[RFC PATCH] mm/damon/core: reset invalid quota->charge_target_from

[RFC PATCH] mm/damon/core: reset invalid quota->charge_target_from

[SJ Park]

DAMOS can suddenly stop working if a target process that quota just
fully charged in it is terminated.  Fix by catching and processing the
corner case.

When DAMOS quota is fully charged, the target and the region to continue
applying the action in the next round is saved in
damos_quota->charge_{target,addr}_from.  In the next round, DAMOS
iterates targets and regions from the beginning.  It skips applying the
action to the regions until it visits and skips the saved target/region.

Virtual address space targets become invalid if the process is
terminated.  Trying to apply the scheme to invalid target is just a
waste of time.  Hence commit 6e4930e33329 ("mm/damon/core: fix wasteful
CPU calls by skipping non-existent targets") made the logic to skip
invalid targets.  However, it does skip before the charged target/region
skipping/updating.

Let's suppose the user runs DAMOS for multiple virtual address spaces
with a quota.  The quota exceeded in the middle of a virtual address
space.  And the process of the address space is terminated.  Then the
charge_target_from points to the invalid target.  The pointer update
logic is skipped for the invalid target, so the charge_target_from is
never updated.  DAMOS action to every target/region is skipped.  From
the user's perspective, it would look like suddenly DAMOS has stopped
working.

No critical leak or crash can happen.  The user could reinstall the
scheme.  But this makes use of DAMOS under certain setups quite
unreliable.

Catch the corner case and reset the pointer in damos_adjust_quota(),
which is called for adjustment of the quota for every round.

This issue was discovered [1] by Sashiko.

[1] https://lore.kernel.org/20260830064708.40CA61F000E9@smtp.kernel.org

Fixes: 6e4930e33329 ("mm/damon/core: fix wasteful CPU calls by skipping non-existent targets")
Cc: <stable@vger.kernel.org> # 7.0.x
Signed-off-by: SJ Park <sj@kernel.org>
---
 mm/damon/core.c | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/mm/damon/core.c b/mm/damon/core.c
index ce8c6f99106ed..2c58be9edbde4 100644
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -3283,6 +3283,22 @@ static void damos_trace_esz(struct damon_ctx *c, struct damos *s,
 	trace_damos_esz(cidx, sidx, quota->esz);
 }
 
+static void damos_reset_invalid_charge_target_from(struct damos_quota *quota,
+		struct damon_ctx *c)
+{
+	struct damon_target *t;
+
+	t = quota->charge_target_from;
+	if (!t)
+		return;
+	if (!c->ops.target_valid)
+		return;
+	if (c->ops.target_valid(t))
+		return;
+	quota->charge_target_from = NULL;
+	quota->charge_addr_from = 0;
+}
+
 static void damos_adjust_quota(struct damon_ctx *c, struct damos *s)
 {
 	struct damos_quota *quota = &s->quota;
@@ -3318,6 +3334,8 @@ static void damos_adjust_quota(struct damon_ctx *c, struct damos *s)
 			damos_trace_esz(c, s, quota);
 	}
 
+	damos_reset_invalid_charge_target_from(quota, c);
+
 	if (!c->ops.get_scheme_score)
 		return;
 

base-commit: 98a7a286b6060dce5e93b37e28f2e0be80304027
-- 
2.47.3

Re: [RFC PATCH] mm/damon/core: reset invalid quota->charge_target_from

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:

Pre-existing issues:
- [High] The new `damos_reset_invalid_charge_target_from()` function fails to reset dangling `charge_target_from` pointers for PADDR operations, permanently breaking `damon_reclaim` functionality across restarts.
--

commit 6f10f4f2fc1ae76229671b5157f9aa34aed9227b
Author: SJ Park <sj@kernel.org>

    mm/damon/core: reset invalid quota->charge_target_from

This commit fixes an issue where DAMOS suddenly stops working if a target
process that a quota just fully charged in is terminated. It catches this
corner case and resets the invalid quota->charge_target_from pointer.

