[PATCH 0/7] damos_apply_scheme() splits a region and apply the action to the

[PATCH 0/7] damos_apply_scheme() splits a region and apply the action to the

[SJ Park]

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.

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

SJ Park (7):
  mm/damon/paddr: respect folio end for DAMOS_STAT
  mm/damon/paddr: respect folio end for DAMOS actions except STAT
  mm/damon/vaddr: respect folio end for DAMOS_STAT
  mm/damon/vaddr: respect folio end for DAMOS_MIGRATE_{HOT,COLD}
  mm/damon/core: handle extreme memory state in damon_get_node_mem_bp()
  mm/damon/core: handle extreme memory state in get_node_memcg_used_bp()
  mm/damon/core: handle extreme memory state in get_in_active_mem_bp()

 mm/damon/core.c  | 15 ++++++++++++++-
 mm/damon/paddr.c |  8 ++++----
 mm/damon/vaddr.c | 10 ++++++++--
 3 files changed, 26 insertions(+), 7 deletions(-)


base-commit: d93d8b0181065f8375af61e3a18bbc7c066a8159
-- 
2.47.3

[PATCH 1/7] mm/damon/paddr: respect folio end for DAMOS_STAT

[SJ Park]

The function for applying DAMOS_STAT in DAMON physical address space
operation set (paddr), namely damon_pa_stat(), applies DAMOS filters to
folios of the given region.  For that, it gets folios of addresses in
the region.  It starts from the region start address and advances the
address by the size of the folio of the address until it goes out of the
region.  If the start address is in the middle of a large folio, and if
the next folios are small, some of the next folios could be skipped. Fix
the issue by advancing the address to exactly the start address of the
next folio.

The user impact is that the DAMOS_STAT-based page level monitoring
results become inaccurate.  Since the page level monitoring is supposed
to provide relatively high precision, this is definitely a problem.  It
is arguably not critical since it is only monitoring quality
degradation.

Fixes: bdbe1d7bc325 ("mm/damon/paddr: increment pa_stat damon address range by folio size")
Cc: <stable@vger.kernel.org> # 6.14.x
Signed-off-by: SJ Park <sj@kernel.org>
---
 mm/damon/paddr.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/damon/paddr.c b/mm/damon/paddr.c
index 5c6c3a597fd0b..2ab7b3842701e 100644
--- a/mm/damon/paddr.c
+++ b/mm/damon/paddr.c
@@ -379,7 +379,7 @@ static unsigned long damon_pa_stat(struct damon_region *r,
 
 		if (!damos_pa_filter_out(s, folio))
 			*sz_filter_passed += folio_size(folio) / addr_unit;
-		addr += folio_size(folio);
+		addr = PFN_PHYS(folio_pfn(folio)) + folio_size(folio);
 		folio_put(folio);
 	}
 	s->last_applied = folio;
-- 
2.47.3

Re: [PATCH 1/7] mm/damon/paddr: respect folio end for DAMOS_STAT

[sashiko-bot]

> The function for applying DAMOS_STAT in DAMON physical address space
> operation set (paddr), namely damon_pa_stat(), applies DAMOS filters to
> folios of the given region.  For that, it gets folios of addresses in
> the region.  It starts from the region start address and advances the
> address by the size of the folio of the address until it goes out of the
> region.  If the start address is in the middle of a large folio, and if
> the next folios are small, some of the next folios could be skipped. Fix
> the issue by advancing the address to exactly the start address of the
> next folio.
> 
> The user impact is that the DAMOS_STAT-based page level monitoring
> results become inaccurate.  Since the page level monitoring is supposed
> to provide relatively high precision, this is definitely a problem.  It
> is arguably not critical since it is only monitoring quality
> degradation.
> 
> Fixes: bdbe1d7bc325 ("mm/damon/paddr: increment pa_stat damon address range by folio size")
> Cc: <stable@vger.kernel.org> # 6.14.x
> 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/20260901131326.97615-1-sj@kernel.org?part=1

