[PATCH v4 0/6] __folio_split() clean up

[PATCH v4 0/6] __folio_split() clean up

[Zi Yan]

Hi Andrew,

This series replaces both [PATCH v3 0/2] __folio_split() clean up
and [PATCH] mm/huge_memory: refactor after-split (page) cache code.

Hi Lorenzo,

I addressed all of your comments except renaming folio to origin_folio,
since I find that might either cause confusion or require a lot of code
churn. folio variable points to the original folio throughout
__folio_split() and using origin_folio in the middle of __folio_split()
is confusing as one might wonder if origin_folio is different from or
the same as folio. The alternative is to rename all folio to origin_folio
in __folio_split(). That seems to be unnecessary code churn.

Hi all,

This patchset refactors __folio_split() and __split_unmapped_folio() to:
1. make __split_unmapped_folio() reusable for splitting unmapped
   folios. It avoids the need for a new boolean unmapped parameter to guard
   mapping-related code when __split_unmapped_folio() is reused to split
   unmapped folios.
2. improve code readability and prevent smatch/coverity checkers from
   complaining about NULL mapping referencing.

An additional benefit for __split_unmapped_folio() refactoring is that
__split_unmapped_folio() could be called on after-split folios by
__folio_split(). It can enable new split methods. For example, at deferred
split time, unmapped subpages can scatter arbitrarily within a large folio,
neither uniform nor non-uniform split can maximize after-split folio orders
for mapped subpages. The hope is that by calling __split_unmapped_folio()
multiple times, a better split result can be achieved.

The patchset is based on mm-new with aforementioned two patchsets
reverted. It passes mm selftests.

Changelog
===
From V3[4]:
1. Split up Patch 1 into incremental changes:
    a. Patch 1 moves code out of __split_unmapped_folio();
    b. Patch 2 removes after_split label in __split_unmapped_folio();
    c. Patch 3 refactors __folio_split() to deduplicate code;
    d. Patch 4 converts VM_BUGs to VM_WARMs;
2. Added "mm/huge_memory: refactor after-split (page) cache code"
   patch[5] to this series.
3. Added remap_flags to make remap_page() call easier to read.
4. Updated Patch 1 commit log to include variable rename information.
5. Converted additional VM_BUGs in __folio_split().
6. Renamed next_folio to end_folio to avoid confusion.
7. Added a comment about start for loop with folio_next(folio) instead
   of just folio plus skipping folio in the loop body.
8. Dropped swapcache folio split check code from __split_unmapped_folio(),
   since the check is already done at the beginning of __folio_split().

From V2[3]:
1. Code format fixes
2. Restructured code to remove after_split goto label.

From V1[2]:
1. Fixed indentations.
2. Used folio_expected_ref_count() to calculate ref_count instead of
   open coding.

[1] https://lore.kernel.org/linux-mm/94D8C1A4-780C-4BEC-A336-7D3613B54845@nvidia.com/
[2] https://lore.kernel.org/linux-mm/20250711030259.3574392-1-ziy@nvidia.com/
[3] https://lore.kernel.org/linux-mm/20250711182355.3592618-1-ziy@nvidia.com/
[4] https://lore.kernel.org/linux-mm/20250714171823.3626213-1-ziy@nvidia.com/
[5] https://lore.kernel.org/linux-mm/20250716171112.3666150-1-ziy@nvidia.com/

Zi Yan (6):
  mm/huge_memory: move unrelated code out of __split_unmapped_folio()
  mm/huge_memory: remove after_split label in __split_unmapped_folio().
  mm/huge_memory: deduplicate code in __folio_split().
  mm/huge_memory: convert VM_BUG* to VM_WARN* in __folio_split.
  mm/huge_memory: get frozen folio refcount with
    folio_expected_ref_count()
  mm/huge_memory: refactor after-split (page) cache code.

 mm/huge_memory.c | 317 ++++++++++++++++++++++++-----------------------
 1 file changed, 165 insertions(+), 152 deletions(-)

-- 
2.47.2

[PATCH v4 1/6] mm/huge_memory: move unrelated code out of __split_unmapped_folio()

[Zi Yan]

remap(), folio_ref_unfreeze(), lru_add_split_folio() are not relevant to
splitting unmapped folio operations. Move them out to __folio_split() so
that __split_unmapped_folio() only handles unmapped folio splits. This
makes __split_unmapped_folio() reusable.

Remove the swapcache folio split check code before __split_unmapped_folio()
call, since it is already checked at the beginning of __folio_split() in
uniform_split_supported() and non_uniform_split_supported().

Along with the code move, there are some variable renames:

1. release is renamed to new_folio,
2. origin_folio is now folio, since __folio_split() has folio pointing to
   the original folio already.

Signed-off-by: Zi Yan <ziy@nvidia.com>
---
 mm/huge_memory.c | 270 +++++++++++++++++++++++------------------------
 1 file changed, 133 insertions(+), 137 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index ce130225a8e5..63eebca07628 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -3385,10 +3385,6 @@ static void __split_folio_to_order(struct folio *folio, int old_order,
  *             order - 1 to new_order).
  * @split_at: in buddy allocator like split, the folio containing @split_at
  *            will be split until its order becomes @new_order.
- * @lock_at: the folio containing @lock_at is left locked for caller.
- * @list: the after split folios will be added to @list if it is not NULL,
- *        otherwise to LRU lists.
- * @end: the end of the file @folio maps to. -1 if @folio is anonymous memory.
  * @xas: xa_state pointing to folio->mapping->i_pages and locked by caller
  * @mapping: @folio->mapping
  * @uniform_split: if the split is uniform or not (buddy allocator like split)
@@ -3414,52 +3410,26 @@ static void __split_folio_to_order(struct folio *folio, int old_order,
  *    @page, which is split in next for loop.
  *
  * After splitting, the caller's folio reference will be transferred to the
- * folio containing @page. The other folios may be freed if they are not mapped.
- *
- * In terms of locking, after splitting,
- * 1. uniform split leaves @page (or the folio contains it) locked;
- * 2. buddy allocator like (non-uniform) split leaves @folio locked.
- *
+ * folio containing @page. The caller needs to unlock and/or free after-split
+ * folios if necessary.
  *
  * For !uniform_split, when -ENOMEM is returned, the original folio might be
  * split. The caller needs to check the input folio.
  */
 static int __split_unmapped_folio(struct folio *folio, int new_order,
-		struct page *split_at, struct page *lock_at,
-		struct list_head *list, pgoff_t end,
-		struct xa_state *xas, struct address_space *mapping,
-		bool uniform_split)
+		struct page *split_at, struct xa_state *xas,
+		struct address_space *mapping, bool uniform_split)
 {
-	struct lruvec *lruvec;
-	struct address_space *swap_cache = NULL;
-	struct folio *origin_folio = folio;
-	struct folio *next_folio = folio_next(folio);
-	struct folio *new_folio;
-	struct folio *next;
 	int order = folio_order(folio);
-	int split_order;
 	int start_order = uniform_split ? new_order : order - 1;
-	int nr_dropped = 0;
-	int ret = 0;
 	bool stop_split = false;
-
-	if (folio_test_swapcache(folio)) {
-		VM_BUG_ON(mapping);
-
-		/* a swapcache folio can only be uniformly split to order-0 */
-		if (!uniform_split || new_order != 0)
-			return -EINVAL;
-
-		swap_cache = swap_address_space(folio->swap);
-		xa_lock(&swap_cache->i_pages);
-	}
+	struct folio *next;
+	int split_order;
+	int ret = 0;
 
 	if (folio_test_anon(folio))
 		mod_mthp_stat(order, MTHP_STAT_NR_ANON, -1);
 
