[PATCH v3 2/3] percpu: Do not trust hint starts when they are not set

[Joonwon Kang <joonwonkang@google.com>]

contig_hint_start can be trusted outside the hint update function since
it will be updated everytime contig_hint is broken. On the other hand,
scan_hint_start might still be invalid anywhere in the code due to the
broken scan_hint not being updated promptly. If those starts are trusted
when they are not set, it could lead to false invalidation or update of
the hints.

Signed-off-by: Joonwon Kang <joonwonkang@google.com>
---
v3: Initial version.

 mm/percpu.c | 15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)

diff --git a/mm/percpu.c b/mm/percpu.c
index 3ecd86096641..f16533ed4a49 100644
--- a/mm/percpu.c
+++ b/mm/percpu.c
@@ -638,6 +638,13 @@ static void pcpu_block_update(struct pcpu_block_md *block, int start, int end)
 	if (end == block->nr_bits)
 		block->right_free = contig;
 
+	if (block->contig_hint == 0) {
+		block->contig_hint = contig;
+		block->contig_hint_start = start;
+		block->scan_hint = 0;
+		return;
+	}
+
 	if (contig > block->contig_hint) {
 		/* promote the old contig_hint to be the new scan_hint */
 		if (start > block->contig_hint_start) {
@@ -845,7 +852,8 @@ static void pcpu_block_update_hint_alloc(struct pcpu_chunk *chunk, int bit_off,
 					PCPU_BITMAP_BLOCK_BITS,
 					s_off + bits);
 
-	if (pcpu_region_overlap(s_block->scan_hint_start,
+	if (s_block->scan_hint &&
+	    pcpu_region_overlap(s_block->scan_hint_start,
 				s_block->scan_hint_start + s_block->scan_hint,
 				s_off,
 				s_off + bits))
@@ -889,7 +897,7 @@ static void pcpu_block_update_hint_alloc(struct pcpu_chunk *chunk, int bit_off,
 			/* reset the block */
 			e_block++;
 		} else {
-			if (e_off > e_block->scan_hint_start)
+			if (e_block->scan_hint && e_off > e_block->scan_hint_start)
 				e_block->scan_hint = 0;
 
 			e_block->left_free = 0;
@@ -922,7 +930,8 @@ static void pcpu_block_update_hint_alloc(struct pcpu_chunk *chunk, int bit_off,
 	if (nr_empty_pages)
 		pcpu_update_empty_pages(chunk, -nr_empty_pages);
 
-	if (pcpu_region_overlap(chunk_md->scan_hint_start,
+	if (chunk_md->scan_hint &&
+	    pcpu_region_overlap(chunk_md->scan_hint_start,
 				chunk_md->scan_hint_start +
 				chunk_md->scan_hint,
 				bit_off,
-- 
2.53.0.1213.gd9a14994de-goog