[PATCH 2/7] mm/damon/paddr: respect folio end for DAMOS actions except STAT

[SJ Park]

A few functions for applying DAMOS actions including pageout,
lru_[de]prio and migrate_{hot,cold} in DAMON physical address space
operation set (paddr) collect folios of the given region by getting the
folios of region-internal addresses.  Then, those functions apply the
action to the collected folios at once.  The collection starts from the
region start address and advances the address by the size of the folio
of the address until it goes out of the region.  If the start address is
in the middle of a large folio, and if the next folios are small, some
of the next folios could be skipped.  Fix the issue by advancing the
address to exactly the start address of the next folio.

The user impact is that DAMOS action is applied to less than expected
amount of memory.  Given the best effort nature of DAMON, it is no big
problem, but it is clearly a bug that is better to be fixed.

The issue was discovered [1] by Sashiko.

[1] https://lore.kernel.org/20260517234112.89245-1-sj@kernel.org

Fixes: 3a06696305e7 ("mm/damon/ops: have damon_get_folio return folio even for tail pages")
Cc: <stable@vger.kernel.org> # 6.15.x
Signed-off-by: SJ Park <sj@kernel.org>
---
 mm/damon/paddr.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/mm/damon/paddr.c b/mm/damon/paddr.c
index 2ab7b3842701e..9ddd1ec8202b7 100644
--- a/mm/damon/paddr.c
+++ b/mm/damon/paddr.c
@@ -264,7 +264,7 @@ static unsigned long damon_pa_pageout(struct damon_region *r,
 		else
 			list_add(&folio->lru, &folio_list);
 put_folio:
-		addr += folio_size(folio);
+		addr = PFN_PHYS(folio_pfn(folio)) + folio_size(folio);
 		folio_put(folio);
 	}
 	if (install_young_filter)
@@ -302,7 +302,7 @@ static inline unsigned long damon_pa_de_activate(
 			folio_deactivate(folio);
 		applied += folio_nr_pages(folio);
 put_folio:
-		addr += folio_size(folio);
+		addr = PFN_PHYS(folio_pfn(folio)) + folio_size(folio);
 		folio_put(folio);
 	}
 	s->last_applied = folio;
@@ -350,7 +350,7 @@ static unsigned long damon_pa_migrate(struct damon_region *r,
 				folio_is_file_lru(folio));
 		list_add(&folio->lru, &folio_list);
 put_folio:
-		addr += folio_size(folio);
+		addr = PFN_PHYS(folio_pfn(folio)) + folio_size(folio);
 		folio_put(folio);
 	}
 	applied = damon_migrate_pages(&folio_list, s->target_nid);
-- 
2.47.3

Re: [PATCH 2/7] mm/damon/paddr: respect folio end for DAMOS actions except STAT

[sashiko-bot]

> A few functions for applying DAMOS actions including pageout,
> lru_[de]prio and migrate_{hot,cold} in DAMON physical address space
> operation set (paddr) collect folios of the given region by getting the
> folios of region-internal addresses.  Then, those functions apply the
> action to the collected folios at once.  The collection starts from the
> region start address and advances the address by the size of the folio
> of the address until it goes out of the region.  If the start address is
> in the middle of a large folio, and if the next folios are small, some
> of the next folios could be skipped.  Fix the issue by advancing the
> address to exactly the start address of the next folio.
> 
> The user impact is that DAMOS action is applied to less than expected
> amount of memory.  Given the best effort nature of DAMON, it is no big
> problem, but it is clearly a bug that is better to be fixed.
> 
> The issue was discovered [1] by Sashiko.
> 
> [1] https://lore.kernel.org/20260517234112.89245-1-sj@kernel.org
> 
> Fixes: 3a06696305e7 ("mm/damon/ops: have damon_get_folio return folio even for tail pages")
> Cc: <stable@vger.kernel.org> # 6.15.x
> 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/20260901131326.97615-1-sj@kernel.org?part=2

[PATCH 3/7] mm/damon/vaddr: respect folio end for DAMOS_STAT

[SJ Park]

For applying DAMOS_STAT action to a region, DAMON virtual address space
operation set (vaddr) calls walk_page_range[_vma]() for the region.  The
pmd walk entry function, namely damon_va_stat_pmd_entry(), applies DAMOS
filters to folios of addresses of the region in the pmd.  It starts from
the walking address and advances the address by the size of the folio of
the address until it goes out of the pmd or the region.

