[PATCH 6.1.y 0/2] mm/damon: porting fixes that cherry-pick failed on 2026-11-07

[PATCH 6.1.y 0/2] mm/damon: porting fixes that cherry-pick failed on 2026-11-07

[SJ Park]

Manually port two DAMON fixes that cherry-pick failed [1,2] and turned
out to really need to be picked.  Note that seven patches [3-9] that
also failed in the same day were having wrong Cc:stable@ tags, so
ignored.

Patches that this series is porting.

[1] https://lore.kernel.org/20261107054458-failed-6.1-90c2fa6adfed@kernel.org
[2] https://lore.kernel.org/20261107055454-failed-6.1-5695abc4e742@kernel.org

Patches that also cherry-pick failed but deliverately ignored.

[3] https://lore.kernel.org/20261107054658-failed-6.1-cec08aad3def@kernel.org
[4] https://lore.kernel.org/20261107054701-failed-6.1-1bfb9384ae0e@kernel.org
[5] https://lore.kernel.org/20261107054705-failed-6.1-6f276422aded@kernel.org
[6] https://lore.kernel.org/20261107054708-failed-6.1-a1c7fdaf1657@kernel.org
[7] https://lore.kernel.org/20261107054711-failed-6.1-409d2ab6d982@kernel.org
[8] https://lore.kernel.org/20261107054714-failed-6.1-1d2397b2c414@kernel.org
[9] https://lore.kernel.org/20261107054721-failed-6.1-1c3352282063@kernel.org

Changes from RFC
- RFC: https://lore.kernel.org/20261107224800.340250-1-sj@kernel.org
- None.

SJ Park (2):
  mm/damon/core: skip applying scheme if region split for quota fails
  mm/damon/core: remove unreachable damos charge_{target,addr}_from

 mm/damon/core.c | 34 ++++++++++++++++++++++++++++++----
 1 file changed, 30 insertions(+), 4 deletions(-)


base-commit: 718bf49618bbf78a08ab69ed0dfffa8e8f917cfa
-- 
2.47.3

[PATCH 6.1.y 1/2] mm/damon/core: skip applying scheme if region split for quota fails

[SJ Park]

Patch series "mm/damon: fix DAMOS bugs in core, paddr and vaddr".

Fix misc bugs of DAMOS.  Patch 1 makes DAMOS less stress memory allocator
under extreme situation.  Patches 2 and 3 fix wrong folios walking in
DAMON_PADDR.  Patches 4 and 5 fix wrong folios walking in DAMON_VADDR.
Patches 6-8 handle extreme and unlikely memory situations that can cause
divide by zero and underflow.

All bugs are discovered by Sashiko.

This patch (of 8):

damos_apply_scheme() splits a region and apply the action to the subregion
if it is needed for not violating the quota.  The split operation
(damon_split_region_at()) could fail for allocation failure.  In the case,
the quota could be violated.  From the user's perspective, DAMOS becomes
more aggressive than expected under the extreme situation.  Handle the
failure.

The user impact is not critical.  The failure of damon_split_region_at()
is unlikely since it is arguably too small to fail.  Also DAMOS being
aggressive is limited to the single region.  Users can set min_nr_regions
to set the maximum size of each region.  If it is reasonably set, the
transient overhead shouldn't be critical.

The issue was discovered [1] by Sashiko.

6.1.y porting notes.  Modify damon_split_region_at() to return failures.
It is a part of commit c60874860762 ("mm/damon/core: handle region split
failure in apply_min_nr_regions()").  Without it, build fails.

Link: https://lore.kernel.org/20260901131850.98037-1-sj@kernel.org
Link: https://lore.kernel.org/20260901131850.98037-2-sj@kernel.org
Link: https://lore.kernel.org/20260718171523.87547-1-sj@kernel.org [1]
Fixes: 2b8a248d5873 ("mm/damon/schemes: implement size quota for schemes application speed control")
Signed-off-by: SJ Park <sj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Usama Arif <usamaarif642@gmail.com>
Cc: <stable@vger.kernel.org> # 5.16.x
(cherry picked from commit 90c2fa6adfed7abe0935506a43cdff3a05c1fff0)
Signed-off-by: SJ Park <sj@kernel.org>
---
 mm/damon/core.c | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/mm/damon/core.c b/mm/damon/core.c
index f0da5cb2b88a0..9206737d05ae7 100644
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -718,7 +718,7 @@ static void kdamond_reset_aggregated(struct damon_ctx *c)
 	}
 }
 
