linux-kernel - [PATCH 03/12] percpu: introduce helper to determine if two regions overlap

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Date:   Wed, 27 Feb 2019 21:18:30 -0500
From:   Dennis Zhou <dennis@...nel.org>
To:     Dennis Zhou <dennis@...nel.org>, Tejun Heo <tj@...nel.org>,
        Christoph Lameter <cl@...ux.com>
Cc:     Vlad Buslov <vladbu@...lanox.com>, kernel-team@...com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: [PATCH 03/12] percpu: introduce helper to determine if two regions overlap

While block hints were always accurate, it's possible when spanning
across blocks that we miss updating the chunk's contig_hint. Rather than
rely on correctness of the boundaries of hints, do a full overlap
comparison.

Signed-off-by: Dennis Zhou <dennis@...nel.org>
---
 mm/percpu.c | 31 +++++++++++++++++++++++++++----
 1 file changed, 27 insertions(+), 4 deletions(-)

diff --git a/mm/percpu.c b/mm/percpu.c
index 69ca51d238b5..b40112b2fc59 100644
--- a/mm/percpu.c
+++ b/mm/percpu.c
@@ -546,6 +546,24 @@ static inline int pcpu_cnt_pop_pages(struct pcpu_chunk *chunk, int bit_off,
 	       bitmap_weight(chunk->populated, page_start);
 }
 
+/*
+ * pcpu_region_overlap - determines if two regions overlap
+ * @a: start of first region, inclusive
+ * @b: end of first region, exclusive
+ * @x: start of second region, inclusive
+ * @y: end of second region, exclusive
+ *
+ * This is used to determine if the hint region [a, b) overlaps with the
+ * allocated region [x, y).
+ */
+static inline bool pcpu_region_overlap(int a, int b, int x, int y)
+{
+	if ((x >= a && x < b) || (y > a && y <= b) ||
+	    (x <= a && y >= b))
+		return true;
+	return false;
+}
+
 /**
  * pcpu_chunk_update - updates the chunk metadata given a free area
  * @chunk: chunk of interest
@@ -710,8 +728,11 @@ static void pcpu_block_update_hint_alloc(struct pcpu_chunk *chunk, int bit_off,
 					PCPU_BITMAP_BLOCK_BITS,
 					s_off + bits);
 
-	if (s_off >= s_block->contig_hint_start &&
-	    s_off < s_block->contig_hint_start + s_block->contig_hint) {
+	if (pcpu_region_overlap(s_block->contig_hint_start,
+				s_block->contig_hint_start +
+				s_block->contig_hint,
+				s_off,
+				s_off + bits)) {
 		/* block contig hint is broken - scan to fix it */
 		pcpu_block_refresh_hint(chunk, s_index);
 	} else {
@@ -764,8 +785,10 @@ static void pcpu_block_update_hint_alloc(struct pcpu_chunk *chunk, int bit_off,
 	 * contig hint is broken.  Otherwise, it means a smaller space
 	 * was used and therefore the chunk contig hint is still correct.
 	 */
-	if (bit_off >= chunk->contig_bits_start  &&
-	    bit_off < chunk->contig_bits_start + chunk->contig_bits)
+	if (pcpu_region_overlap(chunk->contig_bits_start,
+				chunk->contig_bits_start + chunk->contig_bits,
+				bit_off,
+				bit_off + bits))
 		pcpu_chunk_refresh_hint(chunk);
 }
 
-- 
2.17.1