[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@xxxxxxxxxx>
---
 .../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