-static void damon_split_region_at(struct damon_target *t,
+static int damon_split_region_at(struct damon_target *t,
 				  struct damon_region *r, unsigned long sz_r);
 
 static bool __damos_valid_target(struct damon_region *r, struct damos *s)
@@ -810,7 +810,8 @@ static void damon_do_apply_schemes(struct damon_ctx *c,
 						DAMON_MIN_REGION);
 				if (!sz)
 					goto update_stat;
-				damon_split_region_at(t, r, sz);
+				if (damon_split_region_at(t, r, sz))
+					goto update_stat;
 			}
 			ktime_get_coarse_ts64(&begin);
 			sz_applied = c->ops.apply_scheme(c, t, r, s);
@@ -1015,15 +1016,17 @@ static void kdamond_merge_regions(struct damon_ctx *c, unsigned int threshold,
  *
  * r		the region to be split
  * sz_r		size of the first sub-region that will be made
+ *
+ * Return: 0 on success, negative error code otherwise.
  */
-static void damon_split_region_at(struct damon_target *t,
+static int damon_split_region_at(struct damon_target *t,
 				  struct damon_region *r, unsigned long sz_r)
 {
 	struct damon_region *new;
 
 	new = damon_new_region(r->ar.start + sz_r, r->ar.end);
 	if (!new)
-		return;
+		return -ENOMEM;
 
 	r->ar.end = new->ar.start;
 
@@ -1031,6 +1034,7 @@ static void damon_split_region_at(struct damon_target *t,
 	new->last_nr_accesses = r->last_nr_accesses;
 
 	damon_insert_region(new, r, damon_next_region(r), t);
+	return 0;
 }
 
 /* Split every region in the given target into 'nr_subs' regions */
-- 
2.47.3

[PATCH 6.1.y 2/2] mm/damon/core: remove unreachable damos charge_{target,addr}_from

[SJ Park]

damos_quota->charge_{target,addr}_from saves the target and region the
scheme's quota limit is reached.  damos_skip_charged_region() skips the
target and regions before the point.  The target that was assigned to
charge_target_from could be dynamically removed from the context by the
runtime parameters update (aka commit) feature.  In this case,
damos_skip_charged_region() cannot find if the point is passed or not,
so the scheme is effectively disabled.

Similarly, the user could also remove all the regions of the target
using the runtime parameters update feature.  It would be uncommon to do
that, but it is a supported feature.  Because the skip pointer detection
logic (damos_skip_charged_region()) works for each region of targets,
this result in the logic is entirely skipped and the scheme is
effectively disabled again.

The user impact is that the DAMOS scheme could suddenly stop working
under certain uses of the runtime parameters update feature.  It could
surprise users. No crash or leak happens though.  Users could also
simply work around by re-installing the scheme.

Catch the corner cases in damos_adjust_quota() and reset the pointers.

Link: https://lore.kernel.org/20261020023958.108276-1-sj@kernel.org
Fixes: da87878010e5 ("mm/damon/sysfs: support online inputs update")
Signed-off-by: SJ Park <sj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Closes: https://sashiko.dev/#/bug/linux-cd0c55fc-0cc8-45d8-9637-3334811c49b8
Cc: <stable@vger.kernel.org> # 5.19.x
(cherry picked from commit 5695abc4e742b947f586bfb6a080a80ad9957e5d)
Signed-off-by: SJ Park <sj@kernel.org>
---
 mm/damon/core.c | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/mm/damon/core.c b/mm/damon/core.c
index 9206737d05ae7..01f93146b92fe 100644
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -860,6 +860,26 @@ static void damos_set_effective_quota(struct damos_quota *quota)
 	quota->esz = esz;
 }
 
+static void damos_fix_stale_charge_pointers(struct damon_ctx *c,
+		struct damos *s)
+{
+	struct damos_quota *quota;
+	struct damon_target *t;
+
+	quota = &s->quota;
+	if (!quota->charge_target_from)
+		return;
+	damon_for_each_target(t, c) {
+		if (t == quota->charge_target_from) {
+			if (damon_nr_regions(t) > 0)
+				return;
+			break;
+		}
+	}
+	quota->charge_target_from = NULL;
+	quota->charge_addr_from = 0;
+}
+
 static void kdamond_apply_schemes(struct damon_ctx *c)
 {
 	struct damon_target *t;
@@ -890,6 +910,8 @@ static void kdamond_apply_schemes(struct damon_ctx *c)
 			damos_set_effective_quota(quota);
 		}
 
+		damos_fix_stale_charge_pointers(c, s);
+
 		if (!c->ops.get_scheme_score)
 			continue;
 
-- 
2.47.3

Re: [PATCH 6.1.y 1/2] mm/damon/core: skip applying scheme if region split for quota fails

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 1 potential
issue to consider:
- [Low] mm/damon: inflated tried stats in damon_do_apply_schemes() on
  split failure
--

commit 3b087258b4341cba87012b7cb96ed676fd164c12
Author: SJ Park <sj@kernel.org>

mm/damon/core: skip applying scheme if region split for quota fails

This commit updates damon_split_region_at() to return an error code on
allocation failure and skips applying the DAMOS scheme in
damon_do_apply_schemes() when splitting a region to fit the quota fails.

