[PATCH] mm/huge_memory.c: introduce folio_split_unmapped

Balbir Singh
2025-11-14T01:22:43Z

Unmapped was added as a parameter to __folio_split() and related
call sites to support splitting of folios already in the midst
of a migration. This special case arose for device private folio
migration since during migration there could be a disconnect between
source and destination on the folio size.

Introduce folio_split_unmapped() to handle this special case. Also
refactor code and add __folio_freeze_and_split_unmapped() helper that
is common to both __folio_split() and folio_split_unmapped().

This in turn removes the special casing introduced by the unmapped
parameter in __folio_split().

Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: David Hildenbrand <david@redhat.com>
Cc: Zi Yan <ziy@nvidia.com>
Cc: Joshua Hahn <joshua.hahnjy@gmail.com>
Cc: Rakie Kim <rakie.kim@sk.com>
Cc: Byungchul Park <byungchul@sk.com>
Cc: Gregory Price <gourry@gourry.net>
Cc: Ying Huang <ying.huang@linux.alibaba.com>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: "Liam R. Howlett" <Liam.Howlett@oracle.com>
Cc: Nico Pache <npache@redhat.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Dev Jain <dev.jain@arm.com>
Cc: Barry Song <baohua@kernel.org>
Cc: Lyude Paul <lyude@redhat.com>
Cc: Danilo Krummrich <dakr@kernel.org>
Cc: David Airlie <airlied@gmail.com>
Cc: Simona Vetter <simona@ffwll.ch>
Cc: Ralph Campbell <rcampbell@nvidia.com>
Cc: Mika Penttilä <mpenttil@redhat.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Cc: Francois Dugast <francois.dugast@intel.com>

Suggested-by: Zi Yan <ziy@nvidia.com>
Signed-off-by: Balbir Singh <balbirs@nvidia.com>
---
include/linux/huge_mm.h | 5 +-
mm/huge_memory.c | 332 ++++++++++++++++++++++------------------
mm/migrate_device.c | 3 +-
3 files changed, 191 insertions(+), 149 deletions(-)

diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
index e2e91aa1a042..1d439de1ca2c 100644
--- a/include/linux/huge_mm.h
+++ b/include/linux/huge_mm.h
@@ -371,7 +371,8 @@ enum split_type {

bool can_split_folio(struct folio *folio, int caller_pins, int *pextra_pins);
int __split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
- unsigned int new_order, bool unmapped);
+ unsigned int new_order);
+int folio_split_unmapped(struct folio *folio, unsigned int new_order);
int min_order_for_split(struct folio *folio);
int split_folio_to_list(struct folio *folio, struct list_head *list);
bool folio_split_supported(struct folio *folio, unsigned int new_order,
@@ -382,7 +383,7 @@ int folio_split(struct folio *folio, unsigned int new_order, struct page *page,
static inline int split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
unsigned int new_order)
{
- return __split_huge_page_to_list_to_order(page, list, new_order, false);
+ return __split_huge_page_to_list_to_order(page, list, new_order);
}
static inline int split_huge_page_to_order(struct page *page, unsigned int new_order)
{
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index e4dadcb9e90b..afc10079724c 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -3739,6 +3739,151 @@ bool folio_split_supported(struct folio *folio, unsigned int new_order,
return true;
}

