[PATCH v3 1/3] percpu: Fix wrong chunk hints update

[PATCH v3 1/3] percpu: Fix wrong chunk hints update

[Joonwon Kang]

Chunk end offset was set to a block end offset, which could prevent
chunk hints from being updated correctly. It was observed that the chunk
free size gets minus or shorter than the actual free size due to this.
This commit fixes it.

Signed-off-by: Joonwon Kang <joonwonkang@google.com>
---
v3: Initial version.

 mm/percpu.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/percpu.c b/mm/percpu.c
index 81462ce5866e..3ecd86096641 100644
--- a/mm/percpu.c
+++ b/mm/percpu.c
@@ -1054,7 +1054,7 @@ static void pcpu_block_update_hint_free(struct pcpu_chunk *chunk, int bit_off,
 	else
 		pcpu_block_update(&chunk->chunk_md,
 				  pcpu_block_off_to_off(s_index, start),
-				  end);
+				  pcpu_block_off_to_off(e_index, end));
 }
 
 /**
-- 
2.53.0.1213.gd9a14994de-goog

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

[Joonwon Kang]

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

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@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
> 

[PATCH v3 3/3] percpu: Fix hint invariant breakage

[Joonwon Kang]

The invariant "scan_hint_start > contig_hint_start if and only if
scan_hint == contig_hint" should be kept for hint management. However,
it could be broken in some cases:

  - if (new contig == contig_hint == scan_hint) && (contig_hint_start <
    scan_hint_start < new contig start) && the new contig is to become a
    new contig_hint due to its better alignment, then scan_hint should
    be invalidated instead of keeping the old value.

  - if (new contig == contig_hint > scan_hint) && (new contig start <
    contig_hint_start) && the new contig is not to become a new
    contig_hint, then scan_hint should be not updated to the new contig.

This commit mainly fixes this invariant breakage and includes more:

  - Refactor the percpu block update code to make it more visible on
    what to consider, e.g. when the new contig overlaps with the old
    contig_hint or scan_hint.

  - Merge the new contig with other hints when it overlaps with them and
    treat it as a whole free region instead of a separate small region.

  - Fix the invariant breakage and also optimizes scan_hint further.
    Some of the optimization cases when no overlap occurs are:

    - if (new contig > contig_hint > scan_hint) && (scan_hint_start < new
      contig start < contig_hint_start), then keep scan_hint instead of
      invalidating it.

    - if (new contig > contig_hint == scan_hint) && (contig_hint_start <
      new contig start < scan_hint_start), then update scan_hint to the
      old contig_hint instead of invalidating it.

    - if (new contig == contig_hint > scan_hint) && (new contig start <
      contig_hint_start) && the new contig is to become a new contig_hint
      due to its better alignment, then update scan_hint to the old
      contig_hint instead of invalidating or keeping it.

Signed-off-by: Joonwon Kang <joonwonkang@google.com>
---
v2 -> v3: Merge the new contig with other hints when it overlaps with
  them and treat it as a whole free region instead of a separate small
  region.

v1 -> v2: Consider cases where the new contig overlaps with the existing
  contig_hint or scan_hint.

 mm/percpu.c | 130 ++++++++++++++++++++++++++++++++++++----------------
 1 file changed, 90 insertions(+), 40 deletions(-)

diff --git a/mm/percpu.c b/mm/percpu.c
index f16533ed4a49..d5b0b4863ffe 100644
--- a/mm/percpu.c
+++ b/mm/percpu.c
@@ -629,7 +629,27 @@ static inline bool pcpu_region_overlap(int a, int b, int x, int y)
  */
 static void pcpu_block_update(struct pcpu_block_md *block, int start, int end)
 {
-	int contig = end - start;
+	int contig;
+	int scan_hint_cand_1 = 0;
+	int scan_hint_cand_1_start = 0;
+	int scan_hint_cand_2 = 0;
+	int scan_hint_cand_2_start = 0;
+	bool overlap_with_contig_hint = pcpu_region_overlap(start, end,
+		block->contig_hint_start,
+		block->contig_hint_start + block->contig_hint);
+	bool overlap_with_scan_hint = pcpu_region_overlap(start, end,
+		block->scan_hint_start,
+		block->scan_hint_start + block->scan_hint);
+
+	if (block->contig_hint && overlap_with_contig_hint) {
+		start = min(start, block->contig_hint_start);
+		end = max(end, block->contig_hint_start + block->contig_hint);
+	}
+	if (block->scan_hint && overlap_with_scan_hint) {
+		start = min(start, block->scan_hint_start);
+		end = max(end, block->scan_hint_start + block->scan_hint);
+	}
+	contig = end - start;
 
