On 7/20/2026 6:03 AM, Lian Wang wrote:
> Add a new DAMOS action, DAMOS_SPLIT, that splits large folios in the
> target region down to a smaller order. A new per-scheme field and sysfs
> file, target_order, selects the split target: 0 for order-0 base pages,
> or 2..HPAGE_PMD_ORDER-1 for a smaller mTHP order (order-1 is rejected at
> store time since anonymous THPs cannot be split to order-1).
>
> Unlike DAMOS_COLLAPSE which builds large folios, splitting only dismantles
> them; it allocates nothing, copies nothing, and no THP admission policy
> applies. khugepaged deliberately refuses to collapse a folio to a
> smaller order, leaving the shrink direction to callers like this one.
>
> This patch adds the DAMOS_SPLIT action, the ``order`` field (placed in
> the existing union alongside target_nid and migrate_dests), its sysfs
> interface, and documentation. The vaddr operations set handler is added
> in the next patch.
>
> Link: https://lore.kernel.org/[email protected]/
> ("mm/khugepaged: skip collapsing mTHP to smaller orders")
>
>
>
> Co-developed-by: Kunwu Chan <[email protected]>
> Signed-off-by: Kunwu Chan <[email protected]>
> Signed-off-by: Lian Wang (Processmission) <[email protected]>
> ---
> .../ABI/testing/sysfs-kernel-mm-damon | 7 +++
> Documentation/mm/damon/design.rst | 5 ++
> include/linux/damon.h | 15 ++++--
> mm/damon/core.c | 2 +
> mm/damon/sysfs-schemes.c | 48 +++++++++++++++++++
> tools/testing/selftests/damon/sysfs.py | 11 +++--
> 6 files changed, 79 insertions(+), 9 deletions(-)
>
> diff --git a/Documentation/ABI/testing/sysfs-kernel-mm-damon
> b/Documentation/ABI/testing/sysfs-kernel-mm-damon
> index 907a504fb64c..ee8e260a631c 100644
> --- a/Documentation/ABI/testing/sysfs-kernel-mm-damon
> +++ b/Documentation/ABI/testing/sysfs-kernel-mm-damon
> @@ -265,6 +265,13 @@ Contact: SJ Park <[email protected]>
> Description: Action's target NUMA node id. Supported by only relevant
> actions.
>
> +What:
> /sys/kernel/mm/damon/admin/kdamonds/<K>/contexts/<C>/schemes/<S>/target_order
> +Date: Jul 2026
> +Contact: SJ Park <[email protected]>
Shouldn't this point to your email address?
> +Description: Target folio order for the ``split`` action. Large folios in
> + the target region are split down to this order. Valid values
> + are 0 (order-0 base pages) and 2..HPAGE_PMD_ORDER-1.
> +
> What:
> /sys/kernel/mm/damon/admin/kdamonds/<K>/contexts/<C>/schemes/<S>/apply_interval_us
> Date: Sep 2023
> Contact: SJ Park <[email protected]>
> diff --git a/Documentation/mm/damon/design.rst
> b/Documentation/mm/damon/design.rst
> index aed6cb1cf483..87a801954b1b 100644
> --- a/Documentation/mm/damon/design.rst
> +++ b/Documentation/mm/damon/design.rst
> @@ -549,6 +549,11 @@ that supports each action are as below.
> Supported by ``vaddr`` and ``fvaddr`` operations set. When
> TRANSPARENT_HUGEPAGE is disabled, the application of the action will just
> fail.
> + - ``split``: Split each large folio in the region down to the order
> specified
> + by the scheme's ``target_order`` (``0`` for order-0 base pages, or
> + ``2..HPAGE_PMD_ORDER-1``). 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 f69442a9d431..88b61b900a36 100644
> --- a/include/linux/damon.h
> +++ b/include/linux/damon.h
> @@ -110,6 +110,7 @@ struct damon_target {
> * @DAMOS_HUGEPAGE: Call ``madvise()`` for the region with MADV_HUGEPAGE.