+static int __folio_freeze_and_split_unmapped(struct folio *folio, unsigned int new_order,
+ struct page *split_at, struct xa_state *xas,
+ struct address_space *mapping, bool do_lru,
+ struct list_head *list, enum split_type split_type,
+ pgoff_t end, int extra_pins)
+{
+ struct folio *end_folio = folio_next(folio);
+ struct folio *new_folio, *next;
+ int old_order = folio_order(folio);
+ int nr_shmem_dropped = 0;
+ int ret = 0;
+ struct deferred_split *ds_queue;
+
+ /* Prevent deferred_split_scan() touching ->_refcount */
+ ds_queue = folio_split_queue_lock(folio);
+ if (folio_ref_freeze(folio, 1 + extra_pins)) {
+ struct swap_cluster_info *ci = NULL;
+ struct lruvec *lruvec;
+ int expected_refs;
+
+ if (old_order > 1) {
+ if (!list_empty(&folio->_deferred_list)) {
+ ds_queue->split_queue_len--;
+ /*
+ * Reinitialize page_deferred_list after removing the
+ * page from the split_queue, otherwise a subsequent
+ * split will see list corruption when checking the
+ * page_deferred_list.
+ */
+ list_del_init(&folio->_deferred_list);
+ }
+ if (folio_test_partially_mapped(folio)) {
+ folio_clear_partially_mapped(folio);
+ mod_mthp_stat(old_order,
+ MTHP_STAT_NR_ANON_PARTIALLY_MAPPED, -1);
+ }
+ }
+ split_queue_unlock(ds_queue);
+ if (mapping) {
+ int nr = folio_nr_pages(folio);
+
+ if (folio_test_pmd_mappable(folio) &&
+ new_order < HPAGE_PMD_ORDER) {
+ if (folio_test_swapbacked(folio)) {
+ __lruvec_stat_mod_folio(folio,
+ NR_SHMEM_THPS, -nr);
+ } else {
+ __lruvec_stat_mod_folio(folio,
+ NR_FILE_THPS, -nr);
+ filemap_nr_thps_dec(mapping);
+ }
+ }
+ }
+
+ if (folio_test_swapcache(folio)) {
+ if (mapping) {
+ VM_WARN_ON_ONCE_FOLIO(mapping, folio);
+ return -EINVAL;
+ }
+
+ ci = swap_cluster_get_and_lock(folio);
+ }
+
+ /* lock lru list/PageCompound, ref frozen by page_ref_freeze */
+ if (do_lru)
+ lruvec = folio_lruvec_lock(folio);
+
+ ret = __split_unmapped_folio(folio, new_order, split_at, xas,
+ mapping, split_type);
+
+ /*
+ * Unfreeze after-split folios and put them back to the right
+ * list. @folio should be kept frozon until page cache
+ * entries are updated with all the other after-split folios
+ * to prevent others seeing stale page cache entries.
+ * As a result, new_folio starts from the next folio of
+ * @folio.
+ */
+ for (new_folio = folio_next(folio); new_folio != end_folio;
+ new_folio = next) {
+ unsigned long nr_pages = folio_nr_pages(new_folio);
+
+ next = folio_next(new_folio);
+
+ zone_device_private_split_cb(folio, new_folio);
+
+ expected_refs = folio_expected_ref_count(new_folio) + 1;
+ folio_ref_unfreeze(new_folio, expected_refs);
+
+ if (do_lru)
+ lru_add_split_folio(folio, new_folio, lruvec, list);
+
+ /*
+ * Anonymous folio with swap cache.
+ * NOTE: shmem in swap cache is not supported yet.
+ */
+ if (ci) {
+ __swap_cache_replace_folio(ci, folio, new_folio);
+ continue;
+ }
+
+ /* Anonymous folio without swap cache */
+ if (!mapping)
+ continue;
+
+ /* Add the new folio to the page cache. */
+ if (new_folio->index < end) {
+ __xa_store(&mapping->i_pages, new_folio->index,
+ new_folio, 0);
+ continue;
+ }
+
+ /* Drop folio beyond EOF: ->index >= end */
+ if (shmem_mapping(mapping))
+ nr_shmem_dropped += nr_pages;
+ else if (folio_test_clear_dirty(new_folio))
+ folio_account_cleaned(
+ new_folio, inode_to_wb(mapping->host));
+ __filemap_remove_folio(new_folio, NULL);
+ folio_put_refs(new_folio, nr_pages);
+ }
+
+ zone_device_private_split_cb(folio, NULL);
+ /*
+ * Unfreeze @folio only after all page cache entries, which
+ * used to point to it, have been updated with new folios.
+ * Otherwise, a parallel folio_try_get() can grab @folio
+ * and its caller can see stale page cache entries.
+ */
+ expected_refs = folio_expected_ref_count(folio) + 1;
+ folio_ref_unfreeze(folio, expected_refs);
+
+ if (do_lru)
+ unlock_page_lruvec(lruvec);
+
+ if (ci)
+ swap_cluster_unlock(ci);
+ } else {
+ split_queue_unlock(ds_queue);
+ return -EAGAIN;
+ }
+
+ return ret;
+}
+
/**
* __folio_split() - split a folio at @split_at to a @new_order folio
* @folio: folio to split
@@ -3747,7 +3892,6 @@ bool folio_split_supported(struct folio *folio, unsigned int new_order,
* @lock_at: a page within @folio to be left locked to caller
* @list: after-split folios will be put on it if non NULL
* @split_type: perform uniform split or not (non-uniform split)
- * @unmapped: The pages are already unmapped, they are migration entries.
*
* It calls __split_unmapped_folio() to perform uniform and non-uniform split.
* It is in charge of checking whether the split is supported or not and
@@ -3763,9 +3907,8 @@ bool folio_split_supported(struct folio *folio, unsigned int new_order,
*/
static int __folio_split(struct folio *folio, unsigned int new_order,
struct page *split_at, struct page *lock_at,
- struct list_head *list, enum split_type split_type, bool unmapped)
+ struct list_head *list, enum split_type split_type)
{
- struct deferred_split *ds_queue;
XA_STATE(xas, &folio->mapping->i_pages, folio->index);
struct folio *end_folio = folio_next(folio);
bool is_anon = folio_test_anon(folio);
@@ -3809,14 +3952,12 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
* is taken to serialise against parallel split or collapse
* operations.
*/
- if (!unmapped) {
- anon_vma = folio_get_anon_vma(folio);
- if (!anon_vma) {
- ret = -EBUSY;
- goto out;
- }
- anon_vma_lock_write(anon_vma);
+ anon_vma = folio_get_anon_vma(folio);
+ if (!anon_vma) {
+ ret = -EBUSY;
+ goto out;
}
+ anon_vma_lock_write(anon_vma);
mapping = NULL;
} else {
unsigned int min_order;
@@ -3882,8 +4023,7 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
goto out_unlock;
}