 	block->first_free = min(block->first_free, start);
 	if (start == 0)
@@ -646,56 +666,86 @@ 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 > block->scan_hint) {
-				block->scan_hint_start =
-					block->contig_hint_start;
-				block->scan_hint = block->contig_hint;
-			} else if (start < block->scan_hint_start) {
-				/*
-				 * The old contig_hint == scan_hint.  But, the
-				 * new contig is larger so hold the invariant
-				 * scan_hint_start < contig_hint_start.
-				 */
-				block->scan_hint = 0;
-			}
-		} else {
-			block->scan_hint = 0;
+		if (!overlap_with_contig_hint) {
+			scan_hint_cand_1 = block->contig_hint;
+			scan_hint_cand_1_start = block->contig_hint_start;
 		}
-		block->contig_hint_start = start;
+
 		block->contig_hint = contig;
+		block->contig_hint_start = start;
 	} else if (contig == block->contig_hint) {
 		if (block->contig_hint_start &&
 		    (!start ||
 		     __ffs(start) > __ffs(block->contig_hint_start))) {
-			/* start has a better alignment so use it */
+			scan_hint_cand_1 = block->contig_hint;
+			scan_hint_cand_1_start = block->contig_hint_start;
+
+			/* Start has a better alignment so use it. */
 			block->contig_hint_start = start;
-			if (start < block->scan_hint_start &&
-			    block->contig_hint > block->scan_hint)
-				block->scan_hint = 0;
-		} else if (start > block->scan_hint_start ||
-			   block->contig_hint > block->scan_hint) {
-			/*
-			 * Knowing contig == contig_hint, update the scan_hint
-			 * if it is farther than or larger than the current
-			 * scan_hint.
-			 */
-			block->scan_hint_start = start;
-			block->scan_hint = contig;
+		} else {
+			if (!overlap_with_contig_hint) {
+				scan_hint_cand_1 = contig;
+				scan_hint_cand_1_start = start;
+			}
 		}
 	} else {
 		/*
-		 * The region is smaller than the contig_hint.  So only update
-		 * the scan_hint if it is larger than or equal and farther than
-		 * the current scan_hint.
+		 * Consider only when the new contig is larger than or equal to
+		 * the old scan hint.
 		 */
-		if ((start < block->contig_hint_start &&
-		     (contig > block->scan_hint ||
-		      (contig == block->scan_hint &&
-		       start > block->scan_hint_start)))) {
-			block->scan_hint_start = start;
-			block->scan_hint = contig;
+		if (contig >= block->scan_hint) {
+			scan_hint_cand_1 = contig;
+			scan_hint_cand_1_start = start;
+		}
+	}
+
+	if (block->scan_hint &&
+	    !pcpu_region_overlap(start, end, block->scan_hint_start,
+				 block->scan_hint_start + block->scan_hint)) {
+		scan_hint_cand_2 = block->scan_hint;
+		scan_hint_cand_2_start = block->scan_hint_start;
+	}
+
+	/* Make scan_hint_cand_1 be the best candidate for the new scan hint. */
+	if ((scan_hint_cand_2 > scan_hint_cand_1) ||
+	    (scan_hint_cand_2 == scan_hint_cand_1 &&
+	     scan_hint_cand_2_start > scan_hint_cand_1_start)) {
+		int tmp_hint = scan_hint_cand_1;
+		int tmp_hint_start = scan_hint_cand_1_start;
+
+		scan_hint_cand_1 = scan_hint_cand_2;
+		scan_hint_cand_1_start = scan_hint_cand_2_start;
+		scan_hint_cand_2 = tmp_hint;
+		scan_hint_cand_2_start = tmp_hint_start;
+	}
+
+	/*
+	 * At this point, it is guaranteed that none of the scan hint
+	 * candidates overlaps with the new contig hint while they may overlap
+	 * with the old scan hint, and that the first candidate is larger in
+	 * size or, it equal, farther than the second one.
+	 */
+
+	if (block->contig_hint > scan_hint_cand_1) {
+		if (scan_hint_cand_1_start < block->contig_hint_start) {
+			block->scan_hint = scan_hint_cand_1;
+			block->scan_hint_start = scan_hint_cand_1_start;
+		} else if (scan_hint_cand_2_start < block->contig_hint_start) {
+			block->scan_hint = scan_hint_cand_2;
+			block->scan_hint_start = scan_hint_cand_2_start;
+		} else {
+			block->scan_hint = 0;
+		}
+	} else if (block->contig_hint == scan_hint_cand_1) {
+		if (scan_hint_cand_1_start > block->contig_hint_start) {
+			block->scan_hint = scan_hint_cand_1;
+			block->scan_hint_start = scan_hint_cand_1_start;
+		} else if (scan_hint_cand_2 < block->contig_hint &&
+			   scan_hint_cand_2_start < scan_hint_cand_1_start) {
+			block->scan_hint = scan_hint_cand_2;
+			block->scan_hint_start = scan_hint_cand_2_start;
+		} else {
+			block->scan_hint = 0;
 		}
 	}
 }
-- 
2.53.0.1213.gd9a14994de-goog

