+ mm-damon-core-move-static-variable-rotation-to-damon_ctx.patch added to mm-unstable branch

[Andrew Morton <akpm@linux-foundation.org>]

The patch titled
     Subject: mm/damon/core: move static variable rotation to damon_ctx
has been added to the -mm mm-unstable branch.  Its filename is
     mm-damon-core-move-static-variable-rotation-to-damon_ctx.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-damon-core-move-static-variable-rotation-to-damon_ctx.patch

This patch will later appear in the mm-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------
From: SJ Park <sj@kernel.org>
Subject: mm/damon/core: move static variable rotation to damon_ctx
Date: Sun, 25 Oct 2026 09:48:06 -0700

damon_split_some_reigons() uses a static variable, rotation, to remember
how many times it was called.  It is again used to determine fair
selection of regions to split.  Since it is a static variable, it can be
indeterministic and therefore less fair.

The user impact is not critical.  No crash, no leak.  DAMON's
best-effort and indeterministic monitoring accuracy could be slightly
more indeterministic when multiple kdamonds are used in parallel.

Fix the problem by moving the static variable into the damon_ctx struct.

Link: https://lore.kernel.org/20261025164808.152048-4-sj@kernel.org
Fixes: 1a582e6e0174 ("mm/damon/core: split a fraction of regions when nr_regions exceeds max/2")
Signed-off-by: SJ Park <sj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Closes: https://sashiko.dev/#/bug/linux-eebfcdde-2881-48ef-8df1-e2019d2a47f2
Cc: Jiayuan Chen <jiayuan.chen@shopee.com>
Cc: <stable@vger.kernel.org> # 7.3.x
---

 include/linux/damon.h |    5 +++++
 mm/damon/core.c       |    5 +++--
 2 files changed, 8 insertions(+), 2 deletions(-)

--- a/include/linux/damon.h~mm-damon-core-move-static-variable-rotation-to-damon_ctx
+++ a/include/linux/damon.h
@@ -963,6 +963,11 @@ struct damon_ctx {
 	 * for more details.
 	 */
 	unsigned long last_nr_regions;
+	/*
+	 * number of damon_split_some_regions() invocations for this context.
+	 * Read damon_split_some_regions() for more details.
+	 */
+	unsigned long region_split_rotation;
 };
 
 /* Get a random number in [@l, @r) using @ctx's lockless PRNG. */
--- a/mm/damon/core.c~mm-damon-core-move-static-variable-rotation-to-damon_ctx
+++ a/mm/damon/core.c
@@ -3753,12 +3753,13 @@ static void damon_split_regions_of(struc
 static void damon_split_some_regions(struct damon_ctx *ctx,
 				     unsigned long split_step)
 {
-	static unsigned long rotation;
 	struct damon_target *t;
 	struct damon_region *r, *next;
-	unsigned long offset = rotation++ % split_step;
+	unsigned long offset;
 	unsigned long idx = 0;
 
+	offset = ctx->region_split_rotation++ % split_step;
+
 	damon_for_each_target(t, ctx) {
 		damon_for_each_region_safe(r, next, t) {
 			unsigned long sz_region, sz_sub;
_

Patches currently in -mm which might be from sj@kernel.org are

mm-damon-sysfs-schemes-hold-damon_sysfs_lock-in-target_metric_store.patch
mm-damon-core-move-static-variable-last_nr_regions-to-damon_ctx.patch
mm-damon-core-move-static-variable-rotation-to-damon_ctx.patch