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.

A future patch introduces the scan_hint which makes the contig_hint
slightly fuzzy as they can at times be smaller than the actual hint.

Signed-off-by: Dennis Zhou <dennis@kernel.org>

diff --git a/mm/percpu.c b/mm/percpu.c
index 769b7583975b23ac34e5477cfef0bf705050d2ca..cbace9e79f2d3ee2bc0d3a1f781ed778a960635a 100644 (file)
--- a/mm/percpu.c
+++ b/mm/percpu.c
@@ -546,6 +546,21 @@ 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)
+{
+       return (a < y) && (x < b);
+}
+
 /**
  * pcpu_chunk_update - updates the chunk metadata given a free area
  * @chunk: chunk of interest
@@ -710,8 +725,11 @@ 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 +782,10 @@ 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);
 }