-	/* lock lru list/PageCompound, ref frozen by page_ref_freeze */
-	lruvec = folio_lruvec_lock(folio);
-
 	folio_clear_has_hwpoisoned(folio);
 
 	/*
@@ -3469,9 +3439,9 @@ static int __split_unmapped_folio(struct folio *folio, int new_order,
 	for (split_order = start_order;
 	     split_order >= new_order && !stop_split;
 	     split_order--) {
-		int old_order = folio_order(folio);
-		struct folio *release;
 		struct folio *end_folio = folio_next(folio);
+		int old_order = folio_order(folio);
+		struct folio *new_folio;
 
 		/* order-1 anonymous folio is not supported */
 		if (folio_test_anon(folio) && split_order == 1)
@@ -3506,113 +3476,32 @@ static int __split_unmapped_folio(struct folio *folio, int new_order,
 
 after_split:
 		/*
-		 * Iterate through after-split folios and perform related
-		 * operations. But in buddy allocator like split, the folio
+		 * Iterate through after-split folios and update folio stats.
+		 * But in buddy allocator like split, the folio
 		 * containing the specified page is skipped until its order
 		 * is new_order, since the folio will be worked on in next
 		 * iteration.
 		 */
-		for (release = folio; release != end_folio; release = next) {
-			next = folio_next(release);
+		for (new_folio = folio; new_folio != end_folio; new_folio = next) {
+			next = folio_next(new_folio);
 			/*
-			 * for buddy allocator like split, the folio containing
-			 * page will be split next and should not be released,
-			 * until the folio's order is new_order or stop_split
-			 * is set to true by the above xas_split() failure.
+			 * for buddy allocator like split, new_folio containing
+			 * @split_at page could be split again, thus do not
+			 * change stats yet. Wait until new_folio's order is
+			 * @new_order or stop_split is set to true by the above
+			 * xas_split() failure.
 			 */
-			if (release == page_folio(split_at)) {
-				folio = release;
+			if (new_folio == page_folio(split_at)) {
+				folio = new_folio;
 				if (split_order != new_order && !stop_split)
 					continue;
 			}
-			if (folio_test_anon(release)) {
-				mod_mthp_stat(folio_order(release),
-						MTHP_STAT_NR_ANON, 1);
-			}
-
-			/*
-			 * origin_folio should be kept frozon until page cache
-			 * entries are updated with all the other after-split
-			 * folios to prevent others seeing stale page cache
-			 * entries.
-			 */
-			if (release == origin_folio)
-				continue;
-
-			folio_ref_unfreeze(release, 1 +
-					((mapping || swap_cache) ?
-						folio_nr_pages(release) : 0));
-
-			lru_add_split_folio(origin_folio, release, lruvec,
-					list);
-
-			/* Some pages can be beyond EOF: drop them from cache */
-			if (release->index >= end) {
-				if (shmem_mapping(mapping))
-					nr_dropped += folio_nr_pages(release);
-				else if (folio_test_clear_dirty(release))
-					folio_account_cleaned(release,
-						inode_to_wb(mapping->host));
-				__filemap_remove_folio(release, NULL);
-				folio_put_refs(release, folio_nr_pages(release));
-			} else if (mapping) {
-				__xa_store(&mapping->i_pages,
-						release->index, release, 0);
-			} else if (swap_cache) {
-				__xa_store(&swap_cache->i_pages,
-						swap_cache_index(release->swap),
-						release, 0);
-			}
+			if (folio_test_anon(new_folio))
+				mod_mthp_stat(folio_order(new_folio),
+					      MTHP_STAT_NR_ANON, 1);
 		}
 	}
 
-	/*
-	 * Unfreeze origin_folio only after all page cache entries, which used
-	 * to point to it, have been updated with new folios. Otherwise,
-	 * a parallel folio_try_get() can grab origin_folio and its caller can
-	 * see stale page cache entries.
-	 */
-	folio_ref_unfreeze(origin_folio, 1 +
-		((mapping || swap_cache) ? folio_nr_pages(origin_folio) : 0));
-
-	unlock_page_lruvec(lruvec);
-
-	if (swap_cache)
-		xa_unlock(&swap_cache->i_pages);
-	if (mapping)
-		xa_unlock(&mapping->i_pages);
-
-	/* Caller disabled irqs, so they are still disabled here */
-	local_irq_enable();
-
-	if (nr_dropped)
-		shmem_uncharge(mapping->host, nr_dropped);
-
-	remap_page(origin_folio, 1 << order,
-			folio_test_anon(origin_folio) ?
-				RMP_USE_SHARED_ZEROPAGE : 0);
-
-	/*
-	 * At this point, folio should contain the specified page.
-	 * For uniform split, it is left for caller to unlock.
-	 * For buddy allocator like split, the first after-split folio is left
-	 * for caller to unlock.
-	 */
-	for (new_folio = origin_folio; new_folio != next_folio; new_folio = next) {
-		next = folio_next(new_folio);
-		if (new_folio == page_folio(lock_at))
-			continue;
-
-		folio_unlock(new_folio);
-		/*
-		 * Subpages may be freed if there wasn't any mapping
-		 * like if add_to_swap() is running on a lru page that
-		 * had its mapping zapped. And freeing these pages
-		 * requires taking the lru_lock so we do the put_page
-		 * of the tail pages after the split is complete.
-		 */
-		free_folio_and_swap_cache(new_folio);
-	}
 	return ret;
 }
 
@@ -3686,6 +3575,11 @@ bool uniform_split_supported(struct folio *folio, unsigned int new_order,
  * It is in charge of checking whether the split is supported or not and
  * preparing @folio for __split_unmapped_folio().
  *
+ * After splitting, the after-split folio containing @lock_at remains locked
+ * and others are unlocked:
+ * 1. for uniform split, @lock_at points to one of @folio's subpages;
+ * 2. for buddy allocator like (non-uniform) split, @lock_at points to @folio.
+ *
  * return: 0: successful, <0 failed (if -ENOMEM is returned, @folio might be
  * split but not to @new_order, the caller needs to check)
  */
@@ -3695,10 +3589,12 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
 {
 	struct deferred_split *ds_queue = get_deferred_split_queue(folio);
 	XA_STATE(xas, &folio->mapping->i_pages, folio->index);
+	struct folio *end_folio = folio_next(folio);
 	bool is_anon = folio_test_anon(folio);
 	struct address_space *mapping = NULL;
 	struct anon_vma *anon_vma = NULL;
 	int order = folio_order(folio);
+	struct folio *new_folio, *next;
 	int extra_pins, ret;
 	pgoff_t end;
 	bool is_hzp;
@@ -3829,6 +3725,10 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
 	/* Prevent deferred_split_scan() touching ->_refcount */
 	spin_lock(&ds_queue->split_queue_lock);
 	if (folio_ref_freeze(folio, 1 + extra_pins)) {
+		struct address_space *swap_cache = NULL;
+		int nr_dropped = 0;
+		struct lruvec *lruvec;
+
 		if (folio_order(folio) > 1 &&
 		    !list_empty(&folio->_deferred_list)) {
 			ds_queue->split_queue_len--;
@@ -3862,9 +3762,105 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
 			}
 		}
 
