[PATCH 6.18.y] mm/damon/core: avoid infinite kdamond_merge_regions() internal loop

[PATCH 6.18.y] mm/damon/core: avoid infinite kdamond_merge_regions() internal loop

[SJ Park]

Patch series "mm/damon: unurgent fixes for infinite loop, NULL de-ref and
races", v1.1.

Sashiko found a few issues in DAMON that could cause infinite loop, NULL
dereference and monitoring results degradation.  The first two sounds
scary but the infinite loop happens only under unreasonable user setup.
The NULL dereference is only in a unit test.  Monitoring results
degradation is trivial since it is only best-effort, and those happens
from only unlikely races.  Still those are bugs that better to fix if
possible.  Fix those.

This patch (of 6):

Due to online parameter update like events, the number of DAMON regions
could be higher than the user-set upper limit.  kdamond_merge_regions()
repeats merge regions until the number meets the limit, while doubling the
merge threshold up to the theoretical maximum threshold.  It is tried only
up to the theoretical maximum threshold because even the aggressive
merging can fail from reducing the number of regions under the
user-defined upper limit.  For example, there could be many user-defined
non-contiguous regions that cannot be merged.

The threshold based loop break condition is evaluated by comparing the
threshold for the next merging try against the theoretical maximum
threshold.  If max_thres is larger than UINT_MAX / 2, doubling the
threshold could make it overflow, and bypass the loop break condition.  In
the case, if the number of regions cannot be reduced under the upper limit
like explained above, the loop will run infinitely.

Prevent the case by doing the break condition check before doubling the
threshold.  Also, prevent the threshold exceeding the maximum threshold,
as it could overflow and apply the wrong merge threshold.

This issue is unlikely to occur in real world, since having the max_thres
higher than UINT_MAX / 2 require unrealistically large aggregation
intervals compared to the sampling interval.  Also, it requires an
unrealistically large number of uncontiguous regions setup.  Nonetheless,
the consequence is bad and the fix is simple.

The issue was discovered [1] by Sashiko.

Link: https://lore.kernel.org/20260715031002.108504-1-sj@kernel.org
Link: https://lore.kernel.org/20260715031002.108504-2-sj@kernel.org
Link: https://lore.kernel.org/20260709145425.96247-1-sj@kernel.org [1]
Fixes: 310d6c15e910 ("mm/damon/core: merge regions aggressively when max_nr_regions is unmet")
Signed-off-by: SJ Park <sj@kernel.org>
Cc: <stable@vger.kernel.org> # 6.10.x
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
(cherry picked from commit 123e4619ab6c8ab1c4cb1d7a58311a2af13929cd)
Signed-off-by: SJ Park <sj@kernel.org>
---
 mm/damon/core.c | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/mm/damon/core.c b/mm/damon/core.c
index 70ac1f08753d1..08e527efd6883 100644
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -2411,15 +2411,20 @@ static void kdamond_merge_regions(struct damon_ctx *c, unsigned int threshold,
 
 	max_thres = c->attrs.aggr_interval /
 		(c->attrs.sample_interval ?  c->attrs.sample_interval : 1);
-	do {
+	while (true) {
 		nr_regions = 0;
 		damon_for_each_target(t, c) {
 			damon_merge_regions_of(t, threshold, sz_limit);
 			nr_regions += damon_nr_regions(t);
 		}
-		threshold = max(1, threshold * 2);
-	} while (nr_regions > c->attrs.max_nr_regions &&
-			threshold / 2 < max_thres);
+		if (nr_regions <= c->attrs.max_nr_regions ||
+				max_thres <= threshold)
+			break;
+		if (threshold < max_thres / 2)
+			threshold = max(1, threshold * 2);
+		else
+			threshold = max_thres;
+	}
 }
 
 /*
-- 
2.47.3

Re: [PATCH 6.18.y] mm/damon/core: avoid infinite kdamond_merge_regions() internal loop

[sashiko-bot]

