[RFC PATCH] mm/damon/reclaim: allow disabling watermarks to force reclaim of cold pages

[RFC PATCH] mm/damon/reclaim: allow disabling watermarks to force reclaim of cold pages

[Jinjie Ruan]

DAMON_RECLAIM runs its scheme only while the free memory rate is in
[wmarks_low, wmarks_mid].  Reclaiming cold pages raises the rate, so the
scheme self-stops as soon as memory looks plentiful again, which is wrong
for a VMM parking an idle sandbox that wants cold anonymous pages
reclaimed immediately.

So add an ignore_wmarks module parameter. When set, the scheme uses
DAMOS_WMARK_NONE, so damos_wmark_wait_us() always returns zero and the
scheme stays active, bounded only by min_age and the quota.

Signed-off-by: Jinjie Ruan <ruanjinjie@huawei.com>
---
 .../admin-guide/mm/damon/reclaim.rst          | 17 ++++++++++++++++
 mm/damon/reclaim.c                            | 20 +++++++++++++++++++
 2 files changed, 37 insertions(+)

diff --git a/Documentation/admin-guide/mm/damon/reclaim.rst b/Documentation/admin-guide/mm/damon/reclaim.rst
index ec7e3e32b4ac..ef57f60a9105 100644
--- a/Documentation/admin-guide/mm/damon/reclaim.rst
+++ b/Documentation/admin-guide/mm/damon/reclaim.rst
@@ -159,6 +159,23 @@ this auto-tuning feature.
 
 Disabled by default.
 
+ignore_wmarks
+-------------
+
+Ignore the watermarks-based scheme activation condition.
+
+If set to ``Y``, DAMON_RECLAIM runs its scheme regardless of the system's
+free memory rate, so the reclaim is bounded only by ``min_age`` and the
+quota.  This is useful when cold pages should be reclaimed even while the
+system still has free memory, e.g. to squeeze the cold anonymous pages of a
+just-parked virtual machine out immediately.
+
+By default, ``N``, so the scheme is activated based on the system's free
+memory rate, as described for the watermarks below.
+
+Changes take effect when the scheme is rebuilt, i.e. on the next ``enabled``
+``0 -> 1`` transition, like the other DAMON_RECLAIM parameters.
+
 wmarks_interval
 ---------------
 
diff --git a/mm/damon/reclaim.c b/mm/damon/reclaim.c
index 45d5557cc575..03b9a57694a6 100644
--- a/mm/damon/reclaim.c
+++ b/mm/damon/reclaim.c
@@ -112,6 +112,23 @@ static struct damos_watermarks damon_reclaim_wmarks = {
 };
 DEFINE_DAMON_MODULES_WMARKS_PARAMS(damon_reclaim_wmarks);
 
+/*
+ * Ignore the watermarks-based scheme activation condition.
+ *
+ * With the default ``N``, DAMON_RECLAIM runs its scheme only while the
+ * system's free memory rate is inside [wmarks_low, wmarks_mid].
+ * Reclaiming cold pages itself raises the free rate, so the scheme
+ * deactivates as soon as memory looks plentiful again.  For a VMM parking
+ * an idle sandbox, cold anonymous pages should be reclaimed immediately
+ * even while the host still has plenty of free memory; setting this ``Y``
+ * makes the scheme ignore the watermarks, so the reclaim is bounded only
+ * by min_age and the quota.
+ */
+static bool ignore_wmarks __read_mostly;
+module_param(ignore_wmarks, bool, 0600);
+MODULE_PARM_DESC(ignore_wmarks,
+	"Ignore watermarks: reclaim cold pages even while free memory is plentiful (default: false)");
+
 static struct damon_attrs damon_reclaim_mon_attrs = {
 	.sample_interval = 5000,	/* 5 ms */
 	.aggr_interval = 100000,	/* 100 ms */
@@ -226,6 +243,9 @@ static int damon_reclaim_apply_parameters(void)
 	if (err)
 		goto out;
 
+	damon_reclaim_wmarks.metric = ignore_wmarks ?
+			DAMOS_WMARK_NONE : DAMOS_WMARK_FREE_MEM_RATE;
+
 	err = -ENOMEM;
 	scheme = damon_reclaim_new_scheme(attrs.aggr_interval);
 	if (!scheme)
-- 
2.53.0

Re: [RFC PATCH] mm/damon/reclaim: allow disabling watermarks to force reclaim of cold pages

[SJ Park]

Hello Jinjie,

On Tue, 20 Oct 2026 21:32:33 +0800 Jinjie Ruan <ruanjinjie@huawei.com> wrote:

> DAMON_RECLAIM runs its scheme only while the free memory rate is in
> [wmarks_low, wmarks_mid].  Reclaiming cold pages raises the rate, so the
> scheme self-stops as soon as memory looks plentiful again, which is wrong
> for a VMM parking an idle sandbox that wants cold anonymous pages
> reclaimed immediately.
> 
> So add an ignore_wmarks module parameter. When set, the scheme uses
> DAMOS_WMARK_NONE, so damos_wmark_wait_us() always returns zero and the
> scheme stays active, bounded only by min_age and the quota.

I agree there could be use cases that doesn't want the watermarks at all.  I'd
like to avoid adding new DAMON_RECLAIM parameters if possible, though.  That's
becasuse DAMON_RECLAIM is introduced to let people use it without complicated
setups.  We will of course add new parameters if really needed, but I want to
see if there is a good alternative.

