scatterlist: atomic sg_mapping_iter() no longer needs disabled IRQs

SG mapping iterator w/ SG_MITER_ATOMIC set required IRQ disabled because
it originally used KM_BIO_SRC_IRQ to allow use from IRQ handlers.
kmap_atomic() has long been updated to handle stacking atomic mapping
requests on per-cpu basis and only requires not sleeping while mapped.

Update sg_mapping_iter such that atomic iterators only require disabling
preemption instead of disabling IRQ.

While at it, convert wte weird @ARG@ notations to @ARG in the comment of
sg_miter_start().
Signed-off-by: NTejun Heo <tj@kernel.org>
Cc: Maxim Levitsky <maximlevitsky@gmail.com>
Cc: Alex Dubov <oakad@yahoo.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

lib/scatterlist.c

@@ -404,14 +404,13 @@ EXPORT_SYMBOL(sg_miter_start);
  * @miter: sg mapping iter to proceed
  *
  * Description:
- *   Proceeds @miter@ to the next mapping.  @miter@ should have been
- *   started using sg_miter_start().  On successful return,
- *   @miter@->page, @miter@->addr and @miter@->length point to the
- *   current mapping.
+ *   Proceeds @miter to the next mapping.  @miter should have been started
+ *   using sg_miter_start().  On successful return, @miter->page,
+ *   @miter->addr and @miter->length point to the current mapping.
  *
  * Context:
- *   IRQ disabled if SG_MITER_ATOMIC.  IRQ must stay disabled till
- *   @miter@ is stopped.  May sleep if !SG_MITER_ATOMIC.
+ *   Preemption disabled if SG_MITER_ATOMIC.  Preemption must stay disabled
+ *   till @miter is stopped.  May sleep if !SG_MITER_ATOMIC.
  *
  * Returns:
  *   true if @miter contains the next mapping.  false if end of sg
@@ -465,7 +464,8 @@ EXPORT_SYMBOL(sg_miter_next);
  *   resources (kmap) need to be released during iteration.
  *
  * Context:
- *   IRQ disabled if the SG_MITER_ATOMIC is set.  Don't care otherwise.
+ *   Preemption disabled if the SG_MITER_ATOMIC is set.  Don't care
+ *   otherwise.
  */
 void sg_miter_stop(struct sg_mapping_iter *miter)
 {
@@ -479,7 +479,7 @@ void sg_miter_stop(struct sg_mapping_iter *miter)
 			flush_kernel_dcache_page(miter->page);
 
 		if (miter->__flags & SG_MITER_ATOMIC) {
-			WARN_ON(!irqs_disabled());
+			WARN_ON_ONCE(preemptible());
 			kunmap_atomic(miter->addr);
 		} else
 			kunmap(miter->page);