[RFC PATCH 0/5] mm/damon: DAMOS quota controller and paddr migration walk fixes

[RFC PATCH 0/5] mm/damon: DAMOS quota controller and paddr migration walk fixes

[Ravi Jonnalagadda]

MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

Hi,

This series carries five fixes for the DAMOS quota controller and the
paddr migration walk.  All five were surfaced during closed-loop tiering
testing on a heterogeneous-memory system (DRAM + CXL on a separate
NUMA node), but each fix is in code paths that benefit any caller --
not scoped to closed-loop tiering or to any specific goal metric.

Test envelope: AMD EPYC dual-socket host with CXL.mem on a separate
NUMA node, two-scheme migrate_hot PULL+PUSH setup driven by
node_eligible_mem_bp (now in linux-next)[1].

What each patch does
====================

Patch 1 - mm/damon/core: fix nr_accesses_bp underflow in damon_moving_sum

  damon_moving_sum() can underflow when a region's access rate drops
  to zero faster than the moving-sum window length.  The internal
  accumulator subtracts an outgoing sample without a lower bound,
  producing a sentinel-large nr_accesses_bp that mis-classifies a
  cold region as hot.  Affects every DAMOS scheme, since
  nr_accesses_bp is used on every access-rate update for every
  region regardless of which scheme or goal metric is active.

Patch 2 - mm/damon/core: cap effective quota size to total monitored memory

  The DAMOS quota tuner can compute an effective size (esz) larger
  than the total monitored memory, because the tuner integrates over
  cumulative deltas without bounding by the actual workload size.
  Once esz exceeds total monitored memory the per-tick "remaining
  quota" arithmetic stops being meaningful: any scheme can apply to
  the entire monitored space and "remaining" stays positive
  indefinitely.  Cap esz at total monitored memory so the controller
  remains within physically realisable bounds.  Tuner-shape and
  goal-metric agnostic.

Patch 3 - mm/damon/core: floor effective quota size at minimum region size

  Symmetric to patch 2: the tuner can also compute esz < min_region_sz,
  causing schemes to attempt zero-byte migrations for many ticks before
  the tuner ramps esz back up.  Observed under the CONSIST tuner with
  a node_eligible_mem_bp goal: ftrace traced esz stuck at 1 byte for
  96 seconds before the first region was tried; the first acted-on
  region appeared at t=113s when esz crossed the min_region_sz
  threshold.  Floor esz at min_region_sz so schemes always have at
  least one region's worth of quota when the tuner asks them to act.

Patch 4 - mm/damon/paddr: skip free pageblocks in migration walk

  damon_pa_migrate() walks every 4KB PFN in a region.  On
  sparsely-populated lower-tier extents (e.g., a 549GB CXL region
  with only ~8GB populated), this is ~144M PFN iterations per scheme
  tick at ~40ns each = ~5.6 seconds of walk per tick.  Use
  pageblock-level free-page detection to skip unpopulated runs of
  pages: only enter the per-page loop for pageblocks that contain at
  least one allocated page.  This brings the walk to
  O(region_size / pageblock_size) skip-check cost plus
  O(populated_pages) per-page work.  On x86 pageblocks are 2MB, so
  the same 549GB/8GB example becomes ~281K pageblock skip-checks
  (microseconds total) plus ~2M per-page visits for the populated
  pages -- ~80ms expected.  Helps any migrate_hot/migrate_cold scheme
  on paddr ops, regardless of what drives them.

Patch 5 - mm/damon/paddr: add time budget to migration page walk

  Densely populated regions (e.g., a busy DRAM range where most
  pageblocks contain at least one allocated page) can still consume
  full ticks even with patch 4 applied.  Add a 100ms wall-clock
  budget with a ktime_get() check every 4096 pages walked
  (~16MB worth).  When the budget expires before reaching the end of
  a region, kdamond returns control; subsequent ticks re-walk the
  region from the start.  Folios already on the target node are
  dropped at migration time, so re-walks only re-do collection work,
  not the migrate itself.  Together with the per-scheme quota cap,
  per-tick work is bounded and the workload converges over multiple
  ticks for dense regions.

  Worst-case migration walk contribution to a tick is bounded at
  100ms per scheme regardless of region size or population density,
  preserving kdamond's ability to service other DAMOS schemes and
  user-space sysfs operations during heavy migration phases.


