kmap_atomic and preemption

From: Vineet Gupta
Date: Wed May 04 2016 - 06:38:03 EST


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.


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

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.