[PATCH] mm/damon/stat: monitor all System RAM resources

[PATCH] mm/damon/stat: monitor all System RAM resources

[SeongJae Park]

DAMON_STAT usage document (Documentation/admin-guide/mm/damon/stat.rst)
says it monitors the system's entire physical memory.  But, it is
monitoring only the biggest System RAM resource of the system.  When
there are multiple System RAM resources, this results in monitoring only
an unexpectedly small fraction of the physical memory.  For example,
suppose the system has a 500 GiB System RAM, 10 MiB non-System RAM, and
500 GiB System RAM resources in order on the physical address space.
DAMON_STAT will monitor only the first 500 GiB System RAM.  This
situation is particularly common on NUMA systems.

Select a physical address range that covers all System RAM areas of the
system, to fix this issue and make it work as documented.

Fixes: 369c415e6073 ("mm/damon: introduce DAMON_STAT module")
Cc: <stable@vger.kernel.org> # 6.17.x
Signed-off-by: SeongJae Park <sj@kernel.org>
---
 mm/damon/stat.c | 51 ++++++++++++++++++++++++++++++++++++++++++++++---
 1 file changed, 48 insertions(+), 3 deletions(-)

diff --git a/mm/damon/stat.c b/mm/damon/stat.c
index f9a2028483b05..3ed71db33e899 100644
--- a/mm/damon/stat.c
+++ b/mm/damon/stat.c
@@ -145,12 +145,57 @@ static int damon_stat_damon_call_fn(void *data)
 	return 0;
 }
 
+struct damon_stat_system_ram_range_walk_arg {
+	bool walked;
+	struct resource res;
+};
+
+static int damon_stat_system_ram_walk_fn(struct resource *res, void *arg)
+{
+	struct damon_stat_system_ram_range_walk_arg *a = arg;
+
+	if (!a->walked) {
+		a->walked = true;
+		a->res.start = res->start;
+	}
+	a->res.end = res->end;
+	return 0;
+}
+
+static unsigned long damon_stat_res_to_core_addr(resource_size_t ra,
+		unsigned long addr_unit)
+{
+	/*
+	 * Use div_u64() for avoiding linking errors related with __udivdi3,
+	 * __aeabi_uldivmod, or similar problems.  This should also improve the
+	 * performance optimization (read div_u64() comment for the detail).
+	 */
+	if (sizeof(ra) == 8 && sizeof(addr_unit) == 4)
+		return div_u64(ra, addr_unit);
+	return ra / addr_unit;
+}
+
+static int damon_stat_set_monitoring_region(struct damon_target *t,
+		unsigned long addr_unit, unsigned long min_region_sz)
+{
+	struct damon_addr_range addr_range;
+	struct damon_stat_system_ram_range_walk_arg arg = {};
+
+	walk_system_ram_res(0, -1, &arg, damon_stat_system_ram_walk_fn);
+	if (!arg.walked)
+		return -EINVAL;
+	addr_range.start = damon_stat_res_to_core_addr(
+			arg.res.start, addr_unit);
+	addr_range.end = damon_stat_res_to_core_addr(
+			arg.res.end + 1, addr_unit);
+	return damon_set_regions(t, &addr_range, addr_unit, min_region_sz);
+}
+
 static struct damon_ctx *damon_stat_build_ctx(void)
 {
 	struct damon_ctx *ctx;
 	struct damon_attrs attrs;
 	struct damon_target *target;
-	unsigned long start = 0, end = 0;
 
 	ctx = damon_new_ctx();
 	if (!ctx)
@@ -180,8 +225,8 @@ static struct damon_ctx *damon_stat_build_ctx(void)
 	if (!target)
 		goto free_out;
 	damon_add_target(ctx, target);
-	if (damon_set_region_biggest_system_ram_default(target, &start, &end,
-				ctx->addr_unit, ctx->min_region_sz))
+	if (damon_stat_set_monitoring_region(target, ctx->addr_unit,
+				ctx->min_region_sz))
 		goto free_out;
 	return ctx;
 free_out:

base-commit: ef772f765091af645985f2bd70390143a9820970
-- 
2.47.3

Re: [PATCH] mm/damon/stat: monitor all System RAM resources

[SeongJae Park]