Testing context
===============

  Hardware:  AMD EPYC dual-socket, CXL.mem on a separate NUMA node.
  Workload: 32GB hot working set across DRAM and CXL nodes.
  DAMON config: paddr ops, two migrate_hot schemes (PULL CXL->DRAM,
                PUSH DRAM->CXL) with complementary address filters,
                node_eligible_mem_bp goal per scheme, temporal
                quota tuner, 1s reset interval.

Each fix in this series was reproduced under the above setup, then
verified via ftrace and per-scheme stats after the fix landed.

References
==========

[1] mm/damon: add node_eligible_mem_bp goal metric
https://lore.kernel.org/damon/20260428030520.701-1-ravis.opensrc@gmail.com/

Ravi Jonnalagadda (5):
  mm/damon/core: fix nr_accesses_bp underflow in damon_moving_sum
  mm/damon/core: cap effective quota size to total monitored memory
  mm/damon/core: floor effective quota size at minimum region size
  mm/damon/paddr: skip free pageblocks in migration walk
  mm/damon/paddr: add time budget to migration page walk

 mm/damon/core.c  | 29 ++++++++++++++++++++++++++++-
 mm/damon/paddr.c | 40 +++++++++++++++++++++++++++++++++++++---
 2 files changed, 65 insertions(+), 4 deletions(-)


base-commit: 0cec77cfd5314c0b3b03530abe1a4b32e991f639
-- 
2.43.0

[RFC PATCH 1/5] mm/damon/core: fix nr_accesses_bp underflow in damon_moving_sum

[Ravi Jonnalagadda]

Guard against unsigned integer underflow when nomvsum/len_window
exceeds mvsum.  When that subtraction wraps, the moving sum returns a
near-ULONG_MAX value and corrupts nr_accesses_bp.

If subtrahend > mvsum, return new_value: this clamps the moving-sum
estimate to the current observation rather than wrapping.

Signed-off-by: Ravi Jonnalagadda <ravis.opensrc@gmail.com>
---
 mm/damon/core.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/mm/damon/core.c b/mm/damon/core.c
index 3a8725e400c6b..9975f3d9ebfe9 100644
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -3449,7 +3449,11 @@ int damon_set_region_system_rams_default(struct damon_target *t,
 static unsigned int damon_moving_sum(unsigned int mvsum, unsigned int nomvsum,
 		unsigned int len_window, unsigned int new_value)
 {
-	return mvsum - nomvsum / len_window + new_value;
+	unsigned int subtrahend = nomvsum / len_window;
+
+	if (subtrahend > mvsum)
+		return new_value;
+	return mvsum - subtrahend + new_value;
 }
 
 /**
-- 
2.43.0

[RFC PATCH 2/5] mm/damon/core: cap effective quota size to total monitored memory

[Ravi Jonnalagadda]

The DAMOS quota goal tuner can compute an effective size (esz) larger
than the total monitored memory because it integrates over cumulative
deltas without bounding by the actual workload size.  Once esz exceeds
total monitored memory, the per-tick "remaining quota" arithmetic
stops being meaningful: any scheme can apply to the entire monitored
space and "remaining" stays positive indefinitely.

Cap esz to the total size of all currently monitored regions as a
final bound after all other quota calculations.  Add
damon_ctx_total_monitored_sz() helper that sums region sizes across
all targets.

The helper runs only inside damos_set_effective_quota(), which is
called at most once per quota reset_interval (default 1s) per scheme,
not per kdamond tick.  Walk cost is O(nr_regions) at that frequency
and is dominated by the enclosing tuner work.

This bound is tuner-shape and goal-metric agnostic: it constrains the
quota controller to physically realisable values regardless of which
tuner or goal metric drives it.