Let's suppose it is for the first pmd of the region, and the region
start address is in the middle of a large folio.  Also, the next folios
are small.  Then, some of the next folios could be skipped.  Fix the
issue by advancing the address to exactly the start address of the next
folio.

The user impact is that the DAMOS_STAT-based page level monitoring
results become inaccurate.  Since the page level monitoring is supposed
to provide relatively high precision, this is definitely a problem.  It
is arguably not critical since it is only monitoring quality
degradation.

The issue was discovered [1] by Sashiko.

[1] https://lore.kernel.org/20260514015053.149396-1-sj@kernel.org

Fixes: 63f39737d1e3 ("mm/damon/vaddr: support stat-purpose DAMOS filters")
Cc: <stable@vger.kernel.org> # 6.18.x
Signed-off-by: SJ Park <sj@kernel.org>
---
 mm/damon/vaddr.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/mm/damon/vaddr.c b/mm/damon/vaddr.c
index 0648400b2d65b..4b0b5edf67952 100644
--- a/mm/damon/vaddr.c
+++ b/mm/damon/vaddr.c
@@ -831,6 +831,8 @@ static int damos_va_stat_pmd_entry(pmd_t *pmd, unsigned long addr,
 		return 0;
 
 	for (; addr < next; pte += nr, addr += nr * PAGE_SIZE) {
+		unsigned long page_idx;
+
 		nr = 1;
 		ptent = ptep_get(pte);
 
@@ -844,7 +846,8 @@ static int damos_va_stat_pmd_entry(pmd_t *pmd, unsigned long addr,
 
 		if (!damos_va_filter_out(s, folio, vma, addr, pte, NULL))
 			*sz_filter_passed += folio_size(folio);
-		nr = folio_nr_pages(folio);
+		page_idx = folio_page_idx(folio, pte_page(ptent));
+		nr = folio_nr_pages(folio) - page_idx;
 		s->last_applied = folio;
 	}
 	pte_unmap_unlock(start_pte, ptl);
-- 
2.47.3

Re: [PATCH 3/7] mm/damon/vaddr: respect folio end for DAMOS_STAT

[sashiko-bot]

> For applying DAMOS_STAT action to a region, DAMON virtual address space
> operation set (vaddr) calls walk_page_range[_vma]() for the region.  The
> pmd walk entry function, namely damon_va_stat_pmd_entry(), applies DAMOS
> filters to folios of addresses of the region in the pmd.  It starts from
> the walking address and advances the address by the size of the folio of
> the address until it goes out of the pmd or the region.
> 
> Let's suppose it is for the first pmd of the region, and the region
> start address is in the middle of a large folio.  Also, the next folios
> are small.  Then, some of the next folios could be skipped.  Fix the
> issue by advancing the address to exactly the start address of the next
> folio.
> 
> The user impact is that the DAMOS_STAT-based page level monitoring
> results become inaccurate.  Since the page level monitoring is supposed
> to provide relatively high precision, this is definitely a problem.  It
> is arguably not critical since it is only monitoring quality
> degradation.
> 
> The issue was discovered [1] by Sashiko.
> 
> [1] https://lore.kernel.org/20260514015053.149396-1-sj@kernel.org
> 
> Fixes: 63f39737d1e3 ("mm/damon/vaddr: support stat-purpose DAMOS filters")
> Cc: <stable@vger.kernel.org> # 6.18.x
> 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/20260901131326.97615-1-sj@kernel.org?part=3

[PATCH 4/7] mm/damon/vaddr: respect folio end for DAMOS_MIGRATE_{HOT,COLD}

[SJ Park]

For applying DAMOS_MIGRATE_{HOT,COLD} actions to a region, DAMON virtual
address space operation set (vaddr) calls walk_page_range[_vma]() for
the region.  The pmd walk entry function, namely
damon_va_migrate_pmd_entry(), collects folios of addresses of the region
in the pmd.  It starts from the walking address and advances the address
by the size of the folio of the address until it goes out of the pmd or
the region.