On Thu, 12 Mar 2026 21:44:47 -0700 SeongJae Park <sj@kernel.org> wrote:

> DAMON_STAT usage document (Documentation/admin-guide/mm/damon/stat.rst)
> says it monitors the system's entire physical memory.  But, it is
> monitoring only the biggest System RAM resource of the system.  When
> there are multiple System RAM resources, this results in monitoring only
> an unexpectedly small fraction of the physical memory.  For example,
> suppose the system has a 500 GiB System RAM, 10 MiB non-System RAM, and
> 500 GiB System RAM resources in order on the physical address space.
> DAMON_STAT will monitor only the first 500 GiB System RAM.  This
> situation is particularly common on NUMA systems.
> 
> Select a physical address range that covers all System RAM areas of the
> system, to fix this issue and make it work as documented.
> 
> Fixes: 369c415e6073 ("mm/damon: introduce DAMON_STAT module")
> Cc: <stable@vger.kernel.org> # 6.17.x
> Signed-off-by: SeongJae Park <sj@kernel.org>
> ---
>  mm/damon/stat.c | 51 ++++++++++++++++++++++++++++++++++++++++++++++---
>  1 file changed, 48 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/damon/stat.c b/mm/damon/stat.c
> index f9a2028483b05..3ed71db33e899 100644
> --- a/mm/damon/stat.c
> +++ b/mm/damon/stat.c
> @@ -145,12 +145,57 @@ static int damon_stat_damon_call_fn(void *data)
>  	return 0;
>  }
>  
> +struct damon_stat_system_ram_range_walk_arg {
> +	bool walked;
> +	struct resource res;
> +};
> +
> +static int damon_stat_system_ram_walk_fn(struct resource *res, void *arg)
> +{
> +	struct damon_stat_system_ram_range_walk_arg *a = arg;
> +
> +	if (!a->walked) {
> +		a->walked = true;
> +		a->res.start = res->start;
> +	}
> +	a->res.end = res->end;
> +	return 0;
> +}
> +
> +static unsigned long damon_stat_res_to_core_addr(resource_size_t ra,
> +		unsigned long addr_unit)
> +{
> +	/*
> +	 * Use div_u64() for avoiding linking errors related with __udivdi3,
> +	 * __aeabi_uldivmod, or similar problems.  This should also improve the
> +	 * performance optimization (read div_u64() comment for the detail).
> +	 */
> +	if (sizeof(ra) == 8 && sizeof(addr_unit) == 4)
> +		return div_u64(ra, addr_unit);
> +	return ra / addr_unit;
> +}
> +
> +static int damon_stat_set_monitoring_region(struct damon_target *t,
> +		unsigned long addr_unit, unsigned long min_region_sz)
> +{
> +	struct damon_addr_range addr_range;
> +	struct damon_stat_system_ram_range_walk_arg arg = {};
> +
> +	walk_system_ram_res(0, -1, &arg, damon_stat_system_ram_walk_fn);
> +	if (!arg.walked)
> +		return -EINVAL;
> +	addr_range.start = damon_stat_res_to_core_addr(
> +			arg.res.start, addr_unit);
> +	addr_range.end = damon_stat_res_to_core_addr(
> +			arg.res.end + 1, addr_unit);
> +	return damon_set_regions(t, &addr_range, addr_unit, min_region_sz);

The third argument of damon_set_regions() is the number of ranges (length of
the array passed by the second argument), so '1' should be passed.  But the
patch is passing 'addr_unit'.  It will not cause a real issue since the value
is set to '1' in this case.  But it is an obvious bug.  I will fix this in the
v2.

FYI, I found this issue from an AI code review result [1] that published on the
internet.  The review also added two more comments, but those seem irrelevant
to me, so ignoring.  I don't know who made the web site.  I only got the url
from a social.kernel.org post [2].  Whoever made the web site, thanks for the
review.

