mm/huge_memory: merge uniform_split_supported() and non_uniform_split_supported()

The functions uniform_split_supported() and non_uniform_split_supported() share significantly similar logic.

The only functional difference is that uniform_split_supported() includes an additional check on the requested @new_order.

The reason for this check comes from the following two aspects:

* some file system or swap cache just supports order-0 folio
* the behavioral difference between uniform/non-uniform split

The behavioral difference between uniform split and non-uniform:

* uniform split splits folio directly to @new_order
* non-uniform split creates after-split folios with orders from folio_order(folio) - 1 to new_order.

This means for non-uniform split or !new_order split we should check the file system and swap cache respectively.

This commit unifies the logic and merge the two functions into a single combined helper, removing redundant code and simplifying the split support checking mechanism.

Signed-off-by Wei Yang <[email protected]>

Cc Zi Yan <[email protected]>

Cc "David Hildenbrand (Red Hat)" <[email protected]>

---
v2:
* remove need_check
* update comment
* add more explanation in change log
* selftests/split_huge_page_test pass

---
  include/linux/huge_mm.h |  8 ++---
  mm/huge_memory.c        | 70 ++++++++++++++++++-----------------------
  2 files changed, 33 insertions(+), 45 deletions(-)

include/linux/huge_mm.h +3 -5

index cbb2243f8e56..79343809a7be 100644
@@ -369,10 +369,8 @@ int __split_huge_page_to_list_to_order(struct page *page, struct list_head *list
  		unsigned int new_order, bool unmapped);
  int min_order_for_split(struct folio *folio);
  int split_folio_to_list(struct folio *folio, struct list_head *list);
-bool uniform_split_supported(struct folio *folio, unsigned int new_order,
-		bool warns);
-bool non_uniform_split_supported(struct folio *folio, unsigned int new_order,
-		bool warns);
+bool folio_split_supported(struct folio *folio, unsigned int new_order,
+		bool uniform_split, bool warns);
  int folio_split(struct folio *folio, unsigned int new_order, struct page *page,
  		struct list_head *list);
 

@@ -403,7 +401,7 @@ static inline int split_huge_page_to_order(struct page *page, unsigned int new_o
  static inline int try_folio_split_to_order(struct folio *folio,
  		struct page *page, unsigned int new_order)
  {
-	if (!non_uniform_split_supported(folio, new_order, /* warns= */ false))
+	if (!folio_split_supported(folio, new_order, /* uniform_split = */ false, /* warns= */ false))
  		return split_huge_page_to_order(&folio->page, new_order);
  	return folio_split(folio, new_order, page, NULL);
  }

mm/huge_memory.c +20 -34

index 381a49c5ac3f..db442e0e3a46 100644
@@ -3666,55 +3666,49 @@ static int __split_unmapped_folio(struct folio *folio, int new_order,
  	return 0;
  }
 
-bool non_uniform_split_supported(struct folio *folio, unsigned int new_order,
-		bool warns)
+bool folio_split_supported(struct folio *folio, unsigned int new_order,
+		bool uniform_split, bool warns)
  {
  	if (folio_test_anon(folio)) {
  		/* order-1 is not supported for anonymous THP. */
  		VM_WARN_ONCE(warns && new_order == 1,
  				"Cannot split to order-1 folio");
  		return new_order != 1;
-	} else if (IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS) &&
-	    !mapping_large_folio_support(folio->mapping)) {
-		/*
-		 * No split if the file system does not support large folio.
-		 * Note that we might still have THPs in such mappings due to
-		 * CONFIG_READ_ONLY_THP_FOR_FS. But in that case, the mapping
-		 * does not actually support large folios properly.
-		 */
-		VM_WARN_ONCE(warns,
-			"Cannot split file folio to non-0 order");
-		return false;
-	}
-
-	/* Only swapping a whole PMD-mapped folio is supported */
-	if (folio_test_swapcache(folio)) {
-		VM_WARN_ONCE(warns,
-			"Cannot split swapcache folio to non-0 order");
-		return false;
-	}
-
-	return true;
-}
-
-/* See comments in non_uniform_split_supported() */
-bool uniform_split_supported(struct folio *folio, unsigned int new_order,
-		bool warns)
-{
-	if (folio_test_anon(folio)) {
-		VM_WARN_ONCE(warns && new_order == 1,
-				"Cannot split to order-1 folio");
-		return new_order != 1;
-	} else  if (new_order) {
+	} else if (!uniform_split || new_order) {
  		if (IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS) &&
  		    !mapping_large_folio_support(folio->mapping)) {
+			/*
+			 * We can always split a folio down to a single page
+			 * (new_order == 0) uniformly.
+			 *
+			 * For any other scenario
+			 *   a) uniform split targeting a large folio
+			 *      (new_order > 0)
+			 *   b) any non-uniform split
+			 * we must confirm that the file system supports large
+			 * folios.
+			 *
+			 * Note that we might still have THPs in such
+			 * mappings, which is created from khugepaged when
+			 * CONFIG_READ_ONLY_THP_FOR_FS is enabled. But in that
+			 * case, the mapping does not actually support large
+			 * folios properly.
+			 */
  			VM_WARN_ONCE(warns,
  				"Cannot split file folio to non-0 order");
  			return false;
  		}
  	}
 

Quoted 138 lines · Wei Yang

The functions uniform_split_supported() and non_uniform_split_supported() share significantly similar logic.

The only functional difference is that uniform_split_supported() includes an additional check on the requested @new_order.

The reason for this check comes from the following two aspects:

* some file system or swap cache just supports order-0 folio
* the behavioral difference between uniform/non-uniform split

The behavioral difference between uniform split and non-uniform:

* uniform split splits folio directly to @new_order
* non-uniform split creates after-split folios with orders from folio_order(folio) - 1 to new_order.

This means for non-uniform split or !new_order split we should check the file system and swap cache respectively.

This commit unifies the logic and merge the two functions into a single combined helper, removing redundant code and simplifying the split support checking mechanism.

Signed-off-by Wei Yang <[email protected]>

Cc Zi Yan <[email protected]>

Cc "David Hildenbrand (Red Hat)" <[email protected]>

---
v2:
* remove need_check
* update comment
* add more explanation in change log
* selftests/split_huge_page_test pass

---
  include/linux/huge_mm.h |  8 ++---
  mm/huge_memory.c        | 70 ++++++++++++++++++-----------------------
  2 files changed, 33 insertions(+), 45 deletions(-)

include/linux/huge_mm.h +3 -5

index cbb2243f8e56..79343809a7be 100644
@@ -369,10 +369,8 @@ int __split_huge_page_to_list_to_order(struct page *page, struct list_head *list
  		unsigned int new_order, bool unmapped);
  int min_order_for_split(struct folio *folio);
  int split_folio_to_list(struct folio *folio, struct list_head *list);
-bool uniform_split_supported(struct folio *folio, unsigned int new_order,
-		bool warns);
-bool non_uniform_split_supported(struct folio *folio, unsigned int new_order,
-		bool warns);
+bool folio_split_supported(struct folio *folio, unsigned int new_order,
+		bool uniform_split, bool warns);
  int folio_split(struct folio *folio, unsigned int new_order, struct page *page,
  		struct list_head *list);

@@ -403,7 +401,7 @@ static inline int split_huge_page_to_order(struct page *page, unsigned int new_o
  static inline int try_folio_split_to_order(struct folio *folio,
  		struct page *page, unsigned int new_order)
  {
-	if (!non_uniform_split_supported(folio, new_order, /* warns= */ false))
+	if (!folio_split_supported(folio, new_order, /* uniform_split = */ false, /* warns= */ false))
  		return split_huge_page_to_order(&folio->page, new_order);
  	return folio_split(folio, new_order, page, NULL);
  }