Let's suppose it is for the first pmd of the region, and the region
start address is in the middle of a large folio.  Also, the next folios
are small.  Then, some of the next folios could be skipped.  Fix the
issue by advancing the address to exactly the start address of the next
folio.

The user impact is that DAMOS action is applied to less than expected
amount of memory.  Given the best effort nature of DAMON, it is no big
problem, but it is clearly a bug that is better to be fixed.

The issue was discovered [1] by Sashiko.

[1] https://lore.kernel.org/20260514015053.149396-1-sj@kernel.org

Fixes: 09efc56a3b1c ("mm/damon/vaddr: consistently use only pmd_entry for damos_migrate")
Cc: <stable@vger.kernel.org> # 6.19.x
Signed-off-by: SJ Park <sj@kernel.org>
---
 mm/damon/vaddr.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/mm/damon/vaddr.c b/mm/damon/vaddr.c
index 4b0b5edf67952..c8c32b2ae0402 100644
--- a/mm/damon/vaddr.c
+++ b/mm/damon/vaddr.c
@@ -669,6 +669,8 @@ static int damos_va_migrate_pmd_entry(pmd_t *pmd, unsigned long addr,
 		return 0;
 
 	for (; addr < next; pte += nr, addr += nr * PAGE_SIZE) {
+		unsigned long page_idx;
+
 		nr = 1;
 		ptent = ptep_get(pte);
 
@@ -681,7 +683,8 @@ static int damos_va_migrate_pmd_entry(pmd_t *pmd, unsigned long addr,
 			continue;
 		damos_va_migrate_dests_add(folio, walk->vma, addr, dests,
 				migration_lists);
-		nr = folio_nr_pages(folio);
+		page_idx = folio_page_idx(folio, pte_page(ptent));
+		nr = folio_nr_pages(folio) - page_idx;
 	}
 	pte_unmap_unlock(start_pte, ptl);
 	return 0;
-- 
2.47.3

Re: [PATCH 4/7] mm/damon/vaddr: respect folio end for DAMOS_MIGRATE_{HOT,COLD}

[sashiko-bot]

> For applying DAMOS_MIGRATE_{HOT,COLD} actions to a region, DAMON virtual
> address space operation set (vaddr) calls walk_page_range[_vma]() for
> the region.  The pmd walk entry function, namely
> damon_va_migrate_pmd_entry(), collects folios of addresses of the region
> in the pmd.  It starts from the walking address and advances the address
> by the size of the folio of the address until it goes out of the pmd or
> the region.
> 
> Let's suppose it is for the first pmd of the region, and the region
> start address is in the middle of a large folio.  Also, the next folios
> are small.  Then, some of the next folios could be skipped.  Fix the
> issue by advancing the address to exactly the start address of the next
> folio.
> 
> The user impact is that DAMOS action is applied to less than expected
> amount of memory.  Given the best effort nature of DAMON, it is no big
> problem, but it is clearly a bug that is better to be fixed.
> 
> The issue was discovered [1] by Sashiko.
> 
> [1] https://lore.kernel.org/20260514015053.149396-1-sj@kernel.org
> 
> Fixes: 09efc56a3b1c ("mm/damon/vaddr: consistently use only pmd_entry for damos_migrate")
> Cc: <stable@vger.kernel.org> # 6.19.x
> 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/20260901131326.97615-1-sj@kernel.org?part=4

[PATCH 5/7] mm/damon/core: handle extreme memory state in damon_get_node_mem_bp()

[SJ Park]

In an extreme and unlikely situation, si_meminfo_node() might let the
caller show zero total ram.  That could cause a divide by zero in
damon_get_node_mem_bp().  It could also show free memory larger than the
total memory.  This could cause underflow and make DAMOS temporarily
make unexpected behavior.  Thanks to safety guards in the auto-tuning
feedback loop, that should not be a real problem, though.  Fix the
problems by respectively returning 100% and 0% for used and free memory
queries in the corner cases.

