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

[Song Liu <songliubraving@fb.com>]

In collapse_file(), after locking the page, it is necessary to recheck
that the page is up-to-date, clean, and pointing to the proper mapping.
If any check fails, abort the collapse.

Fixes: 99cb0dbd47a1 ("mm,thp: add read-only THP support for (non-shmem) FS")
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Hugh Dickins <hughd@google.com>
Cc: William Kucharski <william.kucharski@oracle.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Song Liu <songliubraving@fb.com>
---
 mm/khugepaged.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 0a1b4b484ac5..7da49b643c4d 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1619,6 +1619,14 @@ static void collapse_file(struct mm_struct *mm,
 				result = SCAN_PAGE_LOCK;
 				goto xa_locked;
 			}
+
+			/* double check the page is correct and clean */
+			if (unlikely(!PageUptodate(page)) ||
+			    unlikely(PageDirty(page)) ||
+			    unlikely(page->mapping != mapping)) {
+				result = SCAN_FAIL;
+				goto out_unlock;
+			}
 		}
 
 		/*
-- 
2.17.1