> Patch series "mm/damon: unurgent fixes for infinite loop, NULL de-ref and
> races", v1.1.
> 
> Sashiko found a few issues in DAMON that could cause infinite loop, NULL
> dereference and monitoring results degradation.  The first two sounds
> scary but the infinite loop happens only under unreasonable user setup.
> The NULL dereference is only in a unit test.  Monitoring results
> degradation is trivial since it is only best-effort, and those happens
> from only unlikely races.  Still those are bugs that better to fix if
> possible.  Fix those.
> 
> This patch (of 6):
> 
> Due to online parameter update like events, the number of DAMON regions
> could be higher than the user-set upper limit.  kdamond_merge_regions()
> [ ... ]
> Signed-off-by: SJ Park <sj@kernel.org>
> Cc: <stable@vger.kernel.org> # 6.10.x
> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
> (cherry picked from commit 123e4619ab6c8ab1c4cb1d7a58311a2af13929cd)
> Signed-off-by: SJ Park <sj@kernel.org>

Sashiko has reviewed this patch and found no issues. It looks great!

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260909041502.181947-1-sj@kernel.org?part=1

Re: [PATCH 6.18.y] mm/damon/core: avoid infinite kdamond_merge_regions() internal loop

[Greg KH]

On Tue, Sep 08, 2026 at 09:15:02PM -0700, SJ Park wrote:
> Patch series "mm/damon: unurgent fixes for infinite loop, NULL de-ref and
> races", v1.1.
> 
> Sashiko found a few issues in DAMON that could cause infinite loop, NULL
> dereference and monitoring results degradation.  The first two sounds
> scary but the infinite loop happens only under unreasonable user setup.
> The NULL dereference is only in a unit test.  Monitoring results
> degradation is trivial since it is only best-effort, and those happens
> from only unlikely races.  Still those are bugs that better to fix if
> possible.  Fix those.
> 
> This patch (of 6):
> 
> Due to online parameter update like events, the number of DAMON regions
> could be higher than the user-set upper limit.  kdamond_merge_regions()
> repeats merge regions until the number meets the limit, while doubling the
> merge threshold up to the theoretical maximum threshold.  It is tried only
> up to the theoretical maximum threshold because even the aggressive
> merging can fail from reducing the number of regions under the
> user-defined upper limit.  For example, there could be many user-defined
> non-contiguous regions that cannot be merged.
> 
> The threshold based loop break condition is evaluated by comparing the
> threshold for the next merging try against the theoretical maximum
> threshold.  If max_thres is larger than UINT_MAX / 2, doubling the
> threshold could make it overflow, and bypass the loop break condition.  In
> the case, if the number of regions cannot be reduced under the upper limit
> like explained above, the loop will run infinitely.
> 
> Prevent the case by doing the break condition check before doubling the
> threshold.  Also, prevent the threshold exceeding the maximum threshold,
> as it could overflow and apply the wrong merge threshold.
> 
> This issue is unlikely to occur in real world, since having the max_thres
> higher than UINT_MAX / 2 require unrealistically large aggregation
> intervals compared to the sampling interval.  Also, it requires an
> unrealistically large number of uncontiguous regions setup.  Nonetheless,
> the consequence is bad and the fix is simple.
> 
> The issue was discovered [1] by Sashiko.
> 
> Link: https://lore.kernel.org/20260715031002.108504-1-sj@kernel.org
> Link: https://lore.kernel.org/20260715031002.108504-2-sj@kernel.org
> Link: https://lore.kernel.org/20260709145425.96247-1-sj@kernel.org [1]
> Fixes: 310d6c15e910 ("mm/damon/core: merge regions aggressively when max_nr_regions is unmet")
> Signed-off-by: SJ Park <sj@kernel.org>
> Cc: <stable@vger.kernel.org> # 6.10.x
> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
> (cherry picked from commit 123e4619ab6c8ab1c4cb1d7a58311a2af13929cd)
> Signed-off-by: SJ Park <sj@kernel.org>
> ---
>  mm/damon/core.c | 13 +++++++++----
>  1 file changed, 9 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/damon/core.c b/mm/damon/core.c
> index 70ac1f08753d1..08e527efd6883 100644
> --- a/mm/damon/core.c
> +++ b/mm/damon/core.c
> @@ -2411,15 +2411,20 @@ static void kdamond_merge_regions(struct damon_ctx *c, unsigned int threshold,
>  
>  	max_thres = c->attrs.aggr_interval /
>  		(c->attrs.sample_interval ?  c->attrs.sample_interval : 1);
> -	do {
> +	while (true) {
>  		nr_regions = 0;
>  		damon_for_each_target(t, c) {
>  			damon_merge_regions_of(t, threshold, sz_limit);
>  			nr_regions += damon_nr_regions(t);
>  		}
> -		threshold = max(1, threshold * 2);
> -	} while (nr_regions > c->attrs.max_nr_regions &&
> -			threshold / 2 < max_thres);
> +		if (nr_regions <= c->attrs.max_nr_regions ||
> +				max_thres <= threshold)
> +			break;
> +		if (threshold < max_thres / 2)
> +			threshold = max(1, threshold * 2);
> +		else
> +			threshold = max_thres;
> +	}
>  }
>  
>  /*
> -- 
> 2.47.3
> 
> 