-		ret = __split_unmapped_folio(folio, new_order,
-				split_at, lock_at, list, end, &xas, mapping,
-				uniform_split);
+		if (folio_test_swapcache(folio)) {
+			VM_BUG_ON(mapping);
+
+			swap_cache = swap_address_space(folio->swap);
+			xa_lock(&swap_cache->i_pages);
+		}
+
+		/* lock lru list/PageCompound, ref frozen by page_ref_freeze */
+		lruvec = folio_lruvec_lock(folio);
+
+		ret = __split_unmapped_folio(folio, new_order, split_at, &xas,
+					     mapping, uniform_split);
+
+		/*
+		 * Unfreeze after-split folios and put them back to the right
+		 * list. @folio should be kept frozon until page cache
+		 * entries are updated with all the other after-split folios
+		 * to prevent others seeing stale page cache entries.
+		 * As a result, new_folio starts from the next folio of
+		 * @folio.
+		 */
+		for (new_folio = folio_next(folio); new_folio != end_folio;
+		     new_folio = next) {
+			next = folio_next(new_folio);
+
+			folio_ref_unfreeze(
+				new_folio,
+				1 + ((mapping || swap_cache) ?
+					     folio_nr_pages(new_folio) :
+					     0));
+
+			lru_add_split_folio(folio, new_folio, lruvec, list);
+
+			/* Some pages can be beyond EOF: drop them from cache */
+			if (new_folio->index >= end) {
+				if (shmem_mapping(mapping))
+					nr_dropped += folio_nr_pages(new_folio);
+				else if (folio_test_clear_dirty(new_folio))
+					folio_account_cleaned(
+						new_folio,
+						inode_to_wb(mapping->host));
+				__filemap_remove_folio(new_folio, NULL);
+				folio_put_refs(new_folio,
+					       folio_nr_pages(new_folio));
+			} else if (mapping) {
+				__xa_store(&mapping->i_pages, new_folio->index,
+					   new_folio, 0);
+			} else if (swap_cache) {
+				__xa_store(&swap_cache->i_pages,
+					   swap_cache_index(new_folio->swap),
+					   new_folio, 0);
+			}
+		}
+		/*
+		 * Unfreeze @folio only after all page cache entries, which
+		 * used to point to it, have been updated with new folios.
+		 * Otherwise, a parallel folio_try_get() can grab @folio
+		 * and its caller can see stale page cache entries.
+		 */
+		folio_ref_unfreeze(folio, 1 +
+			((mapping || swap_cache) ? folio_nr_pages(folio) : 0));
+
+		unlock_page_lruvec(lruvec);
+
+		if (swap_cache)
+			xa_unlock(&swap_cache->i_pages);
+		if (mapping)
+			xas_unlock(&xas);
+
+		local_irq_enable();
+
+		if (nr_dropped)
+			shmem_uncharge(mapping->host, nr_dropped);
+
+		remap_page(folio, 1 << order,
+			   !ret && folio_test_anon(folio) ?
+				   RMP_USE_SHARED_ZEROPAGE :
+				   0);
+
+		/*
+		 * Unlock all after-split folios except the one containing
+		 * @lock_at page. If @folio is not split, it will be kept locked.
+		 */
+		for (new_folio = folio; new_folio != end_folio;
+		     new_folio = next) {
+			next = folio_next(new_folio);
+			if (new_folio == page_folio(lock_at))
+				continue;
+
+			folio_unlock(new_folio);
+			/*
+			 * Subpages may be freed if there wasn't any mapping
+			 * like if add_to_swap() is running on a lru page that
+			 * had its mapping zapped. And freeing these pages
+			 * requires taking the lru_lock so we do the put_page
+			 * of the tail pages after the split is complete.
+			 */
+			free_folio_and_swap_cache(new_folio);
+		}
 	} else {
 		spin_unlock(&ds_queue->split_queue_lock);
 fail:
-- 
2.47.2

Re: [PATCH v4 1/6] mm/huge_memory: move unrelated code out of __split_unmapped_folio()

[David Hildenbrand]

On 18.07.25 04:29, Zi Yan wrote:
> remap(), folio_ref_unfreeze(), lru_add_split_folio() are not relevant to
> splitting unmapped folio operations. Move them out to __folio_split() so
> that __split_unmapped_folio() only handles unmapped folio splits. This
> makes __split_unmapped_folio() reusable.
> 
> Remove the swapcache folio split check code before __split_unmapped_folio()
> call, since it is already checked at the beginning of __folio_split() in
> uniform_split_supported() and non_uniform_split_supported().
> 
> Along with the code move, there are some variable renames:
> 
> 1. release is renamed to new_folio,
> 2. origin_folio is now folio, since __folio_split() has folio pointing to
>     the original folio already.
> 
> Signed-off-by: Zi Yan <ziy@nvidia.com>
> ---

Acked-by: David Hildenbrand <david@redhat.com>

-- 
Cheers,

David / dhildenb

Re: [PATCH v4 1/6] mm/huge_memory: move unrelated code out of __split_unmapped_folio()

[Lorenzo Stoakes]

On Thu, Jul 17, 2025 at 10:29:55PM -0400, Zi Yan wrote:
> remap(), folio_ref_unfreeze(), lru_add_split_folio() are not relevant to
> splitting unmapped folio operations. Move them out to __folio_split() so
> that __split_unmapped_folio() only handles unmapped folio splits. This
> makes __split_unmapped_folio() reusable.
>
> Remove the swapcache folio split check code before __split_unmapped_folio()
> call, since it is already checked at the beginning of __folio_split() in
> uniform_split_supported() and non_uniform_split_supported().
>
> Along with the code move, there are some variable renames:
>
> 1. release is renamed to new_folio,
> 2. origin_folio is now folio, since __folio_split() has folio pointing to
>    the original folio already.
>
> Signed-off-by: Zi Yan <ziy@nvidia.com>

OK, have had a careful look through and LGTM. Thanks very much for address
prior review! So:

Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>

> ---
>  mm/huge_memory.c | 270 +++++++++++++++++++++++------------------------
>  1 file changed, 133 insertions(+), 137 deletions(-)
>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index ce130225a8e5..63eebca07628 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -3385,10 +3385,6 @@ static void __split_folio_to_order(struct folio *folio, int old_order,
>   *             order - 1 to new_order).
>   * @split_at: in buddy allocator like split, the folio containing @split_at
>   *            will be split until its order becomes @new_order.
> - * @lock_at: the folio containing @lock_at is left locked for caller.
> - * @list: the after split folios will be added to @list if it is not NULL,
> - *        otherwise to LRU lists.
> - * @end: the end of the file @folio maps to. -1 if @folio is anonymous memory.
>   * @xas: xa_state pointing to folio->mapping->i_pages and locked by caller
>   * @mapping: @folio->mapping
>   * @uniform_split: if the split is uniform or not (buddy allocator like split)
> @@ -3414,52 +3410,26 @@ static void __split_folio_to_order(struct folio *folio, int old_order,
>   *    @page, which is split in next for loop.
>   *
>   * After splitting, the caller's folio reference will be transferred to the
> - * folio containing @page. The other folios may be freed if they are not mapped.
> - *
> - * In terms of locking, after splitting,
> - * 1. uniform split leaves @page (or the folio contains it) locked;
> - * 2. buddy allocator like (non-uniform) split leaves @folio locked.
> - *
> + * folio containing @page. The caller needs to unlock and/or free after-split
> + * folios if necessary.
>   *
>   * For !uniform_split, when -ENOMEM is returned, the original folio might be
>   * split. The caller needs to check the input folio.
>   */
>  static int __split_unmapped_folio(struct folio *folio, int new_order,
> -		struct page *split_at, struct page *lock_at,
> -		struct list_head *list, pgoff_t end,
> -		struct xa_state *xas, struct address_space *mapping,
> -		bool uniform_split)
> +		struct page *split_at, struct xa_state *xas,
> +		struct address_space *mapping, bool uniform_split)
>  {
> -	struct lruvec *lruvec;
> -	struct address_space *swap_cache = NULL;
> -	struct folio *origin_folio = folio;
> -	struct folio *next_folio = folio_next(folio);
> -	struct folio *new_folio;
> -	struct folio *next;
>  	int order = folio_order(folio);
> -	int split_order;
>  	int start_order = uniform_split ? new_order : order - 1;
> -	int nr_dropped = 0;
> -	int ret = 0;
>  	bool stop_split = false;
> -
> -	if (folio_test_swapcache(folio)) {
> -		VM_BUG_ON(mapping);
> -
> -		/* a swapcache folio can only be uniformly split to order-0 */
> -		if (!uniform_split || new_order != 0)
> -			return -EINVAL;
> -
> -		swap_cache = swap_address_space(folio->swap);
> -		xa_lock(&swap_cache->i_pages);
> -	}
> +	struct folio *next;
> +	int split_order;
> +	int ret = 0;
>
>  	if (folio_test_anon(folio))
>  		mod_mthp_stat(order, MTHP_STAT_NR_ANON, -1);
>
> -	/* lock lru list/PageCompound, ref frozen by page_ref_freeze */
> -	lruvec = folio_lruvec_lock(folio);
> -
>  	folio_clear_has_hwpoisoned(folio);
>
>  	/*
> @@ -3469,9 +3439,9 @@ static int __split_unmapped_folio(struct folio *folio, int new_order,
>  	for (split_order = start_order;
>  	     split_order >= new_order && !stop_split;
>  	     split_order--) {
> -		int old_order = folio_order(folio);
> -		struct folio *release;
>  		struct folio *end_folio = folio_next(folio);
> +		int old_order = folio_order(folio);
> +		struct folio *new_folio;
>
>  		/* order-1 anonymous folio is not supported */
>  		if (folio_test_anon(folio) && split_order == 1)
> @@ -3506,113 +3476,32 @@ static int __split_unmapped_folio(struct folio *folio, int new_order,
>
>  after_split:
>  		/*
> -		 * Iterate through after-split folios and perform related
> -		 * operations. But in buddy allocator like split, the folio
> +		 * Iterate through after-split folios and update folio stats.
> +		 * But in buddy allocator like split, the folio
>  		 * containing the specified page is skipped until its order
>  		 * is new_order, since the folio will be worked on in next
>  		 * iteration.
>  		 */
> -		for (release = folio; release != end_folio; release = next) {
> -			next = folio_next(release);
> +		for (new_folio = folio; new_folio != end_folio; new_folio = next) {
> +			next = folio_next(new_folio);
>  			/*
> -			 * for buddy allocator like split, the folio containing
> -			 * page will be split next and should not be released,
> -			 * until the folio's order is new_order or stop_split
> -			 * is set to true by the above xas_split() failure.
> +			 * for buddy allocator like split, new_folio containing
> +			 * @split_at page could be split again, thus do not
> +			 * change stats yet. Wait until new_folio's order is
> +			 * @new_order or stop_split is set to true by the above
> +			 * xas_split() failure.
>  			 */
> -			if (release == page_folio(split_at)) {
> -				folio = release;
> +			if (new_folio == page_folio(split_at)) {
> +				folio = new_folio;
>  				if (split_order != new_order && !stop_split)
>  					continue;
>  			}
> -			if (folio_test_anon(release)) {
> -				mod_mthp_stat(folio_order(release),
> -						MTHP_STAT_NR_ANON, 1);
> -			}
> -
> -			/*
> -			 * origin_folio should be kept frozon until page cache
> -			 * entries are updated with all the other after-split
> -			 * folios to prevent others seeing stale page cache
> -			 * entries.
> -			 */
> -			if (release == origin_folio)
> -				continue;
> -
> -			folio_ref_unfreeze(release, 1 +
> -					((mapping || swap_cache) ?
> -						folio_nr_pages(release) : 0));
> -
> -			lru_add_split_folio(origin_folio, release, lruvec,
> -					list);
> -
> -			/* Some pages can be beyond EOF: drop them from cache */
> -			if (release->index >= end) {
> -				if (shmem_mapping(mapping))
> -					nr_dropped += folio_nr_pages(release);
> -				else if (folio_test_clear_dirty(release))
> -					folio_account_cleaned(release,
> -						inode_to_wb(mapping->host));
> -				__filemap_remove_folio(release, NULL);
> -				folio_put_refs(release, folio_nr_pages(release));
> -			} else if (mapping) {
> -				__xa_store(&mapping->i_pages,
> -						release->index, release, 0);
> -			} else if (swap_cache) {
> -				__xa_store(&swap_cache->i_pages,
> -						swap_cache_index(release->swap),
> -						release, 0);
> -			}
> +			if (folio_test_anon(new_folio))
> +				mod_mthp_stat(folio_order(new_folio),
> +					      MTHP_STAT_NR_ANON, 1);
>  		}
>  	}
>
> -	/*
> -	 * Unfreeze origin_folio only after all page cache entries, which used
> -	 * to point to it, have been updated with new folios. Otherwise,
> -	 * a parallel folio_try_get() can grab origin_folio and its caller can
> -	 * see stale page cache entries.
> -	 */
> -	folio_ref_unfreeze(origin_folio, 1 +
> -		((mapping || swap_cache) ? folio_nr_pages(origin_folio) : 0));
> -
> -	unlock_page_lruvec(lruvec);
> -
> -	if (swap_cache)
> -		xa_unlock(&swap_cache->i_pages);
> -	if (mapping)
> -		xa_unlock(&mapping->i_pages);
> -
> -	/* Caller disabled irqs, so they are still disabled here */
> -	local_irq_enable();
> -
> -	if (nr_dropped)
> -		shmem_uncharge(mapping->host, nr_dropped);
> -
> -	remap_page(origin_folio, 1 << order,
> -			folio_test_anon(origin_folio) ?
> -				RMP_USE_SHARED_ZEROPAGE : 0);
> -
> -	/*
> -	 * At this point, folio should contain the specified page.
> -	 * For uniform split, it is left for caller to unlock.
> -	 * For buddy allocator like split, the first after-split folio is left
> -	 * for caller to unlock.
> -	 */
> -	for (new_folio = origin_folio; new_folio != next_folio; new_folio = next) {
> -		next = folio_next(new_folio);
> -		if (new_folio == page_folio(lock_at))
> -			continue;
> -
> -		folio_unlock(new_folio);
> -		/*
> -		 * Subpages may be freed if there wasn't any mapping
> -		 * like if add_to_swap() is running on a lru page that
> -		 * had its mapping zapped. And freeing these pages
> -		 * requires taking the lru_lock so we do the put_page
> -		 * of the tail pages after the split is complete.
> -		 */
> -		free_folio_and_swap_cache(new_folio);
> -	}
>  	return ret;
>  }
>
> @@ -3686,6 +3575,11 @@ bool uniform_split_supported(struct folio *folio, unsigned int new_order,
>   * It is in charge of checking whether the split is supported or not and
>   * preparing @folio for __split_unmapped_folio().
>   *
> + * After splitting, the after-split folio containing @lock_at remains locked
> + * and others are unlocked:
> + * 1. for uniform split, @lock_at points to one of @folio's subpages;
> + * 2. for buddy allocator like (non-uniform) split, @lock_at points to @folio.
> + *
>   * return: 0: successful, <0 failed (if -ENOMEM is returned, @folio might be
>   * split but not to @new_order, the caller needs to check)
>   */
> @@ -3695,10 +3589,12 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
>  {
>  	struct deferred_split *ds_queue = get_deferred_split_queue(folio);
>  	XA_STATE(xas, &folio->mapping->i_pages, folio->index);
> +	struct folio *end_folio = folio_next(folio);
>  	bool is_anon = folio_test_anon(folio);
>  	struct address_space *mapping = NULL;
>  	struct anon_vma *anon_vma = NULL;
>  	int order = folio_order(folio);
> +	struct folio *new_folio, *next;
>  	int extra_pins, ret;
>  	pgoff_t end;
>  	bool is_hzp;
> @@ -3829,6 +3725,10 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
>  	/* Prevent deferred_split_scan() touching ->_refcount */
>  	spin_lock(&ds_queue->split_queue_lock);
>  	if (folio_ref_freeze(folio, 1 + extra_pins)) {
> +		struct address_space *swap_cache = NULL;
> +		int nr_dropped = 0;
> +		struct lruvec *lruvec;
> +
>  		if (folio_order(folio) > 1 &&
>  		    !list_empty(&folio->_deferred_list)) {
>  			ds_queue->split_queue_len--;
> @@ -3862,9 +3762,105 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
>  			}
>  		}
>
> -		ret = __split_unmapped_folio(folio, new_order,
> -				split_at, lock_at, list, end, &xas, mapping,
> -				uniform_split);
> +		if (folio_test_swapcache(folio)) {
> +			VM_BUG_ON(mapping);
> +
> +			swap_cache = swap_address_space(folio->swap);
> +			xa_lock(&swap_cache->i_pages);
> +		}
> +
> +		/* lock lru list/PageCompound, ref frozen by page_ref_freeze */
> +		lruvec = folio_lruvec_lock(folio);
> +
> +		ret = __split_unmapped_folio(folio, new_order, split_at, &xas,
> +					     mapping, uniform_split);
> +
> +		/*
> +		 * Unfreeze after-split folios and put them back to the right
> +		 * list. @folio should be kept frozon until page cache
> +		 * entries are updated with all the other after-split folios
> +		 * to prevent others seeing stale page cache entries.
> +		 * As a result, new_folio starts from the next folio of
> +		 * @folio.
> +		 */
> +		for (new_folio = folio_next(folio); new_folio != end_folio;
> +		     new_folio = next) {
> +			next = folio_next(new_folio);
> +
> +			folio_ref_unfreeze(
> +				new_folio,
> +				1 + ((mapping || swap_cache) ?
> +					     folio_nr_pages(new_folio) :
> +					     0));
> +
> +			lru_add_split_folio(folio, new_folio, lruvec, list);
> +
> +			/* Some pages can be beyond EOF: drop them from cache */
> +			if (new_folio->index >= end) {
> +				if (shmem_mapping(mapping))
> +					nr_dropped += folio_nr_pages(new_folio);
> +				else if (folio_test_clear_dirty(new_folio))
> +					folio_account_cleaned(
> +						new_folio,
> +						inode_to_wb(mapping->host));
> +				__filemap_remove_folio(new_folio, NULL);
> +				folio_put_refs(new_folio,
> +					       folio_nr_pages(new_folio));
> +			} else if (mapping) {
> +				__xa_store(&mapping->i_pages, new_folio->index,
> +					   new_folio, 0);
> +			} else if (swap_cache) {
> +				__xa_store(&swap_cache->i_pages,
> +					   swap_cache_index(new_folio->swap),
> +					   new_folio, 0);
> +			}
> +		}
> +		/*
> +		 * Unfreeze @folio only after all page cache entries, which
> +		 * used to point to it, have been updated with new folios.
> +		 * Otherwise, a parallel folio_try_get() can grab @folio
> +		 * and its caller can see stale page cache entries.
> +		 */
> +		folio_ref_unfreeze(folio, 1 +
> +			((mapping || swap_cache) ? folio_nr_pages(folio) : 0));
> +
> +		unlock_page_lruvec(lruvec);
> +
> +		if (swap_cache)
> +			xa_unlock(&swap_cache->i_pages);
> +		if (mapping)
> +			xas_unlock(&xas);
> +
> +		local_irq_enable();
> +
> +		if (nr_dropped)
> +			shmem_uncharge(mapping->host, nr_dropped);
> +
> +		remap_page(folio, 1 << order,
> +			   !ret && folio_test_anon(folio) ?
> +				   RMP_USE_SHARED_ZEROPAGE :
> +				   0);
> +
> +		/*
> +		 * Unlock all after-split folios except the one containing
> +		 * @lock_at page. If @folio is not split, it will be kept locked.
> +		 */
> +		for (new_folio = folio; new_folio != end_folio;
> +		     new_folio = next) {
> +			next = folio_next(new_folio);
> +			if (new_folio == page_folio(lock_at))
> +				continue;
> +
> +			folio_unlock(new_folio);
> +			/*
> +			 * Subpages may be freed if there wasn't any mapping
> +			 * like if add_to_swap() is running on a lru page that
> +			 * had its mapping zapped. And freeing these pages
> +			 * requires taking the lru_lock so we do the put_page
> +			 * of the tail pages after the split is complete.
> +			 */
> +			free_folio_and_swap_cache(new_folio);
> +		}
>  	} else {
>  		spin_unlock(&ds_queue->split_queue_lock);
>  fail:
> --
> 2.47.2
>

