Re: [PATCH v6 00/17] mm/huge_memory: clean up and decouple the anon and file split helpers

[Andrew Morton <akpm@linux-foundation.org>]

On Wed, 23 Sep 2026 23:32:31 +0200 Kairui Song via B4 Relay <devnull+kasong.tencent.com@kernel.org> wrote:

> The folio split path handles anon, page cache and swap cache folios in
> one routine.  That mixing is what makes the swap cache split
> restrictions hard to lift and to review. We now support uniform split to
> order-0 only, and no mappingless swap cache folios. And it has left a fair
> number of dead or redundant checks behind.
> 
> This series prepares for lifting those restrictions by cleaning up the
> code first: split the routine into an anon and a file helper, and keep
> all swap cache handling in the anon helper.  The file helper never sees
> a swap cache folio, folio_check_splittable() rejects them up front.
> 
> Apart from two bug fixes (patch 1 and 2) and a slight adjustment of anon
> splitting (patch 12), this is a pure cleanup.

Thanks, I've updated mm.git's mm-unstable branch to this version.

> Changes in v6:
> - Comments updated and make a few variables const as suggested by
>   [David Hildenbrand]
> - Link to v5: https://patch.msgid.link/20260915-swap-thp-cleanup-v5-0-39878b37dfb0@tencent.com

Here's how v6 altered mm.git:


 mm/huge_memory.c |   19 +++++++++----------
 1 file changed, 9 insertions(+), 10 deletions(-)

--- a/mm/huge_memory.c~b
+++ a/mm/huge_memory.c
@@ -3538,6 +3538,11 @@ void vma_adjust_trans_huge(struct vm_are
 		split_huge_pmd_if_needed(next, end);
 }
 
+/*
+ * A return value of 0 does not mean that unmapping succeeded. It might
+ * still have failed, but remap_anon_folio() must be called afterwards,
+ * for anon folios.
+ */
 static int unmap_folio(struct folio *folio)
 {
 	enum ttu_flags ttu_flags = TTU_RMAP_LOCKED | TTU_SYNC |
@@ -3555,7 +3560,6 @@ static int unmap_folio(struct folio *fol
 	/*
 	 * Anon pages need migration entries to preserve them, but file
 	 * pages can simply be left unmapped, then faulted back on demand.
-	 * If that is ever changed (perhaps for mlock), update remap_anon_folio().
 	 */
 	if (folio_test_anon(folio))
 		try_to_migrate(folio, ttu_flags);
@@ -4025,7 +4029,7 @@ static int __folio_freeze_split_anon(str
 {
 	struct folio *end_folio = folio_next(folio);
 	struct swap_cluster_info *ci = NULL;
-	int old_order = folio_order(folio);
+	const int old_order = folio_order(folio);
 	struct folio *new_folio, *next;
 	struct anon_vma *anon_vma = NULL;
 	enum ttu_flags ttu_flags = 0;
@@ -4048,8 +4052,7 @@ static int __folio_freeze_split_anon(str
 	 * the reference nor the lock. Anything else needs a reference
 	 * first and folio_ref_freeze() below catches it.
 	 *
-	 * Note a swapped-out THP counts as unmapped here as swap PTEs do
-	 * not contribute mapcount, and they are splittable.
+	 * Note that entirely swapped-out THPs are unmapped but can be split.
 	 */
 	if (folio_mapped(folio)) {
 		anon_vma = folio_get_anon_vma(folio);
@@ -4133,10 +4136,10 @@ static int __folio_freeze_split_file(str
 		unsigned int new_order, struct page *split_at,
 		struct list_head *list, enum split_type split_type)
 {
+	const long old_nr_pages = folio_nr_pages(folio);
 	struct address_space *mapping = folio->mapping;
 	XA_STATE(xas, &mapping->i_pages, folio->index);
 	struct folio *end_folio = folio_next(folio);
-	long old_nr_pages = folio_nr_pages(folio);
 	struct mem_cgroup *memcg, *old_memcg;
 	struct folio *new_folio, *next;
 	int nr_shmem_dropped = 0;
@@ -4166,7 +4169,7 @@ static int __folio_freeze_split_file(str
 	mapping_set_update(&xas, mapping);
 
 	if (split_type == SPLIT_TYPE_UNIFORM) {
-		int old_order = folio_order(folio);
+		const int old_order = folio_order(folio);
 
 		xas_set_order(&xas, folio->index, new_order);
 		xas_split_alloc(&xas, folio, old_order, gfp);
@@ -4269,10 +4272,6 @@ static int __folio_freeze_split_file(str
 	folio_ref_unfreeze(folio, folio_nr_pages(folio) + 1);
 	lruvec_unlock(lruvec);
 fail:
-	/*
-	 * If we want to use try_to_migrate() on file in unmap_folio,
-	 * remember to add remap_anon_folio() and adapt it.
-	 */
 	xas_unlock_irq(&xas);
 fail_mmap_unlock:
 	if (nr_shmem_dropped)
_