mm/huge_memory.c +20 -34

index 381a49c5ac3f..db442e0e3a46 100644
@@ -3666,55 +3666,49 @@ static int __split_unmapped_folio(struct folio *folio, int new_order,
  	return 0;
  }
-bool non_uniform_split_supported(struct folio *folio, unsigned int new_order,
-		bool warns)
+bool folio_split_supported(struct folio *folio, unsigned int new_order,
+		bool uniform_split, bool warns)
  {
  	if (folio_test_anon(folio)) {
  		/* order-1 is not supported for anonymous THP. */
  		VM_WARN_ONCE(warns && new_order == 1,
  				"Cannot split to order-1 folio");
  		return new_order != 1;
-	} else if (IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS) &&
-	    !mapping_large_folio_support(folio->mapping)) {
-		/*
-		 * No split if the file system does not support large folio.
-		 * Note that we might still have THPs in such mappings due to
-		 * CONFIG_READ_ONLY_THP_FOR_FS. But in that case, the mapping
-		 * does not actually support large folios properly.
-		 */
-		VM_WARN_ONCE(warns,
-			"Cannot split file folio to non-0 order");
-		return false;
-	}
-
-	/* Only swapping a whole PMD-mapped folio is supported */
-	if (folio_test_swapcache(folio)) {
-		VM_WARN_ONCE(warns,
-			"Cannot split swapcache folio to non-0 order");
-		return false;
-	}
-
-	return true;
-}
-
-/* See comments in non_uniform_split_supported() */
-bool uniform_split_supported(struct folio *folio, unsigned int new_order,
-		bool warns)
-{
-	if (folio_test_anon(folio)) {
-		VM_WARN_ONCE(warns && new_order == 1,
-				"Cannot split to order-1 folio");
-		return new_order != 1;
-	} else  if (new_order) {
+	} else if (!uniform_split || new_order) {
  		if (IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS) &&
  		    !mapping_large_folio_support(folio->mapping)) {
+			/*
+			 * We can always split a folio down to a single page
+			 * (new_order == 0) uniformly.
+			 *
+			 * For any other scenario
+			 *   a) uniform split targeting a large folio
+			 *      (new_order > 0)
+			 *   b) any non-uniform split
+			 * we must confirm that the file system supports large
+			 * folios.
+			 *
+			 * Note that we might still have THPs in such
+			 * mappings, which is created from khugepaged when
+			 * CONFIG_READ_ONLY_THP_FOR_FS is enabled. But in that
+			 * case, the mapping does not actually support large
+			 * folios properly.
+			 */
  			VM_WARN_ONCE(warns,
  				"Cannot split file folio to non-0 order");
  			return false;
  		}
  	}

mm/huge_memory.c static int __split_unmapped_folio(struct folio *folio, int new_order, · line 3709

Quoted 10 lines

mm/huge_memory.c +9 -1

-	if (new_order && folio_test_swapcache(folio)) {
+	/*
+	 * swapcache folio could only be split to order 0
+	 *
+	 * non-uniform split creates after-split folios with orders from
+	 * folio_order(folio) - 1 to new_order, making it not suitable for any
+	 * swapcache folio split. Only uniform split to order-0 can be used
+	 * here.
+	 */
+	if ((!uniform_split || new_order) && folio_test_swapcache(folio)) {

Staring at the existing code, how would we reach the folio_test_swapcache() test for anon folios?

At the beginning of the function we have

if (folio_test_anon()) {
	...
	return new_order != 1;
}

Aren't we missing a check for anon folios that are in the swapcache?

Signature · 3 lines

Cheers

David

Quoted 138 lines · Wei Yang

The functions uniform_split_supported() and non_uniform_split_supported() share significantly similar logic.

The only functional difference is that uniform_split_supported() includes an additional check on the requested @new_order.

The reason for this check comes from the following two aspects:

* some file system or swap cache just supports order-0 folio
* the behavioral difference between uniform/non-uniform split

The behavioral difference between uniform split and non-uniform:

* uniform split splits folio directly to @new_order
* non-uniform split creates after-split folios with orders from folio_order(folio) - 1 to new_order.

