[PATCH v4 2/4] 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>
---
v4: Remove the separate contig_hint == 0 case and handle the case in the
    following code. Add more checks for scan_hint.
v3: Initial version.

 mm/percpu.c | 20 +++++++++++++-------
 1 file changed, 13 insertions(+), 7 deletions(-)

diff --git a/mm/percpu.c b/mm/percpu.c
index 3ecd86096641..0f6dd5260f56 100644
--- a/mm/percpu.c
+++ b/mm/percpu.c
@@ -640,12 +640,13 @@ static void pcpu_block_update(struct pcpu_block_md *block, int start, int end)
 
 	if (contig > block->contig_hint) {
 		/* promote the old contig_hint to be the new scan_hint */
-		if (start > block->contig_hint_start) {
+		if (block->contig_hint && start > block->contig_hint_start) {
 			if (block->contig_hint > block->scan_hint) {
 				block->scan_hint_start =
 					block->contig_hint_start;
 				block->scan_hint = block->contig_hint;
-			} else if (start < block->scan_hint_start) {
+			} else if (block->scan_hint &&
+				   start < block->scan_hint_start) {
 				/*
 				 * The old contig_hint == scan_hint.  But, the
 				 * new contig is larger so hold the invariant
@@ -664,10 +665,12 @@ static void pcpu_block_update(struct pcpu_block_md *block, int start, int end)
 		     __ffs(start) > __ffs(block->contig_hint_start))) {
 			/* start has a better alignment so use it */
 			block->contig_hint_start = start;
-			if (start < block->scan_hint_start &&
+			if (block->scan_hint &&
+			    start < block->scan_hint_start &&
 			    block->contig_hint > block->scan_hint)
 				block->scan_hint = 0;
-		} else if (start > block->scan_hint_start ||
+		} else if ((block->scan_hint &&
+			    start > block->scan_hint_start) ||
 			   block->contig_hint > block->scan_hint) {
 			/*
 			 * Knowing contig == contig_hint, update the scan_hint
@@ -845,7 +848,8 @@ 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 +893,8 @@ 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 +927,8 @@ 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,
-- 
2.54.0.545.g6539524ca2-goog