Re: [PATCH v6 2/4] percpu: Do not trust hint starts when they are not set

[Dennis Zhou]

On Wed, May 13, 2026 at 08:51:14AM +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>
> ---
> v5-v6: No change.
> 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,
> -- 
> 2.54.0.563.g4f69b47b94-goog
> 

Reviewed-by: Dennis Zhou <dennis@xxxxxxxxxx>

Thanks,
Dennis