Signed-off-by: Ravi Jonnalagadda <ravis.opensrc@gmail.com>
---
 mm/damon/core.c | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/mm/damon/core.c b/mm/damon/core.c
index 9975f3d9ebfe9..fd1db234ca304 100644
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -2614,6 +2614,19 @@ static void damos_goal_tune_esz_bp_temporal(struct damon_ctx *c,
 		quota->esz_bp = ULONG_MAX;
 }
 
+/* Sum of all monitored region sizes across all targets in @ctx. */
+static unsigned long damon_ctx_total_monitored_sz(struct damon_ctx *ctx)
+{
+	struct damon_target *t;
+	struct damon_region *r;
+	unsigned long total = 0;
+
+	damon_for_each_target(t, ctx)
+		damon_for_each_region(r, t)
+			total += damon_sz_region(r);
+	return total;
+}
+
 /*
  * Called only if quota->ms, or quota->sz are set, or quota->goals is not empty
  */
@@ -2621,6 +2634,7 @@ static void damos_set_effective_quota(struct damon_ctx *ctx, struct damos *s)
 {
 	struct damos_quota *quota = &s->quota;
 	unsigned long throughput;
+	unsigned long total_sz;
 	unsigned long esz = ULONG_MAX;
 
 	if (!quota->ms && list_empty(&quota->goals)) {
@@ -2649,6 +2663,11 @@ static void damos_set_effective_quota(struct damon_ctx *ctx, struct damos *s)
 	if (quota->sz && quota->sz < esz)
 		esz = quota->sz;
 
+	/* Safety cap: never migrate more than total monitored memory */
+	total_sz = damon_ctx_total_monitored_sz(ctx);
+	if (total_sz && esz > total_sz)
+		esz = total_sz;
+
 	quota->esz = esz;
 }
 
-- 
2.43.0

[RFC PATCH 3/5] mm/damon/core: floor effective quota size at minimum region size

[Ravi Jonnalagadda]

The CONSIST quota goal tuner initializes esz_bp to 0, producing an
effective quota size (esz) of 1 byte on the first tick.
damos_quota_is_full() rejects all regions when esz < min_region_sz
(default PAGE_SIZE = 4096), so no regions can be tried and no
feedback reaches the tuner — a bootstrapping deadlock.

Floor esz at ctx->min_region_sz after the tuner computes it, guarded
by an esz != 0 check.  The guard preserves the temporal tuner's
intentional stop behavior: when score >= 10000 (goal met), temporal
sets esz_bp = 0 to halt migration; the floor must not override that.

Signed-off-by: Ravi Jonnalagadda <ravis.opensrc@gmail.com>
---
 mm/damon/core.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/mm/damon/core.c b/mm/damon/core.c
index fd1db234ca304..d33c4360cbd60 100644
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -2650,6 +2650,10 @@ static void damos_set_effective_quota(struct damon_ctx *ctx, struct damos *s)
 		esz = quota->esz_bp / 10000;
 	}
 
+	/* avoid cold-start deadlock, but respect tuner stop signal (esz=0) */
+	if (esz)
+		esz = max_t(unsigned long, esz, ctx->min_region_sz);
+
 	if (quota->ms) {
 		if (quota->total_charged_ns)
 			throughput = mult_frac(quota->total_charged_sz,
-- 
2.43.0

[RFC PATCH 4/5] mm/damon/paddr: skip free pageblocks in migration walk

[Ravi Jonnalagadda]

damon_pa_migrate() walks every PFN in a region linearly, calling
damon_get_folio() for each one.  On sparse physical address spaces
(e.g., CXL-attached memory), a single DAMON region can span hundreds
of gigabytes where most memory is free and sitting in the buddy
allocator.  Most page lookups are fruitless and dominate kdamond
tick time.

Check at pageblock boundaries (2MB on x86_64) whether the block is
entirely free.  If the first page of a pageblock is a buddy page at
pageblock_order or higher, the entire block is free and can be
skipped.  Similarly skip pageblocks where pfn_to_online_page() returns
NULL.

This reduces the iteration from O(region_sz / PAGE_SIZE) to
O(region_sz / pageblock_sz) + O(populated_pages).

buddy_order_unsafe() is used without zone->lock.  A transient false
positive (block becomes non-free between the PageBuddy and order
checks) costs at most one tick of missed candidates on that block;
the next tick re-scans.  No correctness consequence as DAMON walks
are best-effort.

