[PATCH v3] mm,thp: recheck each page before collapsing file THP

[Song Liu]

In collapse_file(), after locking the page, it is necessary to recheck
that the page is up-to-date. Add PageUptodate() check for both shmem THP
and file THP.

Current khugepaged should not try to collapse dirty file THP, because it
is limited to read only text. Add a PageDirty check and warning for file
THP. This is added after page_mapping() check, because if the page is
truncated, it might be dirty.

Fixes: 99cb0dbd47a1 ("mm,thp: add read-only THP support for (non-shmem) FS")
Cc: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
Cc: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: William Kucharski <william.kucharski@xxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Song Liu <songliubraving@xxxxxx>

---
Changes v1 => v2:
Rearrange the checks per feedback from Johannes, Rik, and Kirill.

Changes v2 => v3:
Remove redudant checks before trylock_page(). (Johannes)
---
 mm/khugepaged.c | 27 +++++++++++++++------------
 1 file changed, 15 insertions(+), 12 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 0a1b4b484ac5..e20bb9d3482d 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1601,17 +1601,6 @@ static void collapse_file(struct mm_struct *mm,
 					result = SCAN_FAIL;
 					goto xa_unlocked;
 				}
-			} else if (!PageUptodate(page)) {
-				xas_unlock_irq(&xas);
-				wait_on_page_locked(page);
-				if (!trylock_page(page)) {
-					result = SCAN_PAGE_LOCK;
-					goto xa_unlocked;
-				}
-				get_page(page);
-			} else if (PageDirty(page)) {
-				result = SCAN_FAIL;
-				goto xa_locked;
 			} else if (trylock_page(page)) {
 				get_page(page);
 				xas_unlock_irq(&xas);
@@ -1626,7 +1615,12 @@ static void collapse_file(struct mm_struct *mm,
 		 * without racing with truncate.
 		 */
 		VM_BUG_ON_PAGE(!PageLocked(page), page);
-		VM_BUG_ON_PAGE(!PageUptodate(page), page);
+
+		/* double check the page is up to date */
+		if (unlikely(!PageUptodate(page))) {
+			result = SCAN_FAIL;
+			goto out_unlock;
+		}
 
 		/*
 		 * If file was truncated then extended, or hole-punched, before
@@ -1642,6 +1636,15 @@ static void collapse_file(struct mm_struct *mm,
 			goto out_unlock;
 		}
 
+		/*
+		 * khugepaged should not try to collapse dirty pages for
+		 * file THP. Show warning if this somehow happens.
+		 */
+		if (WARN_ON_ONCE(!is_shmem && PageDirty(page))) {
+			result = SCAN_FAIL;
+			goto out_unlock;
+		}
+
 		if (isolate_lru_page(page)) {
 			result = SCAN_DEL_PAGE_LRU;
 			goto out_unlock;
-- 
2.17.1