This means for non-uniform split or !new_order split we should check the file system and swap cache respectively.

This commit unifies the logic and merge the two functions into a single combined helper, removing redundant code and simplifying the split support checking mechanism.

Signed-off-by Wei Yang <[email protected]>

Cc Zi Yan <[email protected]>

Cc "David Hildenbrand (Red Hat)" <[email protected]>

---
v2:
* remove need_check
* update comment
* add more explanation in change log
* selftests/split_huge_page_test pass

---
  include/linux/huge_mm.h |  8 ++---
  mm/huge_memory.c        | 70 ++++++++++++++++++-----------------------
  2 files changed, 33 insertions(+), 45 deletions(-)

include/linux/huge_mm.h +2 -4

index cbb2243f8e56..79343809a7be 100644
@@ -369,10 +369,8 @@ int __split_huge_page_to_list_to_order(struct page *page, struct list_head *list
  		unsigned int new_order, bool unmapped);
  int min_order_for_split(struct folio *folio);
  int split_folio_to_list(struct folio *folio, struct list_head *list);
-bool uniform_split_supported(struct folio *folio, unsigned int new_order,
-		bool warns);
-bool non_uniform_split_supported(struct folio *folio, unsigned int new_order,
-		bool warns);
+bool folio_split_supported(struct folio *folio, unsigned int new_order,
+		bool uniform_split, bool warns);
  int folio_split(struct folio *folio, unsigned int new_order, struct page *page,
  		struct list_head *list);
 @@ -403,7 +401,7 @@ static inline int split_huge_page_to_order(struct page *page, unsigned int new_o

static inline int try_folio_split_to_order(struct folio *folio,
struct page *page, unsigned int new_order)
{

include/linux/huge_mm.h +1 -1

-	if (!non_uniform_split_supported(folio, new_order, /* warns= */ false))
+	if (!folio_split_supported(folio, new_order, /* uniform_split = */ false, /* warns= */ false))
  		return split_huge_page_to_order(&folio->page, new_order);
  	return folio_split(folio, new_order, page, NULL);
  }

mm/huge_memory.c +20 -33

index 381a49c5ac3f..db442e0e3a46 100644
@@ -3666,55 +3666,49 @@ static int __split_unmapped_folio(struct folio *folio, int new_order,
  	return 0;
  }
 -bool non_uniform_split_supported(struct folio *folio, unsigned int new_order,
-		bool warns)
+bool folio_split_supported(struct folio *folio, unsigned int new_order,
+		bool uniform_split, bool warns)
  {
  	if (folio_test_anon(folio)) {
  		/* order-1 is not supported for anonymous THP. */
  		VM_WARN_ONCE(warns && new_order == 1,
  				"Cannot split to order-1 folio");
  		return new_order != 1;
-	} else if (IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS) &&
-	    !mapping_large_folio_support(folio->mapping)) {
-		/*
-		 * No split if the file system does not support large folio.
-		 * Note that we might still have THPs in such mappings due to
-		 * CONFIG_READ_ONLY_THP_FOR_FS. But in that case, the mapping
-		 * does not actually support large folios properly.
-		 */
-		VM_WARN_ONCE(warns,
-			"Cannot split file folio to non-0 order");
-		return false;
-	}
-
-	/* Only swapping a whole PMD-mapped folio is supported */
-	if (folio_test_swapcache(folio)) {
-		VM_WARN_ONCE(warns,
-			"Cannot split swapcache folio to non-0 order");
-		return false;
-	}
-
-	return true;
-}
-
-/* See comments in non_uniform_split_supported() */
-bool uniform_split_supported(struct folio *folio, unsigned int new_order,
-		bool warns)
-{
-	if (folio_test_anon(folio)) {
-		VM_WARN_ONCE(warns && new_order == 1,
-				"Cannot split to order-1 folio");
-		return new_order != 1;
-	} else  if (new_order) {
+	} else if (!uniform_split || new_order) {
  		if (IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS) &&
  		    !mapping_large_folio_support(folio->mapping)) {
+			/*
+			 * We can always split a folio down to a single page
+			 * (new_order == 0) uniformly.
+			 *
+			 * For any other scenario
+			 *   a) uniform split targeting a large folio
+			 *      (new_order > 0)
+			 *   b) any non-uniform split
+			 * we must confirm that the file system supports large
+			 * folios.
+			 *
+			 * Note that we might still have THPs in such
+			 * mappings, which is created from khugepaged when
+			 * CONFIG_READ_ONLY_THP_FOR_FS is enabled. But in that
+			 * case, the mapping does not actually support large
+			 * folios properly.
+			 */
  			VM_WARN_ONCE(warns,
  				"Cannot split file folio to non-0 order");
  			return false;
  		}
  	}