Signed-off-by: Ravi Jonnalagadda <ravis.opensrc@gmail.com>
---
 mm/damon/paddr.c | 25 ++++++++++++++++++++++---
 1 file changed, 22 insertions(+), 3 deletions(-)

diff --git a/mm/damon/paddr.c b/mm/damon/paddr.c
index c4738cd5e221e..e844c990987b9 100644
--- a/mm/damon/paddr.c
+++ b/mm/damon/paddr.c
@@ -258,13 +258,32 @@ static unsigned long damon_pa_migrate(struct damon_region *r,
 		unsigned long addr_unit, struct damos *s,
 		unsigned long *sz_filter_passed)
 {
-	phys_addr_t addr, applied;
+	phys_addr_t addr, end, applied;
 	LIST_HEAD(folio_list);
 	struct folio *folio = NULL;
+	unsigned long pfn;
 
 	addr = damon_pa_phys_addr(r->ar.start, addr_unit);
-	while (addr < damon_pa_phys_addr(r->ar.end, addr_unit)) {
-		folio = damon_get_folio(PHYS_PFN(addr));
+	end = damon_pa_phys_addr(r->ar.end, addr_unit);
+	while (addr < end) {
+		pfn = PHYS_PFN(addr);
+
+		/* Skip pageblocks that are entirely free. */
+		if (IS_ALIGNED(pfn, pageblock_nr_pages)) {
+			struct page *page = pfn_to_online_page(pfn);
+
+			if (!page) {
+				addr += pageblock_nr_pages * PAGE_SIZE;
+				continue;
+			}
+			if (PageBuddy(page) &&
+			    buddy_order_unsafe(page) >= pageblock_order) {
+				addr += pageblock_nr_pages * PAGE_SIZE;
+				continue;
+			}
+		}
+
+		folio = damon_get_folio(pfn);
 		if (damon_pa_invalid_damos_folio(folio, s)) {
 			addr += PAGE_SIZE;
 			continue;
-- 
2.43.0

[RFC PATCH 5/5] mm/damon/paddr: add time budget to migration page walk

[Ravi Jonnalagadda]

On populated physical address ranges the pageblock skip optimization
alone is insufficient — most pageblocks contain at least one allocated
page, so the walk still iterates millions of PFNs.

Add a 100ms wall-clock time budget to damon_pa_migrate().  Once the
deadline is reached, the walk breaks out and migrates whatever folios
have been collected so far.

The time check is amortized by only calling ktime_get() every 4096
pages (~16MB of address space), adding negligible overhead to the
fast path.

Signed-off-by: Ravi Jonnalagadda <ravis.opensrc@gmail.com>
---
 mm/damon/paddr.c | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/mm/damon/paddr.c b/mm/damon/paddr.c
index e844c990987b9..a2565287bc10f 100644
--- a/mm/damon/paddr.c
+++ b/mm/damon/paddr.c
@@ -14,6 +14,7 @@
 #include <linux/swap.h>
 #include <linux/memory-tiers.h>
 #include <linux/mm_inline.h>
+#include <linux/ktime.h>
 
 #include "../internal.h"
 #include "ops-common.h"
@@ -254,6 +255,14 @@ static unsigned long damon_pa_deactivate_pages(struct damon_region *r,
 	return damon_pa_de_activate(r, addr_unit, s, false, sz_filter_passed);
 }
 
+/* Maximum wall-clock time to spend in a single migration walk (ns) */
+#define DAMON_PA_MIGRATE_BUDGET_NS	(100 * NSEC_PER_MSEC)
+
+/* Check the time budget every 4096 pages (~16MB) to amortize ktime_get(). */
+#define DAMON_PA_MIGRATE_TIME_CHECK_PAGES	4096
+#define DAMON_PA_MIGRATE_TIME_CHECK_MASK	\
+	(DAMON_PA_MIGRATE_TIME_CHECK_PAGES - 1)
+
 static unsigned long damon_pa_migrate(struct damon_region *r,
 		unsigned long addr_unit, struct damos *s,
 		unsigned long *sz_filter_passed)
@@ -262,6 +271,7 @@ static unsigned long damon_pa_migrate(struct damon_region *r,
 	LIST_HEAD(folio_list);
 	struct folio *folio = NULL;
 	unsigned long pfn;
+	ktime_t deadline = ktime_add_ns(ktime_get(), DAMON_PA_MIGRATE_BUDGET_NS);
 
 	addr = damon_pa_phys_addr(r->ar.start, addr_unit);
 	end = damon_pa_phys_addr(r->ar.end, addr_unit);
@@ -283,6 +293,11 @@ static unsigned long damon_pa_migrate(struct damon_region *r,
 			}
 		}
 
