linux-kernel - [PATCH] mm/vmalloc: reduce half comparison during pcpu_get_vm

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Date:   Thu,  3 Aug 2017 14:38:22 +0800
From:   Wei Yang <richard.weiyang@...il.com>
To:     akpm@...ux-foundation.org, mhocko@...e.com
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Wei Yang <richard.weiyang@...il.com>
Subject: [PATCH] mm/vmalloc: reduce half comparison during pcpu_get_vm_areas()

In pcpu_get_vm_areas(), it checks each range is not overlapped. To make
sure it is, only (N^2)/2 comparison is necessary, while current code does
N^2 times. By starting from the next range, it achieves the goal and the
continue could be removed.

At the mean time, other two work in this patch:
*  the overlap check of two ranges could be done with one clause
*  one typo in comment is fixed.

Signed-off-by: Wei Yang <richard.weiyang@...il.com>
---
 mm/vmalloc.c | 10 +++-------
 1 file changed, 3 insertions(+), 7 deletions(-)

diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index 8087451cb332..f33c8350fd83 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -2457,7 +2457,7 @@ static unsigned long pvm_determine_end(struct vmap_area **pnext,
  * matching slot.  While scanning, if any of the areas overlaps with
  * existing vmap_area, the base address is pulled down to fit the
  * area.  Scanning is repeated till all the areas fit and then all
- * necessary data structres are inserted and the result is returned.
+ * necessary data structures are inserted and the result is returned.
  */
 struct vm_struct **pcpu_get_vm_areas(const unsigned long *offsets,
 				     const size_t *sizes, int nr_vms,
@@ -2485,15 +2485,11 @@ struct vm_struct **pcpu_get_vm_areas(const unsigned long *offsets,
 		if (start > offsets[last_area])
 			last_area = area;
 
-		for (area2 = 0; area2 < nr_vms; area2++) {
+		for (area2 = area + 1; area2 < nr_vms; area2++) {
 			unsigned long start2 = offsets[area2];
 			unsigned long end2 = start2 + sizes[area2];
 
-			if (area2 == area)
-				continue;
-
-			BUG_ON(start2 >= start && start2 < end);
-			BUG_ON(end2 <= end && end2 > start);
+			BUG_ON(start2 < end && start < end2);
 		}
 	}
 	last_end = offsets[last_area] + sizes[last_area];
-- 
2.11.0