Link: https://lore.kernel.org/20260901131850.98037-1-sj@kernel.org
Link: https://lore.kernel.org/20260901131850.98037-2-sj@kernel.org
Link: https://lore.kernel.org/20260718171523.87547-1-sj@kernel.org [1]

> diff --git a/mm/damon/core.c b/mm/damon/core.c
> index f0da5cb2b88a0..9206737d05ae7 100644
> --- a/mm/damon/core.c
> +++ b/mm/damon/core.c

[ ... ]

> @@ -810,7 +810,8 @@ static void damon_do_apply_schemes(struct damon_ctx *c,
>  						DAMON_MIN_REGION);
>  				if (!sz)
>  					goto update_stat;
> -				damon_split_region_at(t, r, sz);
> +				if (damon_split_region_at(t, r, sz))
> +					goto update_stat;
>  			}
>  			ktime_get_coarse_ts64(&begin);
>  			sz_applied = c->ops.apply_scheme(c, t, r, s);

[Severity: Low]
Does jumping to update_stat on damon_split_region_at() failure inflate
s->stat.nr_tried and s->stat.sz_tried?

When damon_split_region_at() fails, sz is the non-zero remaining quota
size, c->ops.apply_scheme() is skipped, and quota->charged_sz is not
advanced:

mm/damon/core.c:damon_do_apply_schemes() {
	...
update_stat:
		s->stat.nr_tried++;
		s->stat.sz_tried += sz;
	...
}

Because quota->charged_sz remains unchanged, subsequent matching regions
can repeatedly add sz to s->stat.sz_tried if allocation continues to fail.

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261108202759.117377-1-sj@kernel.org?part=1

Re: [PATCH 6.1.y 1/2] mm/damon/core: skip applying scheme if region split for quota fails

[SJ Park]

On Sun, 08 Nov 2026 20:36:18 +0000 sashiko-bot@kernel.org wrote:

> Thank you for your contribution! Sashiko AI review found 1 potential
> issue to consider:
> - [Low] mm/damon: inflated tried stats in damon_do_apply_schemes() on
>   split failure
> --
> 
> commit 3b087258b4341cba87012b7cb96ed676fd164c12
> Author: SJ Park <sj@kernel.org>
> 
> mm/damon/core: skip applying scheme if region split for quota fails
> 
> This commit updates damon_split_region_at() to return an error code on
> allocation failure and skips applying the DAMOS scheme in
> damon_do_apply_schemes() when splitting a region to fit the quota fails.
> 
> Link: https://lore.kernel.org/20260901131850.98037-1-sj@kernel.org
> Link: https://lore.kernel.org/20260901131850.98037-2-sj@kernel.org
> Link: https://lore.kernel.org/20260718171523.87547-1-sj@kernel.org [1]
> 
> > diff --git a/mm/damon/core.c b/mm/damon/core.c
> > index f0da5cb2b88a0..9206737d05ae7 100644
> > --- a/mm/damon/core.c
> > +++ b/mm/damon/core.c
> 
> [ ... ]
> 
> > @@ -810,7 +810,8 @@ static void damon_do_apply_schemes(struct damon_ctx *c,
> >  						DAMON_MIN_REGION);
> >  				if (!sz)
> >  					goto update_stat;
> > -				damon_split_region_at(t, r, sz);
> > +				if (damon_split_region_at(t, r, sz))
> > +					goto update_stat;
> >  			}
> >  			ktime_get_coarse_ts64(&begin);
> >  			sz_applied = c->ops.apply_scheme(c, t, r, s);
> 
> [Severity: Low]
> Does jumping to update_stat on damon_split_region_at() failure inflate
> s->stat.nr_tried and s->stat.sz_tried?
> 
> When damon_split_region_at() fails, sz is the non-zero remaining quota
> size, c->ops.apply_scheme() is skipped, and quota->charged_sz is not
> advanced:
> 
> mm/damon/core.c:damon_do_apply_schemes() {
> 	...
> update_stat:
> 		s->stat.nr_tried++;
> 		s->stat.sz_tried += sz;
> 	...
> }
> 
> Because quota->charged_sz remains unchanged, subsequent matching regions
> can repeatedly add sz to s->stat.sz_tried if allocation continues to fail.

Trivial user impact issue.  The same behavior exists in the mainline.  No
blocker for this patch.

> 
> -- 
> Sashiko AI review · https://sashiko.dev/#/patchset/20261108202759.117377-1-sj@kernel.org?part=1
> 


Thanks,
SJ