[to-be-updated] mm-khugepaged-add-folio-dirty-check-after-try_to_unmap.patch removed from -mm tree

[Andrew Morton <akpm@linux-foundation.org>]

The quilt patch titled
     Subject: mm/khugepaged: add folio dirty check after try_to_unmap()
has been removed from the -mm tree.  Its filename was
     mm-khugepaged-add-folio-dirty-check-after-try_to_unmap.patch

This patch was dropped because an updated version will be issued

------------------------------------------------------
From: Zi Yan <ziy@nvidia.com>
Subject: mm/khugepaged: add folio dirty check after try_to_unmap()
Date: Thu, 23 Apr 2026 22:49:05 -0400

This check ensures the correctness of collapse read-only THPs for FSes
after READ_ONLY_THP_FOR_FS is enabled by default for all FSes supporting
PMD THP pagecache.

READ_ONLY_THP_FOR_FS only supports read-only fd and uses mapping->nr_thps
and inode->i_writecount to prevent any write to read-only to-be-collapsed
folios.  In upcoming commits, READ_ONLY_THP_FOR_FS will be removed and the
aforementioned mechanism will go away too.  To ensure khugepaged functions
as expected after the changes, skip if any folio is dirty after
try_to_unmap(), since a dirty folio means this read-only folio got some
writes via mmap can happen between try_to_unmap() and try_to_unmap_flush()
via cached TLB entries and khugepaged does not support writable pagecache
folio collapse yet.

Link: https://lore.kernel.org/20260424024915.28758-3-ziy@nvidia.com
Signed-off-by: Zi Yan <ziy@nvidia.com>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Barry Song <baohua@kernel.org>
Cc: Chris Mason <clm@fb.com>
Cc: Christian Brauner <brauner@kernel.org>
Cc: David Sterba <dsterba@suse.com>
Cc: Dev Jain <dev.jain@arm.com>
Cc: Jan Kara <jack@suse.cz>
Cc: Lance Yang <lance.yang@linux.dev>
Cc: Liam Howlett <liam@infradead.org>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Nico Pache <npache@redhat.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Song Liu <songliubraving@fb.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/khugepaged.c |   28 ++++++++++++++++++++++++----
 1 file changed, 24 insertions(+), 4 deletions(-)

--- a/mm/khugepaged.c~mm-khugepaged-add-folio-dirty-check-after-try_to_unmap
+++ a/mm/khugepaged.c
@@ -2327,8 +2327,7 @@ static enum scan_result collapse_file(st
 				}
 			} else if (folio_test_dirty(folio)) {
 				/*
-				 * khugepaged only works on read-only fd,
-				 * so this page is dirty because it hasn't
+				 * This page is dirty because it hasn't
 				 * been flushed since first write. There
 				 * won't be new dirty pages.
 				 *
@@ -2386,8 +2385,8 @@ static enum scan_result collapse_file(st
 		if (!is_shmem && (folio_test_dirty(folio) ||
 				  folio_test_writeback(folio))) {
 			/*
-			 * khugepaged only works on read-only fd, so this
-			 * folio is dirty because it hasn't been flushed
+			 * khugepaged only works on clean file-backed folios,
+			 * so this folio is dirty because it hasn't been flushed
 			 * since first write.
 			 */
 			result = SCAN_PAGE_DIRTY_OR_WRITEBACK;
@@ -2429,6 +2428,27 @@ static enum scan_result collapse_file(st
 			xas_unlock_irq(&xas);
 			folio_putback_lru(folio);
 			goto out_unlock;
+		}
+
+		/*
+		 * At this point, the folio is locked and unmapped. If the PTE
+		 * was dirty, try_to_unmap() has transferred the dirty bit to
+		 * the folio and we must not collapse it into a clean
+		 * file-backed folio.
+		 *
+		 * If the folio is clean here, no one can write it until we
+		 * drop the folio lock. A write through a stale TLB entry came
+		 * from a clean PTE and must fault because the PTE has been
+		 * cleared; the fault path has to take the folio lock before
+		 * installing a writable mapping. Buffered write paths also
+		 * have to take the folio lock before modifying file contents
+		 * without a mapping, typically via write_begin_get_folio().
+		 */
+		if (!is_shmem && folio_test_dirty(folio)) {
+			result = SCAN_PAGE_DIRTY_OR_WRITEBACK;
+			xas_unlock_irq(&xas);
+			folio_putback_lru(folio);
+			goto out_unlock;
 		}
 
 		/*
_

Patches currently in -mm which might be from ziy@nvidia.com are

mm-huge_memory-remove-read_only_thp_for_fs-from-file_thp_enabled.patch
mm-khugepaged-remove-read_only_thp_for_fs-check-in-hugepage_enabled.patch
mm-remove-read_only_thp_for_fs-kconfig-option.patch
mm-fs-remove-filemap_nr_thps-functions-and-their-users.patch
fs-remove-nr_thps-from-struct-address_space.patch
mm-huge_memory-remove-folio-split-check-for-read_only_thp_for_fs.patch
mm-truncate-use-folio_split-in-truncate_inode_partial_folio.patch
fs-btrfs-remove-a-comment-referring-to-read_only_thp_for_fs.patch
selftests-mm-remove-read_only_thp_for_fs-in-khugepaged.patch
selftests-mm-remove-read_only_thp_for_fs-code-from-guard-regions.patch