Re: [PATCH v3 3/3] percpu: Fix hint invariant breakage

[Dennis Zhou]

On Fri, Apr 10, 2026 at 05:44:17PM +0000, Joonwon Kang wrote:
> The invariant "scan_hint_start > contig_hint_start if and only if
> scan_hint == contig_hint" should be kept for hint management. However,
> it could be broken in some cases:
> 
>   - if (new contig == contig_hint == scan_hint) && (contig_hint_start <
>     scan_hint_start < new contig start) && the new contig is to become a
>     new contig_hint due to its better alignment, then scan_hint should
>     be invalidated instead of keeping the old value.
> 
>   - if (new contig == contig_hint > scan_hint) && (new contig start <
>     contig_hint_start) && the new contig is not to become a new
>     contig_hint, then scan_hint should be not updated to the new contig.
> 
> This commit mainly fixes this invariant breakage and includes more:
> 
>   - Refactor the percpu block update code to make it more visible on
>     what to consider, e.g. when the new contig overlaps with the old
>     contig_hint or scan_hint.
> 

Could you please split this up between the fixing the scan_hint issues
and then refactoring. That way this commit isn't doing too much.

>   - Merge the new contig with other hints when it overlaps with them and
>     treat it as a whole free region instead of a separate small region.
> 
>   - Fix the invariant breakage and also optimizes scan_hint further.
>     Some of the optimization cases when no overlap occurs are:
> 
>     - if (new contig > contig_hint > scan_hint) && (scan_hint_start < new
>       contig start < contig_hint_start), then keep scan_hint instead of
>       invalidating it.
> 
>     - if (new contig > contig_hint == scan_hint) && (contig_hint_start <
>       new contig start < scan_hint_start), then update scan_hint to the
>       old contig_hint instead of invalidating it.
> 
>     - if (new contig == contig_hint > scan_hint) && (new contig start <
>       contig_hint_start) && the new contig is to become a new contig_hint
>       due to its better alignment, then update scan_hint to the old
>       contig_hint instead of invalidating or keeping it.
> 
> Signed-off-by: Joonwon Kang <joonwonkang@google.com>
> ---
> v2 -> v3: Merge the new contig with other hints when it overlaps with
>   them and treat it as a whole free region instead of a separate small
>   region.
> 
> v1 -> v2: Consider cases where the new contig overlaps with the existing
>   contig_hint or scan_hint.
> 
>  mm/percpu.c | 130 ++++++++++++++++++++++++++++++++++++----------------
>  1 file changed, 90 insertions(+), 40 deletions(-)
> 
> diff --git a/mm/percpu.c b/mm/percpu.c
> index f16533ed4a49..d5b0b4863ffe 100644
> --- a/mm/percpu.c
> +++ b/mm/percpu.c
> @@ -629,7 +629,27 @@ static inline bool pcpu_region_overlap(int a, int b, int x, int y)
>   */
>  static void pcpu_block_update(struct pcpu_block_md *block, int start, int end)
>  {
> -	int contig = end - start;
> +	int contig;
> +	int scan_hint_cand_1 = 0;
> +	int scan_hint_cand_1_start = 0;
> +	int scan_hint_cand_2 = 0;
> +	int scan_hint_cand_2_start = 0;

I'm not really a fan of this cand_1 and cand_2. Re-reading all this
code. I think I really should have introduced something like:

struct pcpu_region {
    int start;
    int size;
}

It's really easy to mix contig_hint and contig_hint_start. If you don't
mind, it would be ideal if we could introduce it but it might be a
non-trivial amount of refactor.

