[PATCH 7.2 v3 02/12] mm/khugepaged: add folio dirty check after try_to_unmap()

[Zi Yan]

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.

Signed-off-by: Zi Yan <ziy@xxxxxxxxxx>
---
 mm/khugepaged.c | 25 +++++++++++++++++++++----
 1 file changed, 21 insertions(+), 4 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 3eb5d982d3d3..1c0fdc81d276 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1979,8 +1979,7 @@ 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.
 				 *
@@ -2038,8 +2037,8 @@ 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;
@@ -2083,6 +2082,24 @@ static enum scan_result collapse_file(struct mm_struct *mm, unsigned long addr,
 			goto out_unlock;
 		}
 
+		/*
+		 * At this point, the folio is locked, unmapped. Make sure the
+		 * folio is clean, so that no one else is able to write to it,
+		 * since that would require taking the folio lock first.
+		 * Otherwise that means the folio was pointed by a dirty PTE and
+		 * some CPU might have a valid TLB entry with dirty bit set
+		 * still pointing to this folio and writes can happen without
+		 * causing a page table walk and folio lock acquisition before
+		 * the try_to_unmap_flush() below is done. After the collapse,
+		 * file-backed folio is not set as dirty and can be discarded
+		 * before any new write marks the folio dirty, causing data
+		 * corruption.
+		 */
+		if (!is_shmem && folio_test_dirty(folio)) {
+			result = SCAN_PAGE_DIRTY_OR_WRITEBACK;
+			goto out_unlock;
+		}
+
 		/*
 		 * Accumulate the folios that are being collapsed.
 		 */
-- 
2.43.0