[PATCH v6 02/14] mm/khugepaged: add folio dirty check after try_to_unmap()

[Zi Yan]

This check ensures the correctness of read-only PMD folio collapse after
it is enabled for all FSes supporting PMD pagecache folios and replaces
READ_ONLY_THP_FOR_FS.

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 at that point means this read-only
folio can get writes between try_to_unmap() and try_to_unmap_flush() via
cached TLB entries and khugepaged does not support writable pagecache
folio collapse yet.

Signed-off-by: Zi Yan <ziy@xxxxxxxxxx>
Reviewed-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
Acked-by: David Hildenbrand (Arm) <david@xxxxxxxxxx>
Reviewed-by: Lance Yang <lance.yang@xxxxxxxxx>
Reviewed-by: Nico Pache <npache@xxxxxxxxxx>
Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Cc: Barry Song <baohua@xxxxxxxxxx>
Cc: Chris Mason <clm@xxxxxx>
Cc: Christian Brauner <brauner@xxxxxxxxxx>
Cc: David Sterba <dsterba@xxxxxxxx>
Cc: Dev Jain <dev.jain@xxxxxxx>
Cc: Jan Kara <jack@xxxxxxx>
Cc: Liam Howlett <liam@xxxxxxxxxxxxx>
Cc: Lorenzo Stoakes <ljs@xxxxxxxxxx>
Cc: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxx>
Cc: Mike Rapoport <rppt@xxxxxxxxxx>
Cc: Ryan Roberts <ryan.roberts@xxxxxxx>
Cc: Shuah Khan <shuah@xxxxxxxxxx>
Cc: Song Liu <songliubraving@xxxxxx>
Cc: Suren Baghdasaryan <surenb@xxxxxxxxxx>
Cc: Vlastimil Babka <vbabka@xxxxxxxxxx>
---
 mm/khugepaged.c | 28 ++++++++++++++++++++++++----
 1 file changed, 24 insertions(+), 4 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index afd61168d915c..2ebbfbd260ec4 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -2335,8 +2335,7 @@ static enum scan_result collapse_file(struct mm_struct *mm, unsigned long addr,
 				}
 			} 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.
 				 *
@@ -2394,8 +2393,8 @@ static enum scan_result collapse_file(struct mm_struct *mm, unsigned long addr,
 		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;
@@ -2439,6 +2438,27 @@ static enum scan_result collapse_file(struct mm_struct *mm, unsigned long addr,
 			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;
+		}
+
 		/*
 		 * Accumulate the folios that are being collapsed.
 		 */
-- 
2.53.0