[1] https://sashiko.dev/#/patchset/20260313044449.4038-1-sj%40kernel.org
[2] https://social.kernel.org/notice/B4E8DdeZY07PseemfI


Thanks,
SJ

[...]

Re: [PATCH] mm/damon/stat: monitor all System RAM resources

[SeongJae Park]

On Fri, 13 Mar 2026 16:40:22 -0700 SeongJae Park <sj@kernel.org> wrote:

> On Thu, 12 Mar 2026 21:44:47 -0700 SeongJae Park <sj@kernel.org> wrote:
> 
> > DAMON_STAT usage document (Documentation/admin-guide/mm/damon/stat.rst)
> > says it monitors the system's entire physical memory.  But, it is
> > monitoring only the biggest System RAM resource of the system.  When
> > there are multiple System RAM resources, this results in monitoring only
> > an unexpectedly small fraction of the physical memory.  For example,
> > suppose the system has a 500 GiB System RAM, 10 MiB non-System RAM, and
> > 500 GiB System RAM resources in order on the physical address space.
> > DAMON_STAT will monitor only the first 500 GiB System RAM.  This
> > situation is particularly common on NUMA systems.
> > 
> > Select a physical address range that covers all System RAM areas of the
> > system, to fix this issue and make it work as documented.
> > 
> > Fixes: 369c415e6073 ("mm/damon: introduce DAMON_STAT module")
> > Cc: <stable@vger.kernel.org> # 6.17.x
> > Signed-off-by: SeongJae Park <sj@kernel.org>
> > ---
> >  mm/damon/stat.c | 51 ++++++++++++++++++++++++++++++++++++++++++++++---
> >  1 file changed, 48 insertions(+), 3 deletions(-)
> > 
> > diff --git a/mm/damon/stat.c b/mm/damon/stat.c
> > index f9a2028483b05..3ed71db33e899 100644
> > --- a/mm/damon/stat.c
> > +++ b/mm/damon/stat.c
> > @@ -145,12 +145,57 @@ static int damon_stat_damon_call_fn(void *data)
> >  	return 0;
> >  }
> >  
> > +struct damon_stat_system_ram_range_walk_arg {
> > +	bool walked;
> > +	struct resource res;
> > +};
> > +
> > +static int damon_stat_system_ram_walk_fn(struct resource *res, void *arg)
> > +{
> > +	struct damon_stat_system_ram_range_walk_arg *a = arg;
> > +
> > +	if (!a->walked) {
> > +		a->walked = true;
> > +		a->res.start = res->start;
> > +	}
> > +	a->res.end = res->end;
> > +	return 0;
> > +}
> > +
> > +static unsigned long damon_stat_res_to_core_addr(resource_size_t ra,
> > +		unsigned long addr_unit)
> > +{
> > +	/*
> > +	 * Use div_u64() for avoiding linking errors related with __udivdi3,
> > +	 * __aeabi_uldivmod, or similar problems.  This should also improve the
> > +	 * performance optimization (read div_u64() comment for the detail).
> > +	 */
> > +	if (sizeof(ra) == 8 && sizeof(addr_unit) == 4)
> > +		return div_u64(ra, addr_unit);
> > +	return ra / addr_unit;
> > +}
> > +
> > +static int damon_stat_set_monitoring_region(struct damon_target *t,
> > +		unsigned long addr_unit, unsigned long min_region_sz)
> > +{
> > +	struct damon_addr_range addr_range;
> > +	struct damon_stat_system_ram_range_walk_arg arg = {};
> > +
> > +	walk_system_ram_res(0, -1, &arg, damon_stat_system_ram_walk_fn);
> > +	if (!arg.walked)
> > +		return -EINVAL;
> > +	addr_range.start = damon_stat_res_to_core_addr(
> > +			arg.res.start, addr_unit);
> > +	addr_range.end = damon_stat_res_to_core_addr(
> > +			arg.res.end + 1, addr_unit);
> > +	return damon_set_regions(t, &addr_range, addr_unit, min_region_sz);
> 
> The third argument of damon_set_regions() is the number of ranges (length of
> the array passed by the second argument), so '1' should be passed.  But the
> patch is passing 'addr_unit'.  It will not cause a real issue since the value
> is set to '1' in this case.  But it is an obvious bug.  I will fix this in the
> v2.