> * @DAMOS_NOHUGEPAGE: Call ``madvise()`` for the region with
> MADV_NOHUGEPAGE.
> * @DAMOS_COLLAPSE: Call ``madvise()`` for the region with MADV_COLLAPSE.
> + * @DAMOS_SPLIT: Split each large folio in the region to a smaller order.
> * @DAMOS_LRU_PRIO: Prioritize the region on its LRU lists.
> * @DAMOS_LRU_DEPRIO: Deprioritize the region on its LRU lists.
> * @DAMOS_MIGRATE_HOT: Migrate the regions prioritizing warmer regions.
> @@ -130,6 +131,7 @@ enum damos_action {
> DAMOS_HUGEPAGE,
> DAMOS_NOHUGEPAGE,
> DAMOS_COLLAPSE,
> + DAMOS_SPLIT,
> DAMOS_LRU_PRIO,
> DAMOS_LRU_DEPRIO,
> DAMOS_MIGRATE_HOT,
> @@ -582,10 +584,15 @@ struct damos {
> struct damos_quota quota;
> struct damos_watermarks wmarks;
> union {
> - struct {
> - int target_nid;
> - struct damos_migrate_dests migrate_dests;
> - };
> + int target_nid;
> + struct damos_migrate_dests migrate_dests;
Shouldn't we keep target_nid and migrate_dests together using a
struct, as in the initial code?
> + /*
> + * @order: target folio order for DAMOS_SPLIT.
> + * Split large folios down to this order. 0 for
> + * order-0 base pages, 2..HPAGE_PMD_ORDER-1 for
> + * smaller mTHP. Order-1 is rejected.
> + */
> + unsigned int order;
> };
> struct list_head core_filters;
> struct list_head ops_filters;
> diff --git a/mm/damon/core.c b/mm/damon/core.c
> index 806a67d02a6e..394c62455ad5 100644
> --- a/mm/damon/core.c
> +++ b/mm/damon/core.c
> @@ -693,6 +693,7 @@ struct damos *damon_new_scheme(struct
> damos_access_pattern *pattern,
> return NULL;
> scheme->pattern = *pattern;
> scheme->action = action;
> + scheme->order = 0;
> scheme->apply_interval_us = apply_interval_us;
> /*
> * next_apply_sis will be set when kdamond starts. While kdamond is
> @@ -1446,6 +1447,7 @@ static int damos_commit(struct damos *dst, struct damos
> *src)
>
> dst->pattern = src->pattern;
> dst->action = src->action;
> + dst->order = src->order;
> dst->apply_interval_us = src->apply_interval_us;
>
> err = damos_commit_quota(&dst->quota, &src->quota);
> diff --git a/mm/damon/sysfs-schemes.c b/mm/damon/sysfs-schemes.c
> index 32f495a96b17..725ed81ccf6c 100644
> --- a/mm/damon/sysfs-schemes.c
> +++ b/mm/damon/sysfs-schemes.c
> @@ -4,7 +4,9 @@
> */
>
> #include <linux/slab.h>
> +#include <linux/mm.h>
> #include <linux/numa.h>
> +#include <linux/huge_mm.h>
>
> #include "sysfs-common.h"
>
> @@ -2260,6 +2262,7 @@ struct damon_sysfs_scheme {
> struct damon_sysfs_stats *stats;
> struct damon_sysfs_scheme_regions *tried_regions;
> int target_nid;
> + unsigned int target_order;
> struct damos_sysfs_dests *dests;
> };
>
> @@ -2293,6 +2296,10 @@ static struct damos_sysfs_action_name
> damos_sysfs_action_names[] = {
> .action = DAMOS_COLLAPSE,
> .name = "collapse",
> },
> + {
> + .action = DAMOS_SPLIT,
> + .name = "split",
> + },
> {
> .action = DAMOS_LRU_PRIO,
> .name = "lru_prio",
> @@ -2326,6 +2333,7 @@ static struct damon_sysfs_scheme
> *damon_sysfs_scheme_alloc(
> scheme->action = action;
> scheme->apply_interval_us = apply_interval_us;
> scheme->target_nid = NUMA_NO_NODE;
> + scheme->target_order = 0;
> return scheme;
> }
>
> @@ -2645,6 +2653,40 @@ static ssize_t target_nid_store(struct kobject *kobj,
> return err ? err : count;
> }
>
> +static ssize_t target_order_show(struct kobject *kobj,
> + struct kobj_attribute *attr, char *buf)
> +{
> + struct damon_sysfs_scheme *scheme = container_of(kobj,
> + struct damon_sysfs_scheme, kobj);
> +
> + return sysfs_emit(buf, "%u\n", scheme->target_order);
> +}
> +
> +static ssize_t target_order_store(struct kobject *kobj,
> + struct kobj_attribute *attr, const char *buf, size_t count)
> +{
> + struct damon_sysfs_scheme *scheme = container_of(kobj,
> + struct damon_sysfs_scheme, kobj);
> + unsigned int val;
> + int err;
Move the variable declaration to the top of the function.
