
        pagefault_disable();
        if (!PageHighMem(page))
                return page_address(page);

        /* do the highmem foo ... */
..
}

I would really like to implement a inline fastpath for !PageHighMem(page) case 
and
do the highmem foo out-of-line.

Is preemption disabling a requirement of kmap_atomic() callers independent of
where page is or is it only needed when page is in highmem and can trigger page
faults or TLB Misses between kmap_atomic() and kunmap_atomic and wants 
protection
against reschedules etc.

-Vineet