mm/huge_memory.c static int __split_unmapped_folio(struct folio *folio, int new_order, · line 3711

Quoted 10 lines

mm/huge_memory.c +9 -0

 -	if (new_order && folio_test_swapcache(folio)) {
+	/*
+	 * swapcache folio could only be split to order 0
+	 *
+	 * non-uniform split creates after-split folios with orders from
+	 * folio_order(folio) - 1 to new_order, making it not suitable for any
+	 * swapcache folio split. Only uniform split to order-0 can be used
+	 * here.
+	 */
+	if ((!uniform_split || new_order) && folio_test_swapcache(folio)) {

Staring at the existing code, how would we reach the folio_test_swapcache() test for anon folios?

At the beginning of the function we have

if (folio_test_anon()) {
	...
	return new_order != 1;
}

Aren't we missing a check for anon folios that are in the swapcache?

The functions uniform_split_supported() and non_uniform_split_supported() share significantly similar logic.

The only functional difference is that uniform_split_supported() includes an additional check on the requested @new_order.

The reason for this check comes from the following two aspects:

* some file system or swap cache just supports order-0 folio
* the behavioral difference between uniform/non-uniform split

The behavioral difference between uniform split and non-uniform:

* uniform split splits folio directly to @new_order
* non-uniform split creates after-split folios with orders from folio_order(folio) - 1 to new_order.

This means for non-uniform split or !new_order split we should check the file system and swap cache respectively.

This commit unifies the logic and merge the two functions into a single combined helper, removing redundant code and simplifying the split support checking mechanism.

Signed-off-by Wei Yang <[email protected]>

Cc Zi Yan <[email protected]>

Cc "David Hildenbrand (Red Hat)" <[email protected]>

---
v2:
* remove need_check
* update comment
* add more explanation in change log
* selftests/split_huge_page_test pass

---
 include/linux/huge_mm.h |  8 ++---
 mm/huge_memory.c        | 70 ++++++++++++++++++-----------------------
 2 files changed, 33 insertions(+), 45 deletions(-)

include/linux/huge_mm.h +3 -5

index cbb2243f8e56..79343809a7be 100644
@@ -369,10 +369,8 @@ int __split_huge_page_to_list_to_order(struct page *page, struct list_head *list
 		unsigned int new_order, bool unmapped);
 int min_order_for_split(struct folio *folio);
 int split_folio_to_list(struct folio *folio, struct list_head *list);
-bool uniform_split_supported(struct folio *folio, unsigned int new_order,
-		bool warns);
-bool non_uniform_split_supported(struct folio *folio, unsigned int new_order,
-		bool warns);
+bool folio_split_supported(struct folio *folio, unsigned int new_order,
+		bool uniform_split, bool warns);
 int folio_split(struct folio *folio, unsigned int new_order, struct page *page,
 		struct list_head *list);
 

@@ -403,7 +401,7 @@ static inline int split_huge_page_to_order(struct page *page, unsigned int new_o
 static inline int try_folio_split_to_order(struct folio *folio,
 		struct page *page, unsigned int new_order)
 {
-	if (!non_uniform_split_supported(folio, new_order, /* warns= */ false))
+	if (!folio_split_supported(folio, new_order, /* uniform_split = */ false, /* warns= */ false))
 		return split_huge_page_to_order(&folio->page, new_order);
 	return folio_split(folio, new_order, page, NULL);
 }

mm/huge_memory.c +0 -0

index 381a49c5ac3f..db442e0e3a46 100644

Quoted 70 lines · Wei Yang

The functions uniform_split_supported() and non_uniform_split_supported() share significantly similar logic.

