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

Joonwon Kang
2026-04-10T17:44:28Z

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