Have you considered some other alternatives?  For example, could respectively
setting wmarks_high, wmarks_mid and wmarks_low to 100%, 100% and 0% effectively
disable watermarks?  Have you tried that?  If that works, how about updating
document to clearly explain the usage?  Also, have you considered using DAMON
sysfs interface instead of DAMON_RECLAIM?


Thanks,
SJ

[...]

Re: [RFC PATCH] mm/damon/reclaim: allow disabling watermarks to force reclaim of cold pages

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 2 potential issue(s) to consider:
- [Low] mm/damon/reclaim: missing kernel-parameters.txt entry for ignore_wmarks
- [Low] mm/damon/reclaim: inaccurate ignore_wmarks parameter apply documentation
--

commit 8e407c81e469d1e743666b3331a460c372ffea07
Author: Jinjie Ruan <ruanjinjie@huawei.com>

mm/damon/reclaim: allow disabling watermarks to force reclaim of cold pages

This commit adds the ignore_wmarks module parameter to DAMON_RECLAIM so
that the scheme can use DAMOS_WMARK_NONE and reclaim cold pages even when
free memory is plentiful.

> diff --git a/Documentation/admin-guide/mm/damon/reclaim.rst b/Documentation/admin-guide/mm/damon/reclaim.rst
> index ec7e3e32b4ac6..ef57f60a91059 100644
> --- a/Documentation/admin-guide/mm/damon/reclaim.rst
> +++ b/Documentation/admin-guide/mm/damon/reclaim.rst
> @@ -159,6 +159,23 @@ this auto-tuning feature.
>  
>  Disabled by default.
>  
> +ignore_wmarks
> +-------------
> +
> +Ignore the watermarks-based scheme activation condition.
> +
> +If set to ``Y``, DAMON_RECLAIM runs its scheme regardless of the system's
> +free memory rate, so the reclaim is bounded only by ``min_age`` and the
> +quota.  This is useful when cold pages should be reclaimed even while the
> +system still has free memory, e.g. to squeeze the cold anonymous pages of a
> +just-parked virtual machine out immediately.
> +
> +By default, ``N``, so the scheme is activated based on the system's free
> +memory rate, as described for the watermarks below.
> +
> +Changes take effect when the scheme is rebuilt, i.e. on the next ``enabled``
> +``0 -> 1`` transition, like the other DAMON_RECLAIM parameters.

[Severity: Low]
Is this description of when changes take effect accurate?

In mm/damon/reclaim.c, ignore_wmarks is applied in
damon_reclaim_apply_parameters() and committed via damon_commit_ctx().
Because damon_reclaim_commit_inputs_store() invokes
damon_reclaim_commit_inputs_fn() -> damon_reclaim_apply_parameters():

    static int damon_reclaim_commit_inputs_fn(void *arg)
    {
        return damon_reclaim_apply_parameters();
    }

writing Y to commit_inputs also applies ignore_wmarks changes online while
DAMON_RECLAIM is already running, without requiring an enabled 0 -> 1
transition.

Could the documentation note that changes can also be applied online via
commit_inputs?

>  wmarks_interval
>  ---------------
>  
> diff --git a/mm/damon/reclaim.c b/mm/damon/reclaim.c
> index 014b0779ea6dd..effd963f3c783 100644
> --- a/mm/damon/reclaim.c
> +++ b/mm/damon/reclaim.c
> @@ -112,6 +112,23 @@ static struct damos_watermarks damon_reclaim_wmarks = {
>  };
>  DEFINE_DAMON_MODULES_WMARKS_PARAMS(damon_reclaim_wmarks);
>  
> +/*
> + * Ignore the watermarks-based scheme activation condition.
> + *
> + * With the default ``N``, DAMON_RECLAIM runs its scheme only while the
> + * system's free memory rate is inside [wmarks_low, wmarks_mid].
> + * Reclaiming cold pages itself raises the free rate, so the scheme
> + * deactivates as soon as memory looks plentiful again.  For a VMM parking
> + * an idle sandbox, cold anonymous pages should be reclaimed immediately
> + * even while the host still has plenty of free memory; setting this ``Y``
> + * makes the scheme ignore the watermarks, so the reclaim is bounded only
> + * by min_age and the quota.
> + */
> +static bool ignore_wmarks __read_mostly;
> +module_param(ignore_wmarks, bool, 0600);
> +MODULE_PARM_DESC(ignore_wmarks,
> +	"Ignore watermarks: reclaim cold pages even while free memory is plentiful (default: false)");

[Severity: Low]
Since mm/damon/reclaim.c defines MODULE_PARAM_PREFIX as "damon_reclaim.",
registering ignore_wmarks via module_param() exposes
damon_reclaim.ignore_wmarks as a kernel command-line parameter at boot.

Should this patch also document damon_reclaim.ignore_wmarks in
Documentation/admin-guide/kernel-parameters.txt?

[ ... ]

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261020133233.3920818-1-ruanjinjie@huawei.com?part=1

Re: [RFC PATCH] mm/damon/reclaim: allow disabling watermarks to force reclaim of cold pages

[Aveline Noir]

Hi SJ,

> setting wmarks_high, wmarks_mid and wmarks_low to 100%, 100% and 0%
> effectively disable watermarks? Have you tried that?

That worked in my test with swap enabled. Agree that documenting this
combination might be useful.

Thanks,
Aveline Noir