Re: [PATCH] mm/huge_memory.c: introduce split_unmapped_folio_to_order
From: David Hildenbrand (Red Hat)
Date: Wed Nov 12 2025 - 05:00:09 EST
On 12.11.25 05:46, Balbir Singh wrote:
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 split_unmapped_folio_to_order() to handle this special case.
This in turn removes the special casing introduced by the unmapped
parameter in __folio_split().
As raised recently, I would hope that we can find a way to make all these splitting functions look more similar in the long term, ideally starting with "folio_split" / "folio_try_split".
What about
folio_split_unmapped()
Do we really have to spell out the "to order" part in the function name?
And if it's more a mostly-internal helper, maybe
__folio_split_unmapped()
subject: "mm/huge_memory: introduce ..."
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: David Hildenbrand <david@xxxxxxxxxx>
Cc: Zi Yan <ziy@xxxxxxxxxx>
Cc: Joshua Hahn <joshua.hahnjy@xxxxxxxxx>
Cc: Rakie Kim <rakie.kim@xxxxxx>
Cc: Byungchul Park <byungchul@xxxxxx>
Cc: Gregory Price <gourry@xxxxxxxxxx>
Cc: Ying Huang <ying.huang@xxxxxxxxxxxxxxxxx>
Cc: Alistair Popple <apopple@xxxxxxxxxx>
Cc: Oscar Salvador <osalvador@xxxxxxx>
Cc: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>
Cc: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
Cc: "Liam R. Howlett" <Liam.Howlett@xxxxxxxxxx>
Cc: Nico Pache <npache@xxxxxxxxxx>
Cc: Ryan Roberts <ryan.roberts@xxxxxxx>
Cc: Dev Jain <dev.jain@xxxxxxx>
Cc: Barry Song <baohua@xxxxxxxxxx>
Cc: Lyude Paul <lyude@xxxxxxxxxx>
Cc: Danilo Krummrich <dakr@xxxxxxxxxx>
Cc: David Airlie <airlied@xxxxxxxxx>
Cc: Simona Vetter <simona@xxxxxxxx>
Cc: Ralph Campbell <rcampbell@xxxxxxxxxx>
Cc: Mika Penttilä <mpenttil@xxxxxxxxxx>
Cc: Matthew Brost <matthew.brost@xxxxxxxxx>
Cc: Francois Dugast <francois.dugast@xxxxxxxxx>
Suggested-by: Zi Yan <ziy@xxxxxxxxxx>
Signed-off-by: Balbir Singh <balbirs@xxxxxxxxxx>
---
include/linux/huge_mm.h | 5 +-
mm/huge_memory.c | 135 ++++++++++++++++++++++++++++++++++------
mm/migrate_device.c | 3 +-
3 files changed, 120 insertions(+), 23 deletions(-)
diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
index e2e91aa1a042..9155e683c08a 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 split_unmapped_folio_to_order(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 0184cd915f44..942bd8410c54 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -3747,7 +3747,6 @@ 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
@@ -3763,7 +3762,7 @@ 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)
Yeah, nice to see that go.
{
struct deferred_split *ds_queue;
XA_STATE(xas, &folio->mapping->i_pages, folio->index);
@@ -3809,14 +3808,12 @@ 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 +3879,7 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
goto out_unlock;
}
- if (!unmapped)
- unmap_folio(folio);
+ unmap_folio(folio);
Hm, I would have hoped that we could factor out the core logic and reuse it for the new helper, instead of duplicating code.
Did you look into that?
--
Cheers
David