Message-ID: <20250711030259.3574392-1-ziy@nvidia.com>
Date: Thu, 10 Jul 2025 23:02:59 -0400
From: Zi Yan <ziy@...dia.com>
To: Balbir Singh <balbirs@...dia.com>,
	David Hildenbrand <david@...hat.com>,
	linux-mm@...ck.org
Cc: Andrew Morton <akpm@...ux-foundation.org>,
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
	Zi Yan <ziy@...dia.com>,
	Baolin Wang <baolin.wang@...ux.alibaba.com>,
	"Liam R. Howlett" <Liam.Howlett@...cle.com>,
	Nico Pache <npache@...hat.com>,
	Ryan Roberts <ryan.roberts@....com>,
	Dev Jain <dev.jain@....com>,
	Barry Song <baohua@...nel.org>,
	linux-kernel@...r.kernel.org
Subject: [PATCH] mm/huge_memory: move unrelated code out of __split_unmapped_folio()

remap(), folio_ref_unfreeze(), lru_add_split_folio() are not related to
splitting unmapped folio operations. Move them out to the caller, so that
__split_unmapped_folio() only splits unmapped folios. This makes
__split_unmapped_folio() reusable.

Convert VM_BUG_ON(mapping) to use VM_WARN_ON_ONCE_FOLIO().

Signed-off-by: Zi Yan <ziy@...dia.com>
---
Based on the prior discussion[1], this patch makes
__split_unmapped_folio() reusable for splitting unmapped folios without
adding a new boolean unmapped parameter to guard mapping related code.

Another potential benefit is that __split_unmapped_folio() could be
called on after-split folios by __folio_split() to perform 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.
Hopefully, performing __split_unmapped_folio() multiple times can
achieve the optimal split result.

It passed mm selftests.

[1] https://lore.kernel.org/linux-mm/94D8C1A4-780C-4BEC-A336-7D3613B54845@nvidia.com/
---

 mm/huge_memory.c | 275 ++++++++++++++++++++++++-----------------------
 1 file changed, 139 insertions(+), 136 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 3eb1c34be601..d97145dfa6c8 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -3396,10 +3396,6 @@ 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)
@@ -3425,51 +3421,27 @@ 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);
-	}
-
 	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);
 
@@ -3480,9 +3452,9 @@ 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)
@@ -3517,113 +3489,34 @@ 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
+			 * 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),
+			if (folio_test_anon(new_folio)) {
+				mod_mthp_stat(folio_order(new_folio),
 						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);
-			}
 		}
 	}
 
-	/*
-	 * 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;
 }
 
@@ -3706,11 +3599,14 @@ 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 *next_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;
+	int nr_shmem_dropped = 0;
 	pgoff_t end;
 	bool is_hzp;
 
@@ -3833,13 +3729,18 @@ 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;
+		struct lruvec *lruvec;
+
 		if (folio_order(folio) > 1 &&
 		    !list_empty(&folio->_deferred_list)) {
 			ds_queue->split_queue_len--;
@@ -3873,18 +3774,120 @@ 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)) {
+			if (mapping) {
+				VM_WARN_ON_ONCE_FOLIO(mapping, folio);
+				ret = -EINVAL;
+				goto fail;
+			}
+
+			/*
+			 * a swapcache folio can only be uniformly split to
+			 * order-0
+			 */
+			if (!uniform_split || new_order != 0) {
+				ret = -EINVAL;
+				goto fail;
+			}
+
+			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.
+		 */
+		for (new_folio = folio_next(folio); new_folio != next_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_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) {
+				__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 origin_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);
 	} 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;
 	}
+fail:
+	if (mapping)
+		xas_unlock(&xas);
+
+	local_irq_enable();
+
+	if (nr_shmem_dropped)
+		shmem_uncharge(mapping->host, nr_shmem_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 != 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);
+	}
 
 out_unlock:
 	if (anon_vma) {
-- 
2.47.2

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