mm/page_alloc.c: fix a misleading comment

The comment says that the per-cpu batchsize and zone watermarks are
determined by present_pages which is definitely wrong, they are both
calculated from managed_pages.  Fix it.

Signed-off-by: Yaowei Bai <bywxiaobai@163.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/sysctl/vm.txt

@@ -349,7 +349,7 @@ zone[i]'s protection[j] is calculated by following expression.
 
 (i < j):
   zone[i]->protection[j]
-  = (total sums of present_pages from zone[i+1] to zone[j] on the node)
+  = (total sums of managed_pages from zone[i+1] to zone[j] on the node)
     / lowmem_reserve_ratio[i];
 (i = j):
    (should not be protected. = 0;
@@ -360,7 +360,7 @@ The default values of lowmem_reserve_ratio[i] are
     256 (if zone[i] means DMA or DMA32 zone)
     32  (others).
 As above expression, they are reciprocal number of ratio.
-256 means 1/256. # of protection pages becomes about "0.39%" of total present
+256 means 1/256. # of protection pages becomes about "0.39%" of total managed
 pages of higher zones on the node.
 
 If you would like to protect more pages, smaller values are effective.

mm/page_alloc.c

@@ -6022,7 +6022,7 @@ void __init mem_init_print_info(const char *str)
  * set_dma_reserve - set the specified number of pages reserved in the first zone
  * @new_dma_reserve: The number of pages to mark reserved
  *
- * The per-cpu batchsize and zone watermarks are determined by present_pages.
+ * The per-cpu batchsize and zone watermarks are determined by managed_pages.
  * In the DMA zone, a significant percentage may be consumed by kernel image
  * and other unfreeable allocations which can skew the watermarks badly. This
  * function may optionally be used to account for unfreeable pages in the