[RFC PATCH v2 1/1] mm/damon: support MADV_COLLAPSE via DAMOS_COLLAPSE scheme action

[<gutierrez.asier@huawei-partners.com>]

From: Asier Gutierrez <gutierrez.asier@huawei-partners.com>

This patch set introces a new action: DAMOS_COLLAPSE.

For DAMOS_HUGEPAGE and DAMOS_NOHUGEPAGE to work, khugepaged should be
working, since it relies on hugepage_madvise to add a new slot. This
slot should be picked up by khugepaged and eventually collapse (or
not, if we are using DAMOS_NOHUGEPAGE) the pages. If THP is not
enabled, khugepaged will not be working, and therefore no collapse
will happen.

DAMOS_COLLAPSE eventually calls madvise_collapse, which will collapse
the address range synchronously.

This new action may be required to support autotuning with hugepage as
a goal.

[1] https://lore.kernel.org/lkml/20260314165156.86647-1-sj@kernel.org/

Signed-off-by: Asier Gutierrez <gutierrez.asier@huawei-partners.com>
Reviewed-by: SeongJae Park <sj@kernel.org>
---
 Documentation/mm/damon/design.rst | 4 ++++
 include/linux/damon.h             | 1 +
 mm/damon/sysfs-schemes.c          | 4 ++++
 mm/damon/vaddr.c                  | 3 +++
 4 files changed, 12 insertions(+)

diff --git a/Documentation/mm/damon/design.rst b/Documentation/mm/damon/design.rst
index 29fff20b3c2a..3b1461f42987 100644
--- a/Documentation/mm/damon/design.rst
+++ b/Documentation/mm/damon/design.rst
@@ -463,6 +463,10 @@ that supports each action are as below.
    Supported by ``vaddr`` and ``fvaddr`` operations set.
  - ``nohugepage``: Call ``madvise()`` for the region with ``MADV_NOHUGEPAGE``.
    Supported by ``vaddr`` and ``fvaddr`` operations set.
+ - ``collapse``: Call ``madvise()`` for the region with ``MADV_COLLAPSE``.
+   Supported by ``vaddr`` and ``fvaddr`` operations set. When
+   TRANSPARENT_HUGEPAGE is disabled, the application of the action will just
+   fail.
  - ``lru_prio``: Prioritize the region on its LRU lists.
    Supported by ``paddr`` operations set.
  - ``lru_deprio``: Deprioritize the region on its LRU lists.
diff --git a/include/linux/damon.h b/include/linux/damon.h
index 3a441fbca170..6720dc70c487 100644
--- a/include/linux/damon.h
+++ b/include/linux/damon.h
@@ -140,6 +140,7 @@ enum damos_action {
 	DAMOS_PAGEOUT,
 	DAMOS_HUGEPAGE,
 	DAMOS_NOHUGEPAGE,
+	DAMOS_COLLAPSE,
 	DAMOS_LRU_PRIO,
 	DAMOS_LRU_DEPRIO,
 	DAMOS_MIGRATE_HOT,
diff --git a/mm/damon/sysfs-schemes.c b/mm/damon/sysfs-schemes.c
index 5186966dafb3..aa08a8f885fb 100644
--- a/mm/damon/sysfs-schemes.c
+++ b/mm/damon/sysfs-schemes.c
@@ -2041,6 +2041,10 @@ static struct damos_sysfs_action_name damos_sysfs_action_names[] = {
 		.action = DAMOS_NOHUGEPAGE,
 		.name = "nohugepage",
 	},
+	{
+		.action = DAMOS_COLLAPSE,
+		.name = "collapse",
+	},
 	{
 		.action = DAMOS_LRU_PRIO,
 		.name = "lru_prio",
diff --git a/mm/damon/vaddr.c b/mm/damon/vaddr.c
index b069dbc7e3d2..dd5f2d7027ac 100644
--- a/mm/damon/vaddr.c
+++ b/mm/damon/vaddr.c
@@ -903,6 +903,9 @@ static unsigned long damon_va_apply_scheme(struct damon_ctx *ctx,
 	case DAMOS_NOHUGEPAGE:
 		madv_action = MADV_NOHUGEPAGE;
 		break;
+	case DAMOS_COLLAPSE:
+		madv_action = MADV_COLLAPSE;
+		break;
 	case DAMOS_MIGRATE_HOT:
 	case DAMOS_MIGRATE_COLD:
 		return damos_va_migrate(t, r, scheme, sz_filter_passed);
-- 
2.43.0