+		/* Time budget: keep kdamond responsive on long migration walks. */
+		if (!(pfn & DAMON_PA_MIGRATE_TIME_CHECK_MASK) &&
+		    ktime_after(ktime_get(), deadline))
+			break;
+
 		folio = damon_get_folio(pfn);
 		if (damon_pa_invalid_damos_folio(folio, s)) {
 			addr += PAGE_SIZE;
-- 
2.43.0

Re: [RFC PATCH 1/5] mm/damon/core: fix nr_accesses_bp underflow in damon_moving_sum

[SeongJae Park]

Hello Ravi,

On Sat, 16 May 2026 14:03:53 -0700 Ravi Jonnalagadda <ravis.opensrc@gmail.com> wrote:

> Guard against unsigned integer underflow when nomvsum/len_window
> exceeds mvsum.

How could this happen?  mvsum is assumed to be same to nomvsum at the beginning
of the window.  Hence, even if there is only zero new_value, at the end of the
window, mvsum should be exactly zero.  Of course there could be a bug that
breaks the assumption.

> When that subtraction wraps, the moving sum returns a
> near-ULONG_MAX value and corrupts nr_accesses_bp.
> 
> If subtrahend > mvsum, return new_value: this clamps the moving-sum
> estimate to the current observation rather than wrapping.

I guess you saw this issue in real, and this change should fix the issue.  But
I think we should know why and how mvsum < nomvum / len_window can unexpectedly
happen, and fix that.

Could you share more details about when and how the situation happens?


Thanks,
SJ

[...]

Re: [RFC PATCH 2/5] mm/damon/core: cap effective quota size to total monitored memory

[SeongJae Park]

Hello Ravi,

On Sat, 16 May 2026 14:03:54 -0700 Ravi Jonnalagadda <ravis.opensrc@gmail.com> wrote:

> The DAMOS quota goal tuner can compute an effective size (esz) larger
> than the total monitored memory because it integrates over cumulative
> deltas without bounding by the actual workload size.  Once esz exceeds
> total monitored memory, the per-tick "remaining quota" arithmetic
> stops being meaningful: any scheme can apply to the entire monitored
> space and "remaining" stays positive indefinitely.

Nice finding!

> 
> Cap esz to the total size of all currently monitored regions as a
> final bound after all other quota calculations.  Add
> damon_ctx_total_monitored_sz() helper that sums region sizes across
> all targets.

You could also make an arbitrary cap by setting the static size quota.  That
is, if there are not only quota goal but also the size quota and/or time quota,
and the different types of quotas disagree about the real quota, DAMOS uses
smallest quota.  You could read damos_set_effective_quota() code and kernel-doc
comment of 'struct damos_quota' for more details.

So you could apply the total monitoring region size cap by setting the size
quota of the total monitoring region size.  Could that work for you?

Adding the total monitoring region size cap makes sense to me, and I think that
will make user experience better.  But, if the size quota based cap works, that
could also be handled on user space in an easier and even a betetr way.  If so,
I'd prefer the direction, to reduce kernel code complexity.  What do you think?


Thanks,
SJ

[...]

Re: [RFC PATCH 3/5] mm/damon/core: floor effective quota size at minimum region size

[SeongJae Park]

On Sat, 16 May 2026 14:03:55 -0700 Ravi Jonnalagadda <ravis.opensrc@gmail.com> wrote:

> The CONSIST quota goal tuner initializes esz_bp to 0, producing an
> effective quota size (esz) of 1 byte on the first tick.
> damos_quota_is_full() rejects all regions when esz < min_region_sz
> (default PAGE_SIZE = 4096), so no regions can be tried and no
> feedback reaches the tuner — a bootstrapping deadlock.

That depend on whether the goal is already [over]-achieved.  If the goal is
achieved, the tuner will think no change is needed, so keep the
effectively-zero quota.  If the goal is over-achived, the tuner will think the
DAMOS scheme should be less aggressive, but it is already effectively-zero
quota, so keep having effectively-zero quota.

If the ogal is under-achived, the logic will iteratively increase the internal
esz (esz_bp), until it exceeds the min_region_sz, and finally start making some
effects.

So, unless the goal is already [over]-achieved, there is no deadlock.  If the
goal is already [over]-achieved, why we would want to make DAMOS do something?

Am I missing something?

I'd like to discuss this high level thing first, before digging deep into the
details.


Thanks,
SJ

[...]