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

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Message-ID: <20250716171112.3666150-1-ziy@nvidia.com>
Date: Wed, 16 Jul 2025 13:11:12 -0400
From: Zi Yan <ziy@...dia.com>
To: David Hildenbrand <david@...hat.com>,
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
	Dan Carpenter <dan.carpenter@...aro.org>,
	Antonio Quartulli <antonio@...delbit.com>,
	linux-mm@...ck.org
Cc: Andrew Morton <akpm@...ux-foundation.org>,
	Hugh Dickins <hughd@...gle.com>,
	Kirill Shutemov <k.shutemov@...il.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: refactor after-split (page) cache code.

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.

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@...hat.com>
Signed-off-by: Zi Yan <ziy@...dia.com>
---
 mm/huge_memory.c | 43 ++++++++++++++++++++++++++++---------------
 1 file changed, 28 insertions(+), 15 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 31b5c4e61a57..fe17b0a157cd 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -3804,6 +3804,8 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
 		 */
 		for (new_folio = folio_next(folio); new_folio != next_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;
@@ -3811,25 +3813,36 @@ 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;
+			}
+
+			/* Anonymouse 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