The issue was discovered [1] by Sashiko.

[1] https://lore.kernel.org/20260328133216.9697-1-sj@kernel.org

Fixes: 0e1c773b501f ("mm/damon/core: introduce damos quota goal metrics for memory node utilization")
Cc: <stable@vger.kernel.org> # 6.16.x
Signed-off-by: SJ Park <sj@kernel.org>
---
 mm/damon/core.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/mm/damon/core.c b/mm/damon/core.c
index a6fdb3068262d..ad3657d356fbc 100644
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -2807,6 +2807,13 @@ static __kernel_ulong_t damos_get_node_mem_bp(
 	}
 
 	si_meminfo_node(&i, goal->nid);
+	if (!i.totalram || i.totalram < i.freeram) {
+		if (goal->metric == DAMOS_QUOTA_NODE_MEM_USED_BP)
+			return 10000;
+		else	/* DAMOS_QUOTA_NODE_MEM_FREE_BP */
+			return 0;
+	}
+
 	if (goal->metric == DAMOS_QUOTA_NODE_MEM_USED_BP)
 		numerator = i.totalram - i.freeram;
 	else	/* DAMOS_QUOTA_NODE_MEM_FREE_BP */
-- 
2.47.3

Re: [PATCH 5/7] mm/damon/core: handle extreme memory state in damon_get_node_mem_bp()

[sashiko-bot]

> In an extreme and unlikely situation, si_meminfo_node() might let the
> caller show zero total ram.  That could cause a divide by zero in
> damon_get_node_mem_bp().  It could also show free memory larger than the
> total memory.  This could cause underflow and make DAMOS temporarily
> make unexpected behavior.  Thanks to safety guards in the auto-tuning
> feedback loop, that should not be a real problem, though.  Fix the
> problems by respectively returning 100% and 0% for used and free memory
> queries in the corner cases.
> 
> The issue was discovered [1] by Sashiko.
> 
> [1] https://lore.kernel.org/20260328133216.9697-1-sj@kernel.org
> 
> Fixes: 0e1c773b501f ("mm/damon/core: introduce damos quota goal metrics for memory node utilization")
> Cc: <stable@vger.kernel.org> # 6.16.x
> 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/20260901131326.97615-1-sj@kernel.org?part=5

[PATCH 6/7] mm/damon/core: handle extreme memory state in get_node_memcg_used_bp()

[SJ Park]

In extreme unlikely situations, total memory might be zero.  In less
extreme but still very unlikely situations, lruvec_page_state() calls
might let the caller show used memory larger than total memory.  In the
two cases, damos_get_node_memcg_used_bp() could cause division by zero,
or return underflowed value, respectively.  Handle the cases by
respectively returning 100% and 0% for used and free memory queries in
the corner cases.

This issue was discovered [1] by Sashiko.