> +
> + err = kstrtouint(buf, 0, &val);
> + if (err)
> + return err;
> +
> +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> + /* Valid split targets: 0 (order-0 base pages) or 2..HPAGE_PMD_ORDER-1.
> */
> + if (val != 0 && (val < 2 || val >= HPAGE_PMD_ORDER))
> + return -EINVAL;
> +#else
> + if (val != 0)
> + return -EINVAL;
> +#endif
> +
> + scheme->target_order = val;
> + return count;
> +}
> +
> static void damon_sysfs_scheme_release(struct kobject *kobj)
> {
> kfree(container_of(kobj, struct damon_sysfs_scheme, kobj));
> @@ -2659,10 +2701,14 @@ static struct kobj_attribute
> damon_sysfs_scheme_apply_interval_us_attr =
> static struct kobj_attribute damon_sysfs_scheme_target_nid_attr =
> __ATTR_RW_MODE(target_nid, 0600);
>
> +static struct kobj_attribute damon_sysfs_scheme_target_order_attr =
> + __ATTR_RW_MODE(target_order, 0600);
> +
> static struct attribute *damon_sysfs_scheme_attrs[] = {
> &damon_sysfs_scheme_action_attr.attr,
> &damon_sysfs_scheme_apply_interval_us_attr.attr,
> &damon_sysfs_scheme_target_nid_attr.attr,
> + &damon_sysfs_scheme_target_order_attr.attr,
> NULL,
> };
> ATTRIBUTE_GROUPS(damon_sysfs_scheme);
> @@ -3011,6 +3057,8 @@ static struct damos *damon_sysfs_mk_scheme(
> if (!scheme)
> return NULL;
>
> + scheme->order = sysfs_scheme->target_order;
> +
> err = damos_sysfs_add_quota_score(sysfs_quotas->goals, &scheme->quota);
> if (err) {
> damon_destroy_scheme(scheme);
> diff --git a/tools/testing/selftests/damon/sysfs.py
> b/tools/testing/selftests/damon/sysfs.py
> index 3ffa054b6386..49d3c319dfd8 100755
> --- a/tools/testing/selftests/damon/sysfs.py
> +++ b/tools/testing/selftests/damon/sysfs.py
> @@ -131,11 +131,12 @@ def assert_scheme_committed(scheme, dump):
> 'hugepage': 3,
> 'nohugepage': 4,
> 'collapse': 5,
> - 'lru_prio': 6,
> - 'lru_deprio': 7,
> - 'migrate_hot': 8,
> - 'migrate_cold': 9,
> - 'stat': 10,
> + 'split': 6,
> + 'lru_prio': 7,
> + 'lru_deprio': 8,
> + 'migrate_hot': 9,
> + 'migrate_cold': 10,
> + 'stat': 11,
> }
> assert_true(dump['action'] == action_val[scheme.action], 'action', dump)
> assert_true(dump['apply_interval_us'] == scheme. apply_interval_us,
>
--
Asier Gutierrez
Huawei