- if (!unmapped)
- unmap_folio(folio);
+ unmap_folio(folio);

/* block interrupt reentry in xa_lock and spinlock */
local_irq_disable();
@@ -3900,142 +4040,14 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
}
}

- /* Prevent deferred_split_scan() touching ->_refcount */
- ds_queue = folio_split_queue_lock(folio);
- if (folio_ref_freeze(folio, 1 + extra_pins)) {
- struct swap_cluster_info *ci = NULL;
- struct lruvec *lruvec;
- int expected_refs;
-
- if (old_order > 1) {
- if (!list_empty(&folio->_deferred_list)) {
- ds_queue->split_queue_len--;
- /*
- * Reinitialize page_deferred_list after removing the
- * page from the split_queue, otherwise a subsequent
- * split will see list corruption when checking the
- * page_deferred_list.
- */
- list_del_init(&folio->_deferred_list);
- }
- if (folio_test_partially_mapped(folio)) {
- folio_clear_partially_mapped(folio);
- mod_mthp_stat(old_order,
- MTHP_STAT_NR_ANON_PARTIALLY_MAPPED, -1);
- }
- }
- split_queue_unlock(ds_queue);
- if (mapping) {
- int nr = folio_nr_pages(folio);
-
- if (folio_test_pmd_mappable(folio) &&
- new_order < HPAGE_PMD_ORDER) {
- if (folio_test_swapbacked(folio)) {
- __lruvec_stat_mod_folio(folio,
- NR_SHMEM_THPS, -nr);
- } else {
- __lruvec_stat_mod_folio(folio,
- NR_FILE_THPS, -nr);
- filemap_nr_thps_dec(mapping);
- }
- }
- }
-
- if (folio_test_swapcache(folio)) {
- if (mapping) {
- VM_WARN_ON_ONCE_FOLIO(mapping, folio);
- ret = -EINVAL;
- goto fail;
- }
-
- ci = swap_cluster_get_and_lock(folio);
- }
-
- /* lock lru list/PageCompound, ref frozen by page_ref_freeze */
- lruvec = folio_lruvec_lock(folio);
-
- ret = __split_unmapped_folio(folio, new_order, split_at, &xas,
- mapping, split_type);
-
- /*
- * Unfreeze after-split folios and put them back to the right
- * list. @folio should be kept frozon until page cache
- * entries are updated with all the other after-split folios
- * to prevent others seeing stale page cache entries.
- * As a result, new_folio starts from the next folio of
- * @folio.
- */
- for (new_folio = folio_next(folio); new_folio != end_folio;
- new_folio = next) {
- unsigned long nr_pages = folio_nr_pages(new_folio);
-
- next = folio_next(new_folio);
-
- zone_device_private_split_cb(folio, new_folio);
-
- expected_refs = folio_expected_ref_count(new_folio) + 1;
- folio_ref_unfreeze(new_folio, expected_refs);
-
- if (!unmapped)
- lru_add_split_folio(folio, new_folio, lruvec, list);
-
- /*
- * Anonymous folio with swap cache.
- * NOTE: shmem in swap cache is not supported yet.
- */
- if (ci) {
- __swap_cache_replace_folio(ci, folio, new_folio);
- continue;
- }
-
- /* Anonymous folio without swap cache */
- if (!mapping)
- continue;
-
- /* Add the new folio to the page cache. */
- if (new_folio->index < end) {
- __xa_store(&mapping->i_pages, new_folio->index,
- new_folio, 0);
- continue;
- }
-
- /* Drop folio beyond EOF: ->index >= end */
- if (shmem_mapping(mapping))
- nr_shmem_dropped += nr_pages;
- else if (folio_test_clear_dirty(new_folio))
- folio_account_cleaned(
- new_folio, inode_to_wb(mapping->host));
- __filemap_remove_folio(new_folio, NULL);
- folio_put_refs(new_folio, nr_pages);
- }
-
- zone_device_private_split_cb(folio, NULL);
- /*
- * Unfreeze @folio only after all page cache entries, which
- * used to point to it, have been updated with new folios.
- * Otherwise, a parallel folio_try_get() can grab @folio
- * and its caller can see stale page cache entries.
- */
- expected_refs = folio_expected_ref_count(folio) + 1;
- folio_ref_unfreeze(folio, expected_refs);
-
- unlock_page_lruvec(lruvec);
-
- if (ci)
- swap_cluster_unlock(ci);
- } else {
- split_queue_unlock(ds_queue);
- ret = -EAGAIN;
- }
+ ret = __folio_freeze_and_split_unmapped(folio, new_order, split_at, &xas, mapping,
+ true, list, split_type, end, extra_pins);
fail:
if (mapping)
xas_unlock(&xas);