[1] https://lore.kernel.org/20260329154813.47382-1-sj@kernel.org

Fixes: b74a120bcf50 ("mm/damon/core: implement DAMOS_QUOTA_NODE_MEMCG_USED_BP")
Cc: <stable@vger.kernel.org> # 6.19.x
Signed-off-by: SJ Park <sj@kernel.org>
---
 mm/damon/core.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/mm/damon/core.c b/mm/damon/core.c
index ad3657d356fbc..3060edf5e4fa6 100644
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -2854,6 +2854,12 @@ static unsigned long damos_get_node_memcg_used_bp(
 	mem_cgroup_put(memcg);
 
 	si_meminfo_node(&i, goal->nid);
+	if (!i.totalram || i.totalram < used_pages) {
+		if (goal->metric == DAMOS_QUOTA_NODE_MEMCG_USED_BP)
+			return 10000;
+		else	/* DAMOS_QUOTA_NODE_MEMCG_FREE_BP */
+			return 0;
+	}
 	if (goal->metric == DAMOS_QUOTA_NODE_MEMCG_USED_BP)
 		numerator = used_pages;
 	else	/* DAMOS_QUOTA_NODE_MEMCG_FREE_BP */
-- 
2.47.3

Re: [PATCH 6/7] mm/damon/core: handle extreme memory state in get_node_memcg_used_bp()

[sashiko-bot]

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

Pre-existing issues:
- [High] 32-bit integer overflow in `mult_frac()` leads to wildly inaccurate quota calculations on architectures with 32-bit unsigned longs.
--

commit f507259fcccb65ad40eabec53de3e020582667f6
Author: SJ Park <sj@kernel.org>

mm/damon/core: handle extreme memory state in get_node_memcg_used_bp()

In extreme unlikely situations, total memory might be zero. In less
extreme but still very unlikely situations, lruvec_page_state() calls
might let the caller show used memory larger than total memory. In the
two cases, damos_get_node_memcg_used_bp() could cause division by zero,
or return underflowed value, respectively. Handle the cases by
respectively returning 100% and 0% for used and free memory queries in
the corner cases.

Link: https://lore.kernel.org/20260329154813.47382-1-sj@kernel.org

> diff --git a/mm/damon/core.c b/mm/damon/core.c
> index 32cde20de2792..6288bb3b09e65 100644
> --- a/mm/damon/core.c
> +++ b/mm/damon/core.c
> @@ -2853,6 +2853,12 @@ static unsigned long damos_get_node_memcg_used_bp(
>  	mem_cgroup_put(memcg);
>  
>  	si_meminfo_node(&i, goal->nid);
> +	if (!i.totalram || i.totalram < used_pages) {
> +		if (goal->metric == DAMOS_QUOTA_NODE_MEMCG_USED_BP)
> +			return 10000;
> +		else	/* DAMOS_QUOTA_NODE_MEMCG_FREE_BP */
> +			return 0;
> +	}
>  	if (goal->metric == DAMOS_QUOTA_NODE_MEMCG_USED_BP)
>  		numerator = used_pages;
>  	else	/* DAMOS_QUOTA_NODE_MEMCG_FREE_BP */
>  		numerator = i.totalram - used_pages;
>  	return mult_frac(numerator, 10000, i.totalram);

[Severity: High]
This is a pre-existing issue, but does this calculation overflow on 32-bit
architectures?

The mult_frac(numerator, 10000, i.totalram) macro computes
numerator * 10000 / i.totalram when the numerator is smaller than the
totalram. Because numerator is an unsigned long, this multiplication is
performed using 32-bit arithmetic on 32-bit architectures.

If the numerator exceeds roughly 429,496 pages (about 1.7 gigabytes of
memory), multiplying it by 10,000 exceeds the 32-bit integer maximum of
roughly 4.29 billion. This wraparound yields a massively inaccurate fraction.

This same overflow appears to also affect damos_get_node_mem_bp() and
damos_get_in_active_mem_bp().

Could numerator be cast to a 64-bit type before multiplication to prevent
this issue on larger memory systems with 32-bit kernels?

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260901131326.97615-1-sj@kernel.org?part=6

[PATCH 7/7] mm/damon/core: handle extreme memory state in get_in_active_mem_bp()

[SJ Park]

damos_get_in_active_mem_bp() uses the sum of the active and inactive
memory amount as a denominator.  In an extreme and unlikely environment,
active and inactive memory might be zero.  In this case, hence, it
results in a divide by zero problem.  Avoid it by changing the
denominator to one if it is zero, before it is being used.