I'd say the shortest path forward would be to do just to fix the
scan_hint issues.

> +	bool overlap_with_contig_hint = pcpu_region_overlap(start, end,
> +		block->contig_hint_start,
> +		block->contig_hint_start + block->contig_hint);
> +	bool overlap_with_scan_hint = pcpu_region_overlap(start, end,
> +		block->scan_hint_start,
> +		block->scan_hint_start + block->scan_hint);
> +

This one isn't used again so we should probably just inline it.

> +	if (block->contig_hint && overlap_with_contig_hint) {
> +		start = min(start, block->contig_hint_start);
> +		end = max(end, block->contig_hint_start + block->contig_hint);
> +	}
> +	if (block->scan_hint && overlap_with_scan_hint) {
> +		start = min(start, block->scan_hint_start);
> +		end = max(end, block->scan_hint_start + block->scan_hint);
> +	}
> +	contig = end - start;
>  
>  	block->first_free = min(block->first_free, start);
>  	if (start == 0)
> @@ -646,56 +666,86 @@ 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 > block->scan_hint) {
> -				block->scan_hint_start =
> -					block->contig_hint_start;
> -				block->scan_hint = block->contig_hint;
> -			} else if (start < block->scan_hint_start) {
> -				/*
> -				 * The old contig_hint == scan_hint.  But, the
> -				 * new contig is larger so hold the invariant
> -				 * scan_hint_start < contig_hint_start.
> -				 */
> -				block->scan_hint = 0;
> -			}
> -		} else {
> -			block->scan_hint = 0;
> +		if (!overlap_with_contig_hint) {
> +			scan_hint_cand_1 = block->contig_hint;
> +			scan_hint_cand_1_start = block->contig_hint_start;
>  		}
> -		block->contig_hint_start = start;
> +
>  		block->contig_hint = contig;
> +		block->contig_hint_start = start;
>  	} else if (contig == block->contig_hint) {
>  		if (block->contig_hint_start &&
>  		    (!start ||
>  		     __ffs(start) > __ffs(block->contig_hint_start))) {
> -			/* start has a better alignment so use it */
> +			scan_hint_cand_1 = block->contig_hint;
> +			scan_hint_cand_1_start = block->contig_hint_start;
> +
> +			/* Start has a better alignment so use it. */
>  			block->contig_hint_start = start;
> -			if (start < block->scan_hint_start &&
> -			    block->contig_hint > block->scan_hint)
> -				block->scan_hint = 0;
> -		} else if (start > block->scan_hint_start ||
> -			   block->contig_hint > block->scan_hint) {
> -			/*
> -			 * Knowing contig == contig_hint, update the scan_hint
> -			 * if it is farther than or larger than the current
> -			 * scan_hint.
> -			 */
> -			block->scan_hint_start = start;
> -			block->scan_hint = contig;
> +		} else {
> +			if (!overlap_with_contig_hint) {
> +				scan_hint_cand_1 = contig;
> +				scan_hint_cand_1_start = start;
> +			}
>  		}
>  	} else {
>  		/*
> -		 * The region is smaller than the contig_hint.  So only update
> -		 * the scan_hint if it is larger than or equal and farther than
> -		 * the current scan_hint.
> +		 * Consider only when the new contig is larger than or equal to
> +		 * the old scan hint.
>  		 */
> -		if ((start < block->contig_hint_start &&
> -		     (contig > block->scan_hint ||
> -		      (contig == block->scan_hint &&
> -		       start > block->scan_hint_start)))) {
> -			block->scan_hint_start = start;
> -			block->scan_hint = contig;
> +		if (contig >= block->scan_hint) {
> +			scan_hint_cand_1 = contig;
> +			scan_hint_cand_1_start = start;
> +		}
> +	}
> +
> +	if (block->scan_hint &&
> +	    !pcpu_region_overlap(start, end, block->scan_hint_start,
> +				 block->scan_hint_start + block->scan_hint)) {
> +		scan_hint_cand_2 = block->scan_hint;
> +		scan_hint_cand_2_start = block->scan_hint_start;
> +	}
> +
> +	/* Make scan_hint_cand_1 be the best candidate for the new scan hint. */
> +	if ((scan_hint_cand_2 > scan_hint_cand_1) ||
> +	    (scan_hint_cand_2 == scan_hint_cand_1 &&
> +	     scan_hint_cand_2_start > scan_hint_cand_1_start)) {
> +		int tmp_hint = scan_hint_cand_1;
> +		int tmp_hint_start = scan_hint_cand_1_start;
> +
> +		scan_hint_cand_1 = scan_hint_cand_2;
> +		scan_hint_cand_1_start = scan_hint_cand_2_start;
> +		scan_hint_cand_2 = tmp_hint;
> +		scan_hint_cand_2_start = tmp_hint_start;
> +	}
> +
> +	/*
> +	 * At this point, it is guaranteed that none of the scan hint
> +	 * candidates overlaps with the new contig hint while they may overlap
> +	 * with the old scan hint, and that the first candidate is larger in
> +	 * size or, it equal, farther than the second one.
> +	 */
> +
> +	if (block->contig_hint > scan_hint_cand_1) {
> +		if (scan_hint_cand_1_start < block->contig_hint_start) {
> +			block->scan_hint = scan_hint_cand_1;
> +			block->scan_hint_start = scan_hint_cand_1_start;
> +		} else if (scan_hint_cand_2_start < block->contig_hint_start) {
> +			block->scan_hint = scan_hint_cand_2;
> +			block->scan_hint_start = scan_hint_cand_2_start;
> +		} else {
> +			block->scan_hint = 0;
> +		}
> +	} else if (block->contig_hint == scan_hint_cand_1) {
> +		if (scan_hint_cand_1_start > block->contig_hint_start) {
> +			block->scan_hint = scan_hint_cand_1;
> +			block->scan_hint_start = scan_hint_cand_1_start;
> +		} else if (scan_hint_cand_2 < block->contig_hint &&
> +			   scan_hint_cand_2_start < scan_hint_cand_1_start) {
> +			block->scan_hint = scan_hint_cand_2;
> +			block->scan_hint_start = scan_hint_cand_2_start;
> +		} else {
> +			block->scan_hint = 0;
>  		}


