[PATCH v2] fixup: mm/huge_memory.c: introduce folio_split_unmapped

[Balbir Singh <balbirs@nvidia.com>]

Code refactoring of __folio_split() via helper
__folio_freeze_and_split_unmapped() caused a regression with clang-20
with CONFIG_SHMEM=n, the compiler was not able to optimize away the
call to shmem_uncharge() due to changes in nr_shmem_dropped.
Fix this by adding a stub function for shmem_uncharge when
CONFIG_SHMEM is not defined.

smatch also complained about parameter end being used without
initialization, which is a false positive, but keep the tool happy
by sending in initialized parameters. end is initialized to 0.
smatch still complains about mapping being NULL and nr_shmem_dropped
may not be 0, but that is not true prior to or after the changes.

Add detailed documentation comments for folio_split_unmapped()

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: David Hildenbrand <david@redhat.com>
Signed-off-by: Balbir Singh <balbirs@nvidia.com>
---
This fixup should be squashed into the patch "mm/huge_memory.c:
introduce folio_split_unmapped" in mm/mm-unstable

 include/linux/shmem_fs.h |  6 +++++-
 mm/huge_memory.c         | 30 +++++++++++++++++++++---------
 2 files changed, 26 insertions(+), 10 deletions(-)

diff --git a/include/linux/shmem_fs.h b/include/linux/shmem_fs.h
index 5b368f9549d6..7a412dd6eb4f 100644
--- a/include/linux/shmem_fs.h
+++ b/include/linux/shmem_fs.h
@@ -136,11 +136,16 @@ static inline bool shmem_hpage_pmd_enabled(void)
 
 #ifdef CONFIG_SHMEM
 extern unsigned long shmem_swap_usage(struct vm_area_struct *vma);
+extern void shmem_uncharge(struct inode *inode, long pages);
 #else
 static inline unsigned long shmem_swap_usage(struct vm_area_struct *vma)
 {
 	return 0;
 }
+
+static void shmem_uncharge(struct inode *inode, long pages)
+{
+}
 #endif
 extern unsigned long shmem_partial_swap_usage(struct address_space *mapping,
 						pgoff_t start, pgoff_t end);
@@ -194,7 +199,6 @@ static inline pgoff_t shmem_fallocend(struct inode *inode, pgoff_t eof)
 }
 
 extern bool shmem_charge(struct inode *inode, long pages);
-extern void shmem_uncharge(struct inode *inode, long pages);
 
 #ifdef CONFIG_USERFAULTFD
 #ifdef CONFIG_SHMEM
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 78a31a476ad3..18c12876f5e8 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -3751,6 +3751,7 @@ static int __folio_freeze_and_split_unmapped(struct folio *folio, unsigned int n
 	int ret = 0;
 	struct deferred_split *ds_queue;
 
+	VM_WARN_ON_ONCE(!mapping && end);
 	/* Prevent deferred_split_scan() touching ->_refcount */
 	ds_queue = folio_split_queue_lock(folio);
 	if (folio_ref_freeze(folio, 1 + extra_pins)) {
@@ -3919,7 +3920,7 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
 	int nr_shmem_dropped = 0;
 	int remap_flags = 0;
 	int extra_pins, ret;
-	pgoff_t end;
+	pgoff_t end = 0;
 	bool is_hzp;
 
 	VM_WARN_ON_ONCE_FOLIO(!folio_test_locked(folio), folio);
@@ -4092,16 +4093,27 @@ 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
+/**
+ * folio_split_unmapped() - split a large anon folio that is already unmapped
+ * @folio: folio to split
+ * @new_order: the order of folios after split
+ *
+ * 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.
+ *
+ * anon_vma_lock is not required to be held, mmap_read_lock() or
+ * mmap_write_lock() should be held. @folio is expected to be locked by the
+ * caller. device-private and non device-private folios are supported along
+ * with folios that are in the swapcache. @folio should also be unmapped and
+ * isolated from LRU (if applicable)
  *
- * 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.
+ * Upon return, the folio is not remapped, split folios are not added to LRU,
+ * free_folio_and_swap_cache() is not called, and new folios remain locked.
  *
- * None of the split folios are unlocked
+ * Return: 0 on success, -EAGAIN if the folio cannot be split (e.g., due to
+ *         insufficient reference count or extra pins).
  */
 int folio_split_unmapped(struct folio *folio, unsigned int new_order)
 {
-- 
2.51.1