Does not apply :(

Re: [PATCH 6.18.y] mm/damon/core: avoid infinite kdamond_merge_regions() internal loop

[SJ Park]

On Wed, 9 Sep 2026 15:03:16 +0200 Greg KH <greg@kroah.com> wrote:

> On Tue, Sep 08, 2026 at 09:15:02PM -0700, SJ Park wrote:
> > Patch series "mm/damon: unurgent fixes for infinite loop, NULL de-ref and
> > races", v1.1.
> > 
> > Sashiko found a few issues in DAMON that could cause infinite loop, NULL
> > dereference and monitoring results degradation.  The first two sounds
> > scary but the infinite loop happens only under unreasonable user setup.
> > The NULL dereference is only in a unit test.  Monitoring results
> > degradation is trivial since it is only best-effort, and those happens
> > from only unlikely races.  Still those are bugs that better to fix if
> > possible.  Fix those.
> > 
> > This patch (of 6):
> > 
> > Due to online parameter update like events, the number of DAMON regions
> > could be higher than the user-set upper limit.  kdamond_merge_regions()
> > repeats merge regions until the number meets the limit, while doubling the
> > merge threshold up to the theoretical maximum threshold.  It is tried only
> > up to the theoretical maximum threshold because even the aggressive
> > merging can fail from reducing the number of regions under the
> > user-defined upper limit.  For example, there could be many user-defined
> > non-contiguous regions that cannot be merged.
> > 
> > The threshold based loop break condition is evaluated by comparing the
> > threshold for the next merging try against the theoretical maximum
> > threshold.  If max_thres is larger than UINT_MAX / 2, doubling the
> > threshold could make it overflow, and bypass the loop break condition.  In
> > the case, if the number of regions cannot be reduced under the upper limit
> > like explained above, the loop will run infinitely.
> > 
> > Prevent the case by doing the break condition check before doubling the
> > threshold.  Also, prevent the threshold exceeding the maximum threshold,
> > as it could overflow and apply the wrong merge threshold.
> > 
> > This issue is unlikely to occur in real world, since having the max_thres
> > higher than UINT_MAX / 2 require unrealistically large aggregation
> > intervals compared to the sampling interval.  Also, it requires an
> > unrealistically large number of uncontiguous regions setup.  Nonetheless,
> > the consequence is bad and the fix is simple.
> > 
> > The issue was discovered [1] by Sashiko.
> > 
> > Link: https://lore.kernel.org/20260715031002.108504-1-sj@kernel.org
> > Link: https://lore.kernel.org/20260715031002.108504-2-sj@kernel.org
> > Link: https://lore.kernel.org/20260709145425.96247-1-sj@kernel.org [1]
> > Fixes: 310d6c15e910 ("mm/damon/core: merge regions aggressively when max_nr_regions is unmet")
> > Signed-off-by: SJ Park <sj@kernel.org>
> > Cc: <stable@vger.kernel.org> # 6.10.x
> > Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
> > (cherry picked from commit 123e4619ab6c8ab1c4cb1d7a58311a2af13929cd)
> > Signed-off-by: SJ Park <sj@kernel.org>
> > ---
> >  mm/damon/core.c | 13 +++++++++----
> >  1 file changed, 9 insertions(+), 4 deletions(-)
> > 
> > diff --git a/mm/damon/core.c b/mm/damon/core.c
> > index 70ac1f08753d1..08e527efd6883 100644
> > --- a/mm/damon/core.c
> > +++ b/mm/damon/core.c
> > @@ -2411,15 +2411,20 @@ static void kdamond_merge_regions(struct damon_ctx *c, unsigned int threshold,
> >  
> >  	max_thres = c->attrs.aggr_interval /
> >  		(c->attrs.sample_interval ?  c->attrs.sample_interval : 1);
> > -	do {
> > +	while (true) {
> >  		nr_regions = 0;
> >  		damon_for_each_target(t, c) {
> >  			damon_merge_regions_of(t, threshold, sz_limit);
> >  			nr_regions += damon_nr_regions(t);
> >  		}
> > -		threshold = max(1, threshold * 2);
> > -	} while (nr_regions > c->attrs.max_nr_regions &&
> > -			threshold / 2 < max_thres);
> > +		if (nr_regions <= c->attrs.max_nr_regions ||
> > +				max_thres <= threshold)
> > +			break;
> > +		if (threshold < max_thres / 2)
> > +			threshold = max(1, threshold * 2);
> > +		else
> > +			threshold = max_thres;
> > +	}
> >  }
> >  
> >  /*
> > -- 
> > 2.47.3
> > 
> > 
> 
> Does not apply :(

