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

[Dennis Zhou]

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@xxxxxxxxxx>
> ---
> 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
>