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

Dennis Zhou
2026-04-21T23:29:00Z

On Fri, Apr 10, 2026 at 05:44:16PM +0000, Joonwon Kang wrote:
> 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;
> + }
> +

This change isn't described in the commit log. I'd prefer to clean up
the logic below than adding this additional if empty case.

> 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
>