Seems another patch in 6.18.51..6.18.51-rc1 is causing the conflict.  I just
confirmed the original commit can clealy cherry-picked on 6.18.51-rc1.  Could
you please add that?  Let me know if there is something that I can help :)


Thanks,
SJ

[...]

Re: [PATCH 6.18.y] mm/damon/core: avoid infinite kdamond_merge_regions() internal loop

[Sasha Levin]

> Sashiko found a few issues in DAMON that could cause infinite loop,
> NULL dereference and monitoring results degradation.

Holding this on all five branches for now. 6.18.y is newer than 6.12.y, 6.6.y,
6.1.y, and 5.15.y, and it needs a fix here first, but the patch you sent for
6.18.y does not apply: 6.18.y has since grown a count_age parameter
through the same lines this patch touches.

Could you send an updated 6.18.y version rebased against the current count_age
code? Once that lands, the other four branches are ready to go.

-- 
Thanks,
Sasha

Re: [PATCH 6.18.y] mm/damon/core: avoid infinite kdamond_merge_regions() internal loop

[SJ Park]

On Fri, 11 Sep 2026 07:20:56 -0400 Sasha Levin <sashal@kernel.org> wrote:

> > Sashiko found a few issues in DAMON that could cause infinite loop,
> > NULL dereference and monitoring results degradation.
> 
> Holding this on all five branches for now. 6.18.y is newer than 6.12.y, 6.6.y,
> 6.1.y, and 5.15.y, and it needs a fix here first, but the patch you sent for
> 6.18.y does not apply: 6.18.y has since grown a count_age parameter
> through the same lines this patch touches.
> 
> Could you send an updated 6.18.y version rebased against the current count_age
> code?

Sure!  I just sent [1] it.

[1] https://lore.kernel.org/20260911134904.96369-1-sj@kernel.org

> Once that lands, the other four branches are ready to go.

Cool, let me know if there is anything that I can help!


Thanks,
SJ

[...]