[PATCH v4 2/6] mm/huge_memory: remove after_split label in __split_unmapped_folio().

[Zi Yan]

Checking stop_split instead to avoid the goto statement.

Signed-off-by: Zi Yan <ziy@nvidia.com>
---
 mm/huge_memory.c | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 63eebca07628..e01359008b13 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -3463,18 +3463,18 @@ static int __split_unmapped_folio(struct folio *folio, int new_order,
 				if (xas_error(xas)) {
 					ret = xas_error(xas);
 					stop_split = true;
-					goto after_split;
 				}
 			}
 		}
 
-		folio_split_memcg_refs(folio, old_order, split_order);
-		split_page_owner(&folio->page, old_order, split_order);
-		pgalloc_tag_split(folio, old_order, split_order);
+		if (!stop_split) {
+			folio_split_memcg_refs(folio, old_order, split_order);
+			split_page_owner(&folio->page, old_order, split_order);
+			pgalloc_tag_split(folio, old_order, split_order);
 
-		__split_folio_to_order(folio, old_order, split_order);
+			__split_folio_to_order(folio, old_order, split_order);
+		}
 
-after_split:
 		/*
 		 * Iterate through after-split folios and update folio stats.
 		 * But in buddy allocator like split, the folio
-- 
2.47.2

Re: [PATCH v4 2/6] mm/huge_memory: remove after_split label in __split_unmapped_folio().

[David Hildenbrand]

On 18.07.25 04:29, Zi Yan wrote:
> Checking stop_split instead to avoid the goto statement.
> 
> Signed-off-by: Zi Yan <ziy@nvidia.com>
> ---

Acked-by: David Hildenbrand <david@redhat.com>

-- 
Cheers,

David / dhildenb

Re: [PATCH v4 2/6] mm/huge_memory: remove after_split label in __split_unmapped_folio().

[Lorenzo Stoakes]

On Thu, Jul 17, 2025 at 10:29:56PM -0400, Zi Yan wrote:
> Checking stop_split instead to avoid the goto statement.
>
> Signed-off-by: Zi Yan <ziy@nvidia.com

Thanks, nice + clear!

Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>

> ---
>  mm/huge_memory.c | 12 ++++++------
>  1 file changed, 6 insertions(+), 6 deletions(-)
>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 63eebca07628..e01359008b13 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -3463,18 +3463,18 @@ static int __split_unmapped_folio(struct folio *folio, int new_order,
>  				if (xas_error(xas)) {
>  					ret = xas_error(xas);
>  					stop_split = true;
> -					goto after_split;
>  				}
>  			}
>  		}
>
> -		folio_split_memcg_refs(folio, old_order, split_order);
> -		split_page_owner(&folio->page, old_order, split_order);
> -		pgalloc_tag_split(folio, old_order, split_order);
> +		if (!stop_split) {
> +			folio_split_memcg_refs(folio, old_order, split_order);
> +			split_page_owner(&folio->page, old_order, split_order);
> +			pgalloc_tag_split(folio, old_order, split_order);
>
> -		__split_folio_to_order(folio, old_order, split_order);
> +			__split_folio_to_order(folio, old_order, split_order);
> +		}
>
> -after_split:
>  		/*
>  		 * Iterate through after-split folios and update folio stats.
>  		 * But in buddy allocator like split, the folio
> --
> 2.47.2
>

[PATCH v4 3/6] mm/huge_memory: deduplicate code in __folio_split().

[Zi Yan]

xas unlock, remap_page(), local_irq_enable() are moved out of if branches
to deduplicate the code. While at it, add remap_flags to clean up
remap_page() call site. nr_dropped is renamed to nr_shmem_dropped, as it
becomes a variable at __folio_split() scope.

Signed-off-by: Zi Yan <ziy@nvidia.com>
---
 mm/huge_memory.c | 73 +++++++++++++++++++++++-------------------------
 1 file changed, 35 insertions(+), 38 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index e01359008b13..d36f7bdaeb38 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -3595,6 +3595,8 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
 	struct anon_vma *anon_vma = NULL;
 	int order = folio_order(folio);
 	struct folio *new_folio, *next;
+	int nr_shmem_dropped = 0;
+	int remap_flags = 0;
 	int extra_pins, ret;
 	pgoff_t end;
 	bool is_hzp;
@@ -3718,15 +3720,16 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
 		 */
 		xas_lock(&xas);
 		xas_reset(&xas);
-		if (xas_load(&xas) != folio)
+		if (xas_load(&xas) != folio) {
+			ret = -EAGAIN;
 			goto fail;
+		}
 	}
 
 	/* Prevent deferred_split_scan() touching ->_refcount */
 	spin_lock(&ds_queue->split_queue_lock);
 	if (folio_ref_freeze(folio, 1 + extra_pins)) {
 		struct address_space *swap_cache = NULL;
-		int nr_dropped = 0;
 		struct lruvec *lruvec;
 
 		if (folio_order(folio) > 1 &&
@@ -3798,7 +3801,7 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
 			/* Some pages can be beyond EOF: drop them from cache */
 			if (new_folio->index >= end) {
 				if (shmem_mapping(mapping))
-					nr_dropped += folio_nr_pages(new_folio);
+					nr_shmem_dropped += folio_nr_pages(new_folio);
 				else if (folio_test_clear_dirty(new_folio))
 					folio_account_cleaned(
 						new_folio,
@@ -3828,47 +3831,41 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
 
 		if (swap_cache)
 			xa_unlock(&swap_cache->i_pages);
-		if (mapping)
-			xas_unlock(&xas);
+	} else {
+		spin_unlock(&ds_queue->split_queue_lock);
+		ret = -EAGAIN;
+	}
+fail:
+	if (mapping)
+		xas_unlock(&xas);
+
+	local_irq_enable();
 
-		local_irq_enable();
+	if (nr_shmem_dropped)
+		shmem_uncharge(mapping->host, nr_shmem_dropped);
 
-		if (nr_dropped)
-			shmem_uncharge(mapping->host, nr_dropped);
+	if (!ret && is_anon)
+		remap_flags = RMP_USE_SHARED_ZEROPAGE;
+	remap_page(folio, 1 << order, remap_flags);
 
-		remap_page(folio, 1 << order,
-			   !ret && folio_test_anon(folio) ?
-				   RMP_USE_SHARED_ZEROPAGE :
-				   0);
+	/*
+	 * Unlock all after-split folios except the one containing
+	 * @lock_at page. If @folio is not split, it will be kept locked.
+	 */
+	for (new_folio = folio; new_folio != end_folio; new_folio = next) {
+		next = folio_next(new_folio);
+		if (new_folio == page_folio(lock_at))
+			continue;
 
+		folio_unlock(new_folio);
 		/*
-		 * Unlock all after-split folios except the one containing
-		 * @lock_at page. If @folio is not split, it will be kept locked.
+		 * Subpages may be freed if there wasn't any mapping
+		 * like if add_to_swap() is running on a lru page that
+		 * had its mapping zapped. And freeing these pages
+		 * requires taking the lru_lock so we do the put_page
+		 * of the tail pages after the split is complete.
 		 */
-		for (new_folio = folio; new_folio != end_folio;
-		     new_folio = next) {
-			next = folio_next(new_folio);
-			if (new_folio == page_folio(lock_at))
-				continue;
-
-			folio_unlock(new_folio);
-			/*
-			 * Subpages may be freed if there wasn't any mapping
-			 * like if add_to_swap() is running on a lru page that
-			 * had its mapping zapped. And freeing these pages
-			 * requires taking the lru_lock so we do the put_page
-			 * of the tail pages after the split is complete.
-			 */
-			free_folio_and_swap_cache(new_folio);
-		}
-	} else {
-		spin_unlock(&ds_queue->split_queue_lock);
-fail:
-		if (mapping)
-			xas_unlock(&xas);
-		local_irq_enable();
-		remap_page(folio, folio_nr_pages(folio), 0);
-		ret = -EAGAIN;
+		free_folio_and_swap_cache(new_folio);
 	}
 
 out_unlock:
-- 
2.47.2

Re: [PATCH v4 3/6] mm/huge_memory: deduplicate code in __folio_split().

[David Hildenbrand]

On 18.07.25 04:29, Zi Yan wrote:
> xas unlock, remap_page(), local_irq_enable() are moved out of if branches
> to deduplicate the code. While at it, add remap_flags to clean up
> remap_page() call site. nr_dropped is renamed to nr_shmem_dropped, as it
> becomes a variable at __folio_split() scope.
> 
> Signed-off-by: Zi Yan <ziy@nvidia.com>
> ---

Acked-by: David Hildenbrand <david@redhat.com>

-- 
Cheers,

David / dhildenb

[PATCH v4 4/6] mm/huge_memory: convert VM_BUG* to VM_WARN* in __folio_split.

[Zi Yan]

These VM_BUG* can be handled gracefully without crashing kernel.

Signed-off-by: Zi Yan <ziy@nvidia.com>
---
 mm/huge_memory.c | 16 +++++++++++++---
 1 file changed, 13 insertions(+), 3 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index d36f7bdaeb38..d6ff5e8c89d7 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -3601,8 +3601,14 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
 	pgoff_t end;
 	bool is_hzp;
 
-	VM_BUG_ON_FOLIO(!folio_test_locked(folio), folio);
-	VM_BUG_ON_FOLIO(!folio_test_large(folio), folio);
+	if (!folio_test_locked(folio)) {
+		VM_WARN_ON_ONCE_FOLIO(1, folio);
+		return -EINVAL;
+	}
+	if (!folio_test_large(folio)) {
+		VM_WARN_ON_ONCE_FOLIO(1, folio);
+		return -EINVAL;
+	}
 
 	if (folio != page_folio(split_at) || folio != page_folio(lock_at))
 		return -EINVAL;
@@ -3766,7 +3772,11 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
 		}
 
 		if (folio_test_swapcache(folio)) {
-			VM_BUG_ON(mapping);
+			if (mapping) {
+				VM_WARN_ON_ONCE_FOLIO(mapping, folio);
+				ret = -EINVAL;
+				goto fail;
+			}
 
 			swap_cache = swap_address_space(folio->swap);
 			xa_lock(&swap_cache->i_pages);
-- 
2.47.2

Re: [PATCH v4 4/6] mm/huge_memory: convert VM_BUG* to VM_WARN* in __folio_split.