local_irq_enable();

- if (unmapped)
- return ret;
-
if (nr_shmem_dropped)
shmem_uncharge(mapping->host, nr_shmem_dropped);

@@ -4079,6 +4091,36 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
return ret;
}

+/*
+ * This function is a helper for splitting folios that have already been unmapped.
+ * The use case is that the device or the CPU can refuse to migrate THP pages in
+ * the middle of migration, due to allocation issues on either side
+ *
+ * The high level code is copied from __folio_split, since the pages are anonymous
+ * and are already isolated from the LRU, the code has been simplified to not
+ * burden __folio_split with unmapped sprinkled into the code.
+ *
+ * None of the split folios are unlocked
+ */
+int folio_split_unmapped(struct folio *folio, unsigned int new_order)
+{
+ int extra_pins, ret = 0;
+
+ VM_WARN_ON_FOLIO(folio_mapped(folio), folio);
+ VM_WARN_ON_ONCE_FOLIO(!folio_test_locked(folio), folio);
+ VM_WARN_ON_ONCE_FOLIO(!folio_test_large(folio), folio);
+
+ if (!can_split_folio(folio, 1, &extra_pins))
+ return -EAGAIN;
+
+ local_irq_disable();
+ ret = __folio_freeze_and_split_unmapped(folio, new_order, &folio->page, NULL,
+ NULL, false, NULL, SPLIT_TYPE_UNIFORM,
+ 0, extra_pins);
+ local_irq_enable();
+ return ret;
+}
+
/*
* This function splits a large folio into smaller folios of order @new_order.
* @page can point to any page of the large folio to split. The split operation
@@ -4127,12 +4169,12 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
* with the folio. Splitting to order 0 is compatible with all folios.
*/
int __split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
- unsigned int new_order, bool unmapped)
+ unsigned int new_order)
{
struct folio *folio = page_folio(page);

return __folio_split(folio, new_order, &folio->page, page, list,
- SPLIT_TYPE_UNIFORM, unmapped);
+ SPLIT_TYPE_UNIFORM);
}

/**
@@ -4163,7 +4205,7 @@ int folio_split(struct folio *folio, unsigned int new_order,
struct page *split_at, struct list_head *list)
{
return __folio_split(folio, new_order, split_at, &folio->page, list,
- SPLIT_TYPE_NON_UNIFORM, false);
+ SPLIT_TYPE_NON_UNIFORM);
}

int min_order_for_split(struct folio *folio)
diff --git a/mm/migrate_device.c b/mm/migrate_device.c
index c50abbd32f21..723ef42550dc 100644
--- a/mm/migrate_device.c
+++ b/mm/migrate_device.c
@@ -918,8 +918,7 @@ static int migrate_vma_split_unmapped_folio(struct migrate_vma *migrate,

folio_get(folio);
split_huge_pmd_address(migrate->vma, addr, true);
- ret = __split_huge_page_to_list_to_order(folio_page(folio, 0), NULL,
- 0, true);
+ ret = folio_split_unmapped(folio, 0);
if (ret)
return ret;
migrate->src[idx] &= ~MIGRATE_PFN_COMPOUND;
--
2.51.1