Re: kmap_atomic and preemption

[Peter Zijlstra]

On Wed, May 04, 2016 at 02:16:11PM +0000, Vineet Gupta wrote:
> > static inline void *kmap_atomic(struct page *page)
> > {
> > 	preempt_disable();
> > 	pagefault_disable();
> > 	if (!PageHighMem(page))
> > 		return page_address(page);
> >
> > 	return __kmap_atomic(page);
> > }
> 
> I actually want to return early for !PageHighMem and avoid the pointless 2
> LD-ADD-ST to memory for map and 2 LD-SUB-ST for unmap for regular pages for such
> cases.

So I'm fairly sure people rely on the fact you cannot have pagefault
inside a kmap_atomic().

But you could potentially get away with leaving preemption enabled. Give
it a try, see if something goes *bang* ;-)