This seems easier to invert and do something like this:

Probably something like:

bool overlap_contig_hint = pcpu_region_overlap(new_region, scan_hint);
bool overlap_scan_hint = pcpu_region_overlap(new_region, scan_hint);

if (overlap_scan_hint) {
    scan_hint = 0;
}

if (overlap_contig_hint) {
    contig_hint = new_region;
    return;
}

// Now the new_region is distinct from the contig_hint and then can do
// the standard logic here.

if (new_region > contig_hint) { }
else if (new_region == contig_hint) {}
else {}

What do you think? I think makes cand_1 and cand_2 not necessary.

>  	}
>  }
> -- 
> 2.53.0.1213.gd9a14994de-goog
> 

Thanks,
Dennis

Re: [PATCH v3 1/3] percpu: Fix wrong chunk hints update

[Dennis Zhou]

Hello,

On Fri, Apr 10, 2026 at 05:44:15PM +0000, Joonwon Kang wrote:
> Chunk end offset was set to a block end offset, which could prevent
> chunk hints from being updated correctly. It was observed that the chunk
> free size gets minus or shorter than the actual free size due to this.
> This commit fixes it.
> 
> Signed-off-by: Joonwon Kang <joonwonkang@google.com>
> ---
> v3: Initial version.
> 
>  mm/percpu.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/percpu.c b/mm/percpu.c
> index 81462ce5866e..3ecd86096641 100644
> --- a/mm/percpu.c
> +++ b/mm/percpu.c
> @@ -1054,7 +1054,7 @@ static void pcpu_block_update_hint_free(struct pcpu_chunk *chunk, int bit_off,
>  	else
>  		pcpu_block_update(&chunk->chunk_md,
>  				  pcpu_block_off_to_off(s_index, start),
> -				  end);
> +				  pcpu_block_off_to_off(e_index, end));
>  }

This needs a fixes tag for:

Fixes: 92c14cab4326 ("percpu: convert chunk hints to be based on pcpu_block_md")

Reviewed-by: Dennis Zhou <dennis@kernel.org>

Thanks,
Dennis

>  
>  /**
> -- 
> 2.53.0.1213.gd9a14994de-goog
>