[PATCH 1/2] mm/khugepaged: set THP as uptodate earlier for shmem

[David Stevens <stevensd@chromium.org>]

From: David Stevens <stevensd@chromium.org>

In collapse_file, mark the THP as up-to-date before inserting it into
the page cache. This fixes a race where folio_seek_hole_data would
mistake the THP for an fallocated but unwritten page. This race is
visible to userspace via data temporarily disappearing from
SEEK_DATA/SEEK_HOLE, which can cause data loss for applications that use
lseek to efficiently snapshot sparse shmem.

Fixes: f3f0e1d2150b ("khugepaged: add support of collapse for tmpfs/shmem pages")
Signed-off-by: David Stevens <stevensd@chromium.org>
---
 mm/khugepaged.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 79be13133322..b648f1053d95 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1779,10 +1779,13 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
 	hpage->mapping = mapping;
 
 	/*
-	 * At this point the hpage is locked and not up-to-date.
-	 * It's safe to insert it into the page cache, because nobody would
-	 * be able to map it or use it in another way until we unlock it.
+	 * Mark hpage as up-to-date before inserting it into the page cache to
+	 * prevent it from being mistaken for an fallocated but unwritten page.
+	 * Inserting the unfinished hpage into the page cache is safe because
+	 * it is locked, so nobody can map it or use it in another way until we
+	 * unlock it.
 	 */
+	SetPageUptodate(hpage);
 
 	xas_set(&xas, start);
 	for (index = start; index < end; index++) {
-- 
2.39.1.581.gbfd45094c4-goog