Link: https://lore.kernel.org/20260830064708.40CA61F000E9@smtp.kernel.org

> diff --git a/mm/damon/core.c b/mm/damon/core.c
> index ce8c6f99106ed..2c58be9edbde4 100644
> --- a/mm/damon/core.c
> +++ b/mm/damon/core.c
> @@ -3283,6 +3283,22 @@ static void damos_trace_esz(struct damon_ctx *c, struct damos *s,
>  	trace_damos_esz(cidx, sidx, quota->esz);
>  }
>  
> +static void damos_reset_invalid_charge_target_from(struct damos_quota *quota,
> +		struct damon_ctx *c)
> +{
> +	struct damon_target *t;
> +
> +	t = quota->charge_target_from;
> +	if (!t)
> +		return;
> +	if (!c->ops.target_valid)
> +		return;

[Severity: High]
This is a pre-existing issue, but by returning early here, does this new
function fail to clear dangling charge_target_from pointers for PADDR
operations (like damon_reclaim)?

If damon_reclaim is enabled with quotas and hits quota exhaustion,
quota->charge_target_from is set. If damon_reclaim is then disabled,
damon_stop() destroys the targets, leaving charge_target_from as a dangling
pointer.

When damon_reclaim is re-enabled, a new target is created. Because
target_valid is NULL for PADDR operations, this new function returns early.

Could this lead to damos_skip_charged_region() comparing the new target
against the dangling pointer, evaluating to true, and permanently skipping
all memory reclaim actions?

> +	if (c->ops.target_valid(t))
> +		return;
> +	quota->charge_target_from = NULL;
> +	quota->charge_addr_from = 0;
> +}
>
>  static void damos_adjust_quota(struct damon_ctx *c, struct damos *s)

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260906183654.95634-1-sj@kernel.org?part=1

Re: [RFC PATCH] mm/damon/core: reset invalid quota->charge_target_from

[SJ Park]

On Sun, 06 Sep 2026 18:58:30 +0000 sashiko-bot@kernel.org wrote:

> Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
> 
> Pre-existing issues:
> - [High] The new `damos_reset_invalid_charge_target_from()` function fails to reset dangling `charge_target_from` pointers for PADDR operations, permanently breaking `damon_reclaim` functionality across restarts.
> --
> 
> commit 6f10f4f2fc1ae76229671b5157f9aa34aed9227b
> Author: SJ Park <sj@kernel.org>
> 
>     mm/damon/core: reset invalid quota->charge_target_from
> 
> This commit fixes an issue where DAMOS suddenly stops working if a target
> process that a quota just fully charged in is terminated. It catches this
> corner case and resets the invalid quota->charge_target_from pointer.
> 
> Link: https://lore.kernel.org/20260830064708.40CA61F000E9@smtp.kernel.org
> 
> > diff --git a/mm/damon/core.c b/mm/damon/core.c
> > index ce8c6f99106ed..2c58be9edbde4 100644
> > --- a/mm/damon/core.c
> > +++ b/mm/damon/core.c
> > @@ -3283,6 +3283,22 @@ static void damos_trace_esz(struct damon_ctx *c, struct damos *s,
> >  	trace_damos_esz(cidx, sidx, quota->esz);
> >  }
> >  
> > +static void damos_reset_invalid_charge_target_from(struct damos_quota *quota,
> > +		struct damon_ctx *c)
> > +{
> > +	struct damon_target *t;
> > +
> > +	t = quota->charge_target_from;
> > +	if (!t)
> > +		return;
> > +	if (!c->ops.target_valid)
> > +		return;
> 
> [Severity: High]
> This is a pre-existing issue, but by returning early here, does this new
> function fail to clear dangling charge_target_from pointers for PADDR
> operations (like damon_reclaim)?
> 
> If damon_reclaim is enabled with quotas and hits quota exhaustion,
> quota->charge_target_from is set. If damon_reclaim is then disabled,
> damon_stop() destroys the targets, leaving charge_target_from as a dangling
> pointer.
> 
> When damon_reclaim is re-enabled, a new target is created. Because
> target_valid is NULL for PADDR operations, this new function returns early.
> 
> Could this lead to damos_skip_charged_region() comparing the new target
> against the dangling pointer, evaluating to true, and permanently skipping
> all memory reclaim actions?

The problem doesn't exist.  DAMON_RECLAIM does not reuse the last scheme.  Same
for all DAMON API callers.

> 
> > +	if (c->ops.target_valid(t))
> > +		return;
> > +	quota->charge_target_from = NULL;
> > +	quota->charge_addr_from = 0;
> > +}
> >
> >  static void damos_adjust_quota(struct damon_ctx *c, struct damos *s)
> 
> -- 
> Sashiko AI review · https://sashiko.dev/#/patchset/20260906183654.95634-1-sj@kernel.org?part=1


Thanks,
SJ