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

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

[Balbir Singh]

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

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

[Balbir Singh]

On 11/21/25 00:42, Balbir Singh wrote:
> 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

This is missing an inline attribute, my compilation tests did not catch it, 
but I'll do a new version tomorrow

Balbir

Balbir

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

[kernel test robot]

Hi Balbir,

kernel test robot noticed the following build warnings:

[auto build test WARNING on akpm-mm/mm-everything]
[also build test WARNING on next-20251120]
[cannot apply to drm-misc/drm-misc-next drm-tip/drm-tip linus/master v6.18-rc6]
[If your patch is applied to the wrong git tree, kindly drop us a note.
And when submitting patch, we suggest to use '--base' as documented in
https://git-scm.com/docs/git-format-patch#_base_tree_information]

url:    https://github.com/intel-lab-lkp/linux/commits/Balbir-Singh/fixup-mm-huge_memory-c-introduce-folio_split_unmapped/20251120-214322
base:   https://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm.git mm-everything
patch link:    https://lore.kernel.org/r/20251120134232.3588203-1-balbirs%40nvidia.com
patch subject: [PATCH v2] fixup: mm/huge_memory.c: introduce folio_split_unmapped
config: m68k-allnoconfig (https://download.01.org/0day-ci/archive/20251120/202511202251.PCvSd2p2-lkp@intel.com/config)
compiler: m68k-linux-gcc (GCC) 15.1.0
reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20251120/202511202251.PCvSd2p2-lkp@intel.com/reproduce)

If you fix the issue in a separate patch/commit (i.e. not just a new version of
the same patch/commit), kindly add following tags
| Reported-by: kernel test robot <lkp@intel.com>
| Closes: https://lore.kernel.org/oe-kbuild-all/202511202251.PCvSd2p2-lkp@intel.com/

All warnings (new ones prefixed by >>):

   In file included from mm/filemap.c:39:
>> include/linux/shmem_fs.h:160:13: warning: 'shmem_uncharge' defined but not used [-Wunused-function]
     160 | static void shmem_uncharge(struct inode *inode, long pages)
         |             ^~~~~~~~~~~~~~
--
   mm/shmem.c: In function '__shmem_file_setup':
   mm/shmem.c:5838:23: warning: unused variable 'flags' [-Wunused-variable]
    5838 |         unsigned long flags = (vm_flags & VM_NORESERVE) ? SHMEM_F_NORESERVE : 0;
         |                       ^~~~~
   In file included from mm/shmem.c:36:
   include/linux/shmem_fs.h: At top level:
>> include/linux/shmem_fs.h:160:13: warning: 'shmem_uncharge' defined but not used [-Wunused-function]
     160 | static void shmem_uncharge(struct inode *inode, long pages)
         |             ^~~~~~~~~~~~~~


vim +/shmem_uncharge +160 include/linux/shmem_fs.h

   159	
 > 160	static void shmem_uncharge(struct inode *inode, long pages)
   161	{
   162	}
   163	#endif
   164	extern unsigned long shmem_partial_swap_usage(struct address_space *mapping,
   165							pgoff_t start, pgoff_t end);
   166	

-- 
0-DAY CI Kernel Test Service
https://github.com/intel/lkp-tests/wiki

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

[David Hildenbrand (Red Hat)]

On 11/20/25 14:42, Balbir Singh wrote:
> 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);

I forgot to add, we drop the "extern" on functions whenever we move them.

>   #else
>   static inline unsigned long shmem_swap_usage(struct vm_area_struct *vma)
>   {
>   	return 0;
>   }
> +
> +static void shmem_uncharge(struct inode *inode, long pages)

"static inline void" should resolve the compilation issue.



-- 
Cheers

David

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

[Wei Yang]

On Fri, Nov 21, 2025 at 12:42:32AM +1100, Balbir Singh wrote:
>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

Took a look into its caller:

  __migrate_device_pages()
    migrate_vma_split_unmapped_folio()
      folio_split_unmapped()

I don't see where get the folio lock.

Would you mind giving me a hint where we toke the lock? Seems I missed that.

>+ * 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
>

-- 
Wei Yang
Help you, Help me