[David Hildenbrand]

On 18.07.25 04:29, Zi Yan wrote:
> These VM_BUG* can be handled gracefully without crashing kernel.
> 
> Signed-off-by: Zi Yan <ziy@nvidia.com>
> ---
>   mm/huge_memory.c | 16 +++++++++++++---
>   1 file changed, 13 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index d36f7bdaeb38..d6ff5e8c89d7 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -3601,8 +3601,14 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
>   	pgoff_t end;
>   	bool is_hzp;
>   
> -	VM_BUG_ON_FOLIO(!folio_test_locked(folio), folio);
> -	VM_BUG_ON_FOLIO(!folio_test_large(folio), folio);
> +	if (!folio_test_locked(folio)) {
> +		VM_WARN_ON_ONCE_FOLIO(1, folio);
> +		return -EINVAL;
> +	}
> +	if (!folio_test_large(folio)) {
> +		VM_WARN_ON_ONCE_FOLIO(1, folio);
> +		return -EINVAL;
> +	}

For cases that we handle gracefully you usually want to use

if (WARN_ON_ONCE(..))

because then you get actually notified when that unexpected thing happens.

I am not really sure if recovery is warranted here: smells like a 
straight VM_WARN_ON_ONCE_FOLIO() is sufficient, and catching this early 
during development that something is extremely off.

-- 
Cheers,

David / dhildenb

Re: [PATCH v4 4/6] mm/huge_memory: convert VM_BUG* to VM_WARN* in __folio_split.

[Zi Yan]

On 18 Jul 2025, at 3:22, David Hildenbrand wrote:

> On 18.07.25 04:29, Zi Yan wrote:
>> These VM_BUG* can be handled gracefully without crashing kernel.
>>
>> Signed-off-by: Zi Yan <ziy@nvidia.com>
>> ---
>>   mm/huge_memory.c | 16 +++++++++++++---
>>   1 file changed, 13 insertions(+), 3 deletions(-)
>>
>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>> index d36f7bdaeb38..d6ff5e8c89d7 100644
>> --- a/mm/huge_memory.c
>> +++ b/mm/huge_memory.c
>> @@ -3601,8 +3601,14 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
>>   	pgoff_t end;
>>   	bool is_hzp;
>>  -	VM_BUG_ON_FOLIO(!folio_test_locked(folio), folio);
>> -	VM_BUG_ON_FOLIO(!folio_test_large(folio), folio);
>> +	if (!folio_test_locked(folio)) {
>> +		VM_WARN_ON_ONCE_FOLIO(1, folio);
>> +		return -EINVAL;
>> +	}
>> +	if (!folio_test_large(folio)) {
>> +		VM_WARN_ON_ONCE_FOLIO(1, folio);
>> +		return -EINVAL;
>> +	}
>
> For cases that we handle gracefully you usually want to use
>
> if (WARN_ON_ONCE(..))

Got it.

>
> because then you get actually notified when that unexpected thing happens.
>
> I am not really sure if recovery is warranted here: smells like a straight VM_WARN_ON_ONCE_FOLIO() is sufficient, and catching this early during development that something is extremely off.

OK. I will update it to just VM_WARN_ON_ONCE_FOLIO().

Thanks.

Best Regards,
Yan, Zi

Re: [PATCH v4 4/6] mm/huge_memory: convert VM_BUG* to VM_WARN* in __folio_split.

[Lorenzo Stoakes]

On Thu, Jul 17, 2025 at 10:29:58PM -0400, Zi Yan wrote:
> These VM_BUG* can be handled gracefully without crashing kernel.
>
> Signed-off-by: Zi Yan <ziy@nvidia.com>

This LGTM, but obviously this is predicated on David being happy re: his reply
but from my side:

Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>

> ---
>  mm/huge_memory.c | 16 +++++++++++++---
>  1 file changed, 13 insertions(+), 3 deletions(-)
>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index d36f7bdaeb38..d6ff5e8c89d7 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -3601,8 +3601,14 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
>  	pgoff_t end;
>  	bool is_hzp;
>
> -	VM_BUG_ON_FOLIO(!folio_test_locked(folio), folio);
> -	VM_BUG_ON_FOLIO(!folio_test_large(folio), folio);
> +	if (!folio_test_locked(folio)) {
> +		VM_WARN_ON_ONCE_FOLIO(1, folio);
> +		return -EINVAL;
> +	}
> +	if (!folio_test_large(folio)) {
> +		VM_WARN_ON_ONCE_FOLIO(1, folio);
> +		return -EINVAL;
> +	}
>
>  	if (folio != page_folio(split_at) || folio != page_folio(lock_at))
>  		return -EINVAL;
> @@ -3766,7 +3772,11 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
>  		}
>
>  		if (folio_test_swapcache(folio)) {
> -			VM_BUG_ON(mapping);
> +			if (mapping) {
> +				VM_WARN_ON_ONCE_FOLIO(mapping, folio);
> +				ret = -EINVAL;
> +				goto fail;
> +			}
>
>  			swap_cache = swap_address_space(folio->swap);
>  			xa_lock(&swap_cache->i_pages);
> --
> 2.47.2
>

[PATCH v4 5/6] mm/huge_memory: get frozen folio refcount with folio_expected_ref_count()

[Zi Yan]

Instead of open coding the refcount calculation, use
folio_expected_ref_count() to calculate frozen folio refcount.
Because:

1. __folio_split() does not split a folio with PG_private, so no elevated
   refcount from PG_private;
2. a frozen folio in __folio_split() is fully unmapped, so folio_mapcount()
   in folio_expected_ref_count() is always 0;
3. (mapping || swap_cache) ? folio_nr_pages(folio) is taken care of by
   folio_expected_ref_count() too.

Suggested-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Zi Yan <ziy@nvidia.com>
Acked-by: Balbir Singh <balbirs@nvidia.com>
Acked-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
---
 mm/huge_memory.c | 12 +++++-------
 1 file changed, 5 insertions(+), 7 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index d6ff5e8c89d7..4db67970ae69 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -3737,6 +3737,7 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
 	if (folio_ref_freeze(folio, 1 + extra_pins)) {
 		struct address_space *swap_cache = NULL;
 		struct lruvec *lruvec;
+		int expected_refs;
 
 		if (folio_order(folio) > 1 &&
 		    !list_empty(&folio->_deferred_list)) {
@@ -3800,11 +3801,8 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
 		     new_folio = next) {
 			next = folio_next(new_folio);
 
-			folio_ref_unfreeze(
-				new_folio,
-				1 + ((mapping || swap_cache) ?
-					     folio_nr_pages(new_folio) :
-					     0));
+			expected_refs = folio_expected_ref_count(new_folio) + 1;
+			folio_ref_unfreeze(new_folio, expected_refs);
 
 			lru_add_split_folio(folio, new_folio, lruvec, list);
 
@@ -3834,8 +3832,8 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
 		 * Otherwise, a parallel folio_try_get() can grab @folio
 		 * and its caller can see stale page cache entries.
 		 */
-		folio_ref_unfreeze(folio, 1 +
-			((mapping || swap_cache) ? folio_nr_pages(folio) : 0));
+		expected_refs = folio_expected_ref_count(folio) + 1;
+		folio_ref_unfreeze(folio, expected_refs);
 
 		unlock_page_lruvec(lruvec);
 
-- 
2.47.2

[PATCH v4 6/6] mm/huge_memory: refactor after-split (page) cache code.

[Zi Yan]

Smatch/coverity checkers report NULL mapping referencing issues[1][2][3]
every time the code is modified, because they do not understand that
mapping cannot be NULL when a folio is in page cache in the code.
Refactor the code to make it explicit.