Or, Andrew, if you prefer to, please pick below attaching fixup together.  I
will post v2 on Sunday if this is not picked up with the below fixup by
tomorrow.


Thanks,
SJ

[...]
=== >8 ===
From e5f029441de038289320a7cb99a249800d99471f Mon Sep 17 00:00:00 2001
From: SeongJae Park <sj@kernel.org>
Date: Fri, 13 Mar 2026 16:41:00 -0700
Subject: [PATCH] mm/damon/stat: fix wrong argument to damon_set_regions()

The third argument for damon_set_regions() is the number of ranges in
the second argument, but the code is wrongly passing addr_unit.  Fix it.

Signed-off-by: SeongJae Park <sj@kernel.org>
---
 mm/damon/stat.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/damon/stat.c b/mm/damon/stat.c
index 3ed71db33e899..c5af8ad4bcb65 100644
--- a/mm/damon/stat.c
+++ b/mm/damon/stat.c
@@ -188,7 +188,7 @@ static int damon_stat_set_monitoring_region(struct damon_target *t,
 			arg.res.start, addr_unit);
 	addr_range.end = damon_stat_res_to_core_addr(
 			arg.res.end + 1, addr_unit);
-	return damon_set_regions(t, &addr_range, addr_unit, min_region_sz);
+	return damon_set_regions(t, &addr_range, 1, min_region_sz);
 }
 
 static struct damon_ctx *damon_stat_build_ctx(void)
-- 
2.47.3

Re: [PATCH] mm/damon/stat: monitor all System RAM resources

[Andrew Morton]

On Fri, 13 Mar 2026 16:40:22 -0700 SeongJae Park <sj@kernel.org> wrote:

> FYI, I found this issue from an AI code review result [1] that published on the
> internet.  The review also added two more comments, but those seem irrelevant
> to me, so ignoring.  I don't know who made the web site.  I only got the url
> from a social.kernel.org post [2].  Whoever made the web site,

Sashiko is being led by Roman at Google.

> thanks for the review.

yop, it's good.

> 
> [1] https://sashiko.dev/#/patchset/20260313044449.4038-1-sj%40kernel.org
> [2] https://social.kernel.org/notice/B4E8DdeZY07PseemfI

Re: [PATCH] mm/damon/stat: monitor all System RAM resources

[SeongJae Park]

On Sat, 14 Mar 2026 15:51:09 -0700 Andrew Morton <akpm@linux-foundation.org> wrote:

> On Fri, 13 Mar 2026 16:40:22 -0700 SeongJae Park <sj@kernel.org> wrote:
> 
> > FYI, I found this issue from an AI code review result [1] that published on the
> > internet.  The review also added two more comments, but those seem irrelevant
> > to me, so ignoring.  I don't know who made the web site.  I only got the url
> > from a social.kernel.org post [2].  Whoever made the web site,
> 
> Sashiko is being led by Roman at Google.

Thank you for letting me know this, Anderw!

> 
> > thanks for the review.
> 
> yop, it's good.

I'm now a big fan of it.  It is really good, except the lack of the dark mode
;)  I therefore added features [1] for showing the AI reviews on the terminal to my
mailing tool (hkml).

Anyway, this is really awesome and helpful.  Thank you for developing this,
Roman and the team!

> 
> > 
> > [1] https://sashiko.dev/#/patchset/20260313044449.4038-1-sj%40kernel.org
> > [2] https://social.kernel.org/notice/B4E8DdeZY07PseemfI

[1] https://github.com/sjp38/hackermail/blob/master/USAGE.md#reading-sashikodev-ai-review


Thanks,
SJ