The only functional difference is that uniform_split_supported() includes an additional check on the requested @new_order.

The reason for this check comes from the following two aspects:

* some file system or swap cache just supports order-0 folio
* the behavioral difference between uniform/non-uniform split

The behavioral difference between uniform split and non-uniform:

* uniform split splits folio directly to @new_order
* non-uniform split creates after-split folios with orders from folio_order(folio) - 1 to new_order.

This means for non-uniform split or !new_order split we should check the file system and swap cache respectively.

This commit unifies the logic and merge the two functions into a single combined helper, removing redundant code and simplifying the split support checking mechanism.

Signed-off-by Wei Yang <[email protected]>

Cc Zi Yan <[email protected]>

Cc "David Hildenbrand (Red Hat)" <[email protected]>

---
v2:
* remove need_check
* update comment
* add more explanation in change log
* selftests/split_huge_page_test pass

---
 include/linux/huge_mm.h |  8 ++---
 mm/huge_memory.c        | 70 ++++++++++++++++++-----------------------
 2 files changed, 33 insertions(+), 45 deletions(-)

include/linux/huge_mm.h +3 -5

index cbb2243f8e56..79343809a7be 100644
@@ -369,10 +369,8 @@ int __split_huge_page_to_list_to_order(struct page *page, struct list_head *list
 		unsigned int new_order, bool unmapped);
 int min_order_for_split(struct folio *folio);
 int split_folio_to_list(struct folio *folio, struct list_head *list);
-bool uniform_split_supported(struct folio *folio, unsigned int new_order,
-		bool warns);
-bool non_uniform_split_supported(struct folio *folio, unsigned int new_order,
-		bool warns);
+bool folio_split_supported(struct folio *folio, unsigned int new_order,
+		bool uniform_split, bool warns);
 int folio_split(struct folio *folio, unsigned int new_order, struct page *page,
 		struct list_head *list);
 

@@ -403,7 +401,7 @@ static inline int split_huge_page_to_order(struct page *page, unsigned int new_o
 static inline int try_folio_split_to_order(struct folio *folio,
 		struct page *page, unsigned int new_order)
 {
-	if (!non_uniform_split_supported(folio, new_order, /* warns= */ false))
+	if (!folio_split_supported(folio, new_order, /* uniform_split = */ false, /* warns= */ false))
 		return split_huge_page_to_order(&folio->page, new_order);
 	return folio_split(folio, new_order, page, NULL);
 }

mm/huge_memory.c +0 -0

index 381a49c5ac3f..db442e0e3a46 100644

mm/huge_memory.c static int __split_unmapped_folio(struct folio *folio, int new_order, · line 3670

Quoted 7 lines

mm/huge_memory.c +2 -2

@@ -3666,55 +3666,49 @@ static int __split_unmapped_folio(struct folio *folio, int new_order,
 	return 0;
 }
 
-bool non_uniform_split_supported(struct folio *folio, unsigned int new_order,
-		bool warns)
+bool folio_split_supported(struct folio *folio, unsigned int new_order,
+		bool uniform_split, bool warns)

For this one, David suggested to use a enum

enum split_type {
SPLIT_TYPE_UNIFORM,
SPLIT_TYPE_NON_UNIFORM,
};

in a separate cleanup patch. It is better to send it along with this one, so that:

1. it will be easy for reviewers to keep track of both changes,
2. it also helps resolve the dependency issue, where the cleanup patch goes after this one.

Thanks.

Best Regards,
Yan, Zi