The issue was discovered [1] by Sashiko.

[1] https://lore.kernel.org/20260721034756.147011-1-sj@kernel.org

Fixes: 4835e2871321 ("mm/damon/core: introduce [in]active memory ratio damos quota goal metric")
Cc: <stable@vger.kernel.org> # 7.0.x
Signed-off-by: SJ Park <sj@kernel.org>
---
 mm/damon/core.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/damon/core.c b/mm/damon/core.c
index 3060edf5e4fa6..0df785e72438f 100644
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -3006,7 +3006,7 @@ static unsigned int damos_get_in_active_mem_bp(bool active_ratio)
 		global_node_page_state(NR_LRU_BASE + LRU_ACTIVE_FILE);
 	inactive = global_node_page_state(NR_LRU_BASE + LRU_INACTIVE_ANON) +
 		global_node_page_state(NR_LRU_BASE + LRU_INACTIVE_FILE);
-	total = active + inactive;
+	total = max(active + inactive, 1);
 	if (active_ratio)
 		return mult_frac(active, 10000, total);
 	return mult_frac(inactive, 10000, total);
-- 
2.47.3

Re: [PATCH 7/7] mm/damon/core: handle extreme memory state in get_in_active_mem_bp()

[sashiko-bot]

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

Pre-existing issues:
- [High] 32-bit integer overflow in mult_frac() macro when calculating memory ratio on 32-bit systems, leading to completely incorrect DAMOS quota metrics.
--

commit b2f8512e8018294c2ac5082c2964a0519b9242e8
Author: SJ Park <sj@kernel.org>

mm/damon/core: handle extreme memory state in get_in_active_mem_bp()
    
damos_get_in_active_mem_bp() uses the sum of the active and inactive
memory amount as a denominator.  In an extreme and unlikely environment,
active and inactive memory might be zero.  In this case, hence, it
results in a divide by zero problem.  Avoid it by changing the
denominator to one if it is zero, before it is being used.

[1] https://lore.kernel.org/20260721034756.147011-1-sj@kernel.org

> diff --git a/mm/damon/core.c b/mm/damon/core.c
> index 6288bb3b09e65..fb96848174dcc 100644
> --- a/mm/damon/core.c
> +++ b/mm/damon/core.c
> @@ -3005,7 +3005,7 @@ static unsigned int damos_get_in_active_mem_bp(bool active_ratio)
>  		global_node_page_state(NR_LRU_BASE + LRU_ACTIVE_FILE);
>  	inactive = global_node_page_state(NR_LRU_BASE + LRU_INACTIVE_ANON) +
>  		global_node_page_state(NR_LRU_BASE + LRU_INACTIVE_FILE);
> -	total = active + inactive;
> +	total = max(active + inactive, 1);
>  	if (active_ratio)
>  		return mult_frac(active, 10000, total);

[Severity: High]
This is a pre-existing issue, but does this calculation overflow on
32-bit systems?

If the active memory size exceeds 429,496 pages (about 1.75 GB), multiplying
the 32-bit unsigned long active variable by 10,000 inside the mult_frac()
macro will overflow ULONG_MAX.

This can result in effectively random DAMON quota metrics on 32-bit systems,
leading DAMON to misinterpret memory states and potentially over-reclaim or
under-reclaim memory.

A similar overflow can also happen in damos_quota_score():

mm/damon/core.c:damos_quota_score() {
    ...
    return mult_frac(goal->current_value, 10000, goal->target_value);
}

Here, goal->current_value multiplied by 10,000 overflows for large values,
such as when microseconds of stall time exceed 430ms.

>  	return mult_frac(inactive, 10000, total);

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260901131326.97615-1-sj@kernel.org?part=7

Re: [PATCH 0/7] damos_apply_scheme() splits a region and apply the action to the

[SJ Park]

I sent this series by a mistake.  This has wrong cover letter.  I will resent.
Sorry for the noise.


Thanks,
SJ

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