Re: [PATCH v6 1/4] percpu: Fix wrong chunk hints update

[Andrew Morton]

On Wed, 13 May 2026 08:51:13 +0000 Joonwon Kang <joonwonkang@xxxxxxxxxx> 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.
> 
> Fixes: 92c14cab4326 ("percpu: convert chunk hints to be based on pcpu_block_md")
> Signed-off-by: Joonwon Kang <joonwonkang@xxxxxxxxxx>
> Reviewed-by: Dennis Zhou <dennis@xxxxxxxxxx>

Thanks, I've updated mm.git to this version.  Hopefully we'll be
getting formal review of this soon.

It would be nice to have a formal [0/N] conver letter to introduce this
series.

Below is how v6 altered mm.git, due to alterations in [4/4]:

> v6: Diverge more when the new contig size is the same as the old
>    contig_hint size but greater than the old scan_hint, does not become a
>    new contig_hint and is before the old contig_hint.


 mm/percpu.c |   15 +++++++--------
 1 file changed, 7 insertions(+), 8 deletions(-)

--- a/mm/percpu.c~b
+++ a/mm/percpu.c
@@ -728,19 +728,18 @@ static void pcpu_block_update(struct pcp
 			block->contig_hint.start = free.start;
 		} else {
 			if (block->contig_hint.size > block->scan_hint.size) {
-				if (free.start < block->contig_hint.start) {
+				if (free.start > block->contig_hint.start) {
+					block->scan_hint = free;
+				} else if (block->scan_hint.size &&
+					   free.start < block->scan_hint.start) {
 					/*
 					 * old scan_hint.size < new contig size
 					 * == old contig_hint.size. But, the new
-					 * contig is before the old contig_hint
-					 * so hold the invariant
-					 * scan_hint.start > contig_hint.start
-					 * iff scan_hint.size ==
-					 * contig_hint.size.
+					 * contig is before the old scan_hint
+					 * so invalidate the scan_hint to
+					 * protect the contig_hint.
 					 */
 					block->scan_hint.size = 0;
-				} else {
-					block->scan_hint = free;
 				}
 			} else if (free.start > block->scan_hint.start) {
 				block->scan_hint = free;
_