Re: kmap_atomic and preemption

From: Peter Zijlstra
Date: Wed May 04 2016 - 09:47:56 EST

On Wed, May 04, 2016 at 04:07:40PM +0530, Vineet Gupta wrote:
> 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.

Traditionally kmap_atomic() disables preemption; and the reason is that
the returned pointer must stay valid. This had a side effect in that it
also disabled pagefaults.

We've since de-coupled the pagefault from the preemption thing, so you
could disable pagefaults while leaving preemption enabled.

Now, I've also done preemptible kmap_atomic() on -rt; which appears to
work, suggesting nothing relies on it disabling preemption (on -rt).

So sure; you can try and leave preemption enabled for lowmem pages, see
what comes apart -- if anything. It gives weird semantics for
kmap_atomic() though, and I'm not sure the cost of doing that
preempt_disable/preempt_enable() is worth the pain.

If you want a fast-slow path splt, you can easily do something like:

static inline void *kmap_atomic(struct page *page)
if (!PageHighMem(page))
return page_address(page);

return __kmap_atomic(page);