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mm/damon/core: move static variable rotation to damon_ctx

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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: 1a582e6e01 ("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
This commit is contained in:
SJ Park 2026-10-25 09:48:06 -07:00 committed by Andrew Morton
parent 26ae50a090
commit c4ff3ea306
2 changed files with 8 additions and 2 deletions
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5
include/linux/damon.h
View File

@ -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. */

5
mm/damon/core.c
View File

@ -3753,12 +3753,13 @@ static void damon_split_regions_of(struct damon_ctx *ctx,
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;