mm/huge_memory: move unrelated code out of __split_unmapped_folio()

Patch series "__folio_split() clean up", v5.

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.


This patch (of 6):

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.

Link: https://lkml.kernel.org/r/20250718023000.4044406-1-ziy@nvidia.com
Link: https://lkml.kernel.org/r/20250718023000.4044406-2-ziy@nvidia.com
Link: https://lkml.kernel.org/r/20250718183720.4054515-1-ziy@nvidia.com
Link: https://lkml.kernel.org/r/20250718183720.4054515-2-ziy@nvidia.com
Signed-off-by: Zi Yan <ziy@nvidia.com>
Acked-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Antonio Quartulli <antonio@mandelbit.com>
Cc: Balbir Singh <balbirs@nvidia.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Barry Song <baohua@kernel.org>
Cc: Dan Carpenter <dan.carpenter@linaro.org>
Cc: Dev Jain <dev.jain@arm.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Kirill A. Shutemov <k.shutemov@gmail.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Mariano Pache <npache@redhat.com>
Cc: Mathew Brost <matthew.brost@intel.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 133 insertions, 137 deletions

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index ce130225a8e52..63eebca07628b 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: