[PATCH] damon: reduce stack usage further

[PATCH] damon: reduce stack usage further

[Arnd Bergmann]

From: Arnd Bergmann <arnd@arndb.de>

In a previous patch, I had annotated kdamond_tune_intervals() as
noinline_for_stack in order to not exceed the stack frame warning
limit. Unfortunately, my current linux-next randconfig builds show
a similar problem again with clang-21:

mm/damon/core.c:3953:12: error: stack frame size (1288) exceeds limit (1280) in 'kdamond_fn' [-Werror,-Wframe-larger-than]
 3953 | static int kdamond_fn(void *data)

Do the same thing with kdamond_apply_schemes(), kdamond_merge_regions(),
and kdamond_split_regions(), which also have individually large stacks.
This should work to keep the deepest total stack depth down much more
as well as avoid the warning.

I also tried to reduce the complexity of kdamond_fn() itself by splitting
out the while() loop into a separate function. While this arguably led
to slightly more readable code, it had no effect on the total stack
usage and just made the new function the largest stack user and had
a nonzero risk of me getting the conversion wrong.

Fixes: 5a00cae64de1 ("mm/damon/core: reduce kernel stack usage")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
---
 mm/damon/core.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/mm/damon/core.c b/mm/damon/core.c
index 4687b909d42c..9803b459a0a4 100644
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -3417,7 +3417,7 @@ static void damos_trace_stat(struct damon_ctx *c, struct damos *s)
 	trace_call__damos_stat_after_apply_interval(cidx, sidx, &s->stat);
 }
 
-static void kdamond_apply_schemes(struct damon_ctx *c)
+static noinline_for_stack void kdamond_apply_schemes(struct damon_ctx *c)
 {
 	struct damon_target *t;
 	struct damos *s;
@@ -3573,8 +3573,9 @@ static void damon_merge_regions_of(struct damon_target *t, unsigned int thres,
  * while DAMON is running.  For such a case, repeat merging until the limit is
  * met while increasing @threshold up to possible maximum level.
  */
-static void kdamond_merge_regions(struct damon_ctx *c, unsigned int threshold,
-				  unsigned long sz_limit)
+static noinline_for_stack void kdamond_merge_regions(struct damon_ctx *c,
+						     unsigned int threshold,
+						     unsigned long sz_limit)
 {
 	struct damon_target *t;
 	unsigned int nr_regions;
@@ -3718,7 +3719,7 @@ static void damon_split_some_regions(struct damon_ctx *ctx,
  * split was unnecessarily made, later 'kdamond_merge_regions()' will revert
  * it.
  */
-static void kdamond_split_regions(struct damon_ctx *ctx)
+static noinline_for_stack void kdamond_split_regions(struct damon_ctx *ctx)
 {
 	struct damon_target *t;
 	unsigned long nr_regions = 0;
-- 
2.53.0

Re: [PATCH] damon: reduce stack usage further

[SJ Park]

On Fri, 25 Sep 2026 15:02:46 +0200 Arnd Bergmann <arnd@kernel.org> wrote:

> From: Arnd Bergmann <arnd@arndb.de>
> 
> In a previous patch, I had annotated kdamond_tune_intervals() as
> noinline_for_stack in order to not exceed the stack frame warning
> limit. Unfortunately, my current linux-next randconfig builds show
> a similar problem again with clang-21:
> 
> mm/damon/core.c:3953:12: error: stack frame size (1288) exceeds limit (1280) in 'kdamond_fn' [-Werror,-Wframe-larger-than]
>  3953 | static int kdamond_fn(void *data)
> 
> Do the same thing with kdamond_apply_schemes(), kdamond_merge_regions(),
> and kdamond_split_regions(), which also have individually large stacks.
> This should work to keep the deepest total stack depth down much more
> as well as avoid the warning.
> 
> I also tried to reduce the complexity of kdamond_fn() itself by splitting
> out the while() loop into a separate function. While this arguably led
> to slightly more readable code, it had no effect on the total stack
> usage and just made the new function the largest stack user and had
> a nonzero risk of me getting the conversion wrong.

Looks good to me.  Thank you Arnd!

> 
> Fixes: 5a00cae64de1 ("mm/damon/core: reduce kernel stack usage")
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>

Reviewed-by: SJ Park <sj@kernel.org>

This patch is applied to damon/next [1] tree.  If this patch is not added to
mm.git in short term (~1 week?), I will ask mm.git maintainer (Andrew Morton)
to pick this.  So, no action from your side is needed for now.  If it seems I
also forgot doing that or you cannot wait for my action, please feel free to
ping me or Andrew.

[1] https://origin.kernel.org/doc/html/latest/mm/damon/maintainer-profile.html#scm-trees


Thanks,
SJ

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