[PATCH v2 8/8] mm/khugepaged: flesh out try_to_unmap_flush() comment

[Pedro Falcato]

Flesh out any correctness concerns with try_to_unmap batching, flushing
and its interaction with the rest of collapse_file().

Signed-off-by: Pedro Falcato <pfalcato@xxxxxxx>
---
 mm/khugepaged.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index accd8b4895d3..fd947e698ed0 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -2597,8 +2597,12 @@ static enum scan_result collapse_file(struct mm_struct *mm, unsigned long addr,
 xa_unlocked:
 
 	/*
-	 * Flush before copying the folios, or releasing them in rollback.
-	 * This is a no-op if out_unlock already flushed the batch.
+	 * try_to_unmap() flush must be done now before copying, regardless
+	 * of success or not. In case of success, folios are about to be
+	 * copied and collapsed onto a single large folio. In that case,
+	 * stale TLB entries need to be flushed out, so no racing write may
+	 * get lost. In case of failure, stale TLB entries need to be flushed
+	 * out before putting the folio (which can possibly free it).
 	 */
 	try_to_unmap_flush();
 
-- 
2.56.0