mm/memory.c: remove ZAP_BLOCK_SIZE

ZAP_BLOCK_SIZE became unused in the preemptible-mmu_gather work ("mm:
Remove i_mmap_lock lockbreak").  So zap it.

Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

mm/memory.c

@@ -1290,13 +1290,6 @@ static unsigned long unmap_page_range(struct mmu_gather *tlb,
 	return addr;
 }
 
-#ifdef CONFIG_PREEMPT
-# define ZAP_BLOCK_SIZE	(8 * PAGE_SIZE)
-#else
-/* No preempt: go for improved straight-line efficiency */
-# define ZAP_BLOCK_SIZE	(1024 * PAGE_SIZE)
-#endif
-
 /**
  * unmap_vmas - unmap a range of memory covered by a list of vma's
  * @tlb: address of the caller's struct mmu_gather
@@ -1310,10 +1303,6 @@ static unsigned long unmap_page_range(struct mmu_gather *tlb,
  *
  * Unmap all pages in the vma list.
  *
- * We aim to not hold locks for too long (for scheduling latency reasons).
- * So zap pages in ZAP_BLOCK_SIZE bytecounts.  This means we need to
- * return the ending mmu_gather to the caller.
- *
  * Only addresses between `start' and `end' will be unmapped.
  *
  * The VMA list must be sorted in ascending virtual address order.