Remove "end = -1" for anonymous folios, since after code refactoring, end
is no longer used by anonymous folio handling code.

No functional change is intended.

[1]https://lore.kernel.org/linux-mm/2afe3d59-aca5-40f7-82a3-a6d976fb0f4f@stanley.mountain/
[2]https://lore.kernel.org/oe-kbuild/64b54034-f311-4e7d-b935-c16775dbb642@suswa.mountain/
[3]https://lore.kernel.org/linux-mm/20250716145804.4836-1-antonio@mandelbit.com/

Suggested-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Zi Yan <ziy@nvidia.com>
Acked-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
---
 mm/huge_memory.c | 44 ++++++++++++++++++++++++++++----------------
 1 file changed, 28 insertions(+), 16 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 4db67970ae69..19342660739b 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -3646,7 +3646,6 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
 			ret = -EBUSY;
 			goto out;
 		}
-		end = -1;
 		mapping = NULL;
 		anon_vma_lock_write(anon_vma);
 	} else {
@@ -3799,6 +3798,8 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
 		 */
 		for (new_folio = folio_next(folio); new_folio != end_folio;
 		     new_folio = next) {
+			unsigned long nr_pages = folio_nr_pages(new_folio);
+
 			next = folio_next(new_folio);
 
 			expected_refs = folio_expected_ref_count(new_folio) + 1;
@@ -3806,25 +3807,36 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
 
 			lru_add_split_folio(folio, new_folio, lruvec, list);
 
-			/* Some pages can be beyond EOF: drop them from cache */
-			if (new_folio->index >= end) {
-				if (shmem_mapping(mapping))
-					nr_shmem_dropped += folio_nr_pages(new_folio);
-				else if (folio_test_clear_dirty(new_folio))
-					folio_account_cleaned(
-						new_folio,
-						inode_to_wb(mapping->host));
-				__filemap_remove_folio(new_folio, NULL);
-				folio_put_refs(new_folio,
-					       folio_nr_pages(new_folio));
-			} else if (mapping) {
-				__xa_store(&mapping->i_pages, new_folio->index,
-					   new_folio, 0);
-			} else if (swap_cache) {
+			/*
+			 * Anonymous folio with swap cache.
+			 * NOTE: shmem in swap cache is not supported yet.
+			 */
+			if (swap_cache) {
 				__xa_store(&swap_cache->i_pages,
 					   swap_cache_index(new_folio->swap),
 					   new_folio, 0);
+				continue;
+			}
+
+			/* Anonymous folio without swap cache */
+			if (!mapping)
+				continue;
+
+			/* Add the new folio to the page cache. */
+			if (new_folio->index < end) {
+				__xa_store(&mapping->i_pages, new_folio->index,
+					   new_folio, 0);
+				continue;
 			}
+
+			/* Drop folio beyond EOF: ->index >= end */
+			if (shmem_mapping(mapping))
+				nr_shmem_dropped += nr_pages;
+			else if (folio_test_clear_dirty(new_folio))
+				folio_account_cleaned(
+					new_folio, inode_to_wb(mapping->host));
+			__filemap_remove_folio(new_folio, NULL);
+			folio_put_refs(new_folio, nr_pages);
 		}
 		/*
 		 * Unfreeze @folio only after all page cache entries, which
-- 
2.47.2

Re: [PATCH v4 0/6] __folio_split() clean up

[Lorenzo Stoakes]

On Thu, Jul 17, 2025 at 10:29:54PM -0400, Zi Yan wrote:
> Hi Andrew,
>
> This series replaces both [PATCH v3 0/2] __folio_split() clean up
> and [PATCH] mm/huge_memory: refactor after-split (page) cache code.
>
> Hi Lorenzo,
>
> I addressed all of your comments except renaming folio to origin_folio,
> since I find that might either cause confusion or require a lot of code
> churn. folio variable points to the original folio throughout
> __folio_split() and using origin_folio in the middle of __folio_split()
> is confusing as one might wonder if origin_folio is different from or
> the same as folio. The alternative is to rename all folio to origin_folio
> in __folio_split(). That seems to be unnecessary code churn.

Sounds reasonable! Cheers :)

>
> Hi all,
>
> This patchset refactors __folio_split() and __split_unmapped_folio() to:
> 1. make __split_unmapped_folio() reusable for splitting unmapped
>    folios. It avoids the need for a new boolean unmapped parameter to guard
>    mapping-related code when __split_unmapped_folio() is reused to split
>    unmapped folios.
> 2. improve code readability and prevent smatch/coverity checkers from
>    complaining about NULL mapping referencing.
>
> An additional benefit for __split_unmapped_folio() refactoring is that
> __split_unmapped_folio() could be called on after-split folios by
> __folio_split(). It can enable new split methods. For example, at deferred
> split time, unmapped subpages can scatter arbitrarily within a large folio,
> neither uniform nor non-uniform split can maximize after-split folio orders
> for mapped subpages. The hope is that by calling __split_unmapped_folio()
> multiple times, a better split result can be achieved.
>
> The patchset is based on mm-new with aforementioned two patchsets
> reverted. It passes mm selftests.
>
> Changelog
> ===
> From V3[4]:
> 1. Split up Patch 1 into incremental changes:
>     a. Patch 1 moves code out of __split_unmapped_folio();
>     b. Patch 2 removes after_split label in __split_unmapped_folio();
>     c. Patch 3 refactors __folio_split() to deduplicate code;
>     d. Patch 4 converts VM_BUGs to VM_WARMs;
> 2. Added "mm/huge_memory: refactor after-split (page) cache code"
>    patch[5] to this series.
> 3. Added remap_flags to make remap_page() call easier to read.
> 4. Updated Patch 1 commit log to include variable rename information.
> 5. Converted additional VM_BUGs in __folio_split().
> 6. Renamed next_folio to end_folio to avoid confusion.
> 7. Added a comment about start for loop with folio_next(folio) instead
>    of just folio plus skipping folio in the loop body.
> 8. Dropped swapcache folio split check code from __split_unmapped_folio(),
>    since the check is already done at the beginning of __folio_split().
>
> From V2[3]:
> 1. Code format fixes
> 2. Restructured code to remove after_split goto label.
>
> From V1[2]:
> 1. Fixed indentations.
> 2. Used folio_expected_ref_count() to calculate ref_count instead of
>    open coding.
>
> [1] https://lore.kernel.org/linux-mm/94D8C1A4-780C-4BEC-A336-7D3613B54845@nvidia.com/
> [2] https://lore.kernel.org/linux-mm/20250711030259.3574392-1-ziy@nvidia.com/
> [3] https://lore.kernel.org/linux-mm/20250711182355.3592618-1-ziy@nvidia.com/
> [4] https://lore.kernel.org/linux-mm/20250714171823.3626213-1-ziy@nvidia.com/
> [5] https://lore.kernel.org/linux-mm/20250716171112.3666150-1-ziy@nvidia.com/
>
> Zi Yan (6):
>   mm/huge_memory: move unrelated code out of __split_unmapped_folio()
>   mm/huge_memory: remove after_split label in __split_unmapped_folio().
>   mm/huge_memory: deduplicate code in __folio_split().
>   mm/huge_memory: convert VM_BUG* to VM_WARN* in __folio_split.
>   mm/huge_memory: get frozen folio refcount with
>     folio_expected_ref_count()
>   mm/huge_memory: refactor after-split (page) cache code.
>
>  mm/huge_memory.c | 317 ++++++++++++++++++++++++-----------------------
>  1 file changed, 165 insertions(+), 152 deletions(-)
>
> --
> 2.47.2
>