[PATCH 06/10] mm/khugepaged: collapse_shmem() remember to clear holes

[Hugh Dickins <hughd@google.com>]

Huge tmpfs testing reminds us that there is no __GFP_ZERO in the gfp
flags khugepaged uses to allocate a huge page - in all common cases it
would just be a waste of effort - so collapse_shmem() must remember to
clear out any holes that it instantiates.

The obvious place to do so, where they are put into the page cache tree,
is not a good choice: because interrupts are disabled there.  Leave it
until further down, once success is assured, where the other pages are
copied (before setting PageUptodate).

Fixes: f3f0e1d2150b2 ("khugepaged: add support of collapse for tmpfs/shmem pages")
Signed-off-by: Hugh Dickins <hughd@google.com>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: stable@vger.kernel.org # 4.8+
---
 mm/khugepaged.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 65e82f665c7c..1c402d33547e 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1467,7 +1467,12 @@ static void collapse_shmem(struct mm_struct *mm,
 		 * Replacing old pages with new one has succeeded, now we
 		 * need to copy the content and free the old pages.
 		 */
+		index = start;
 		list_for_each_entry_safe(page, tmp, &pagelist, lru) {
+			while (index < page->index) {
+				clear_highpage(new_page + (index % HPAGE_PMD_NR));
+				index++;
+			}
 			copy_highpage(new_page + (page->index % HPAGE_PMD_NR),
 					page);
 			list_del(&page->lru);
@@ -1477,6 +1482,11 @@ static void collapse_shmem(struct mm_struct *mm,
 			ClearPageActive(page);
 			ClearPageUnevictable(page);
 			put_page(page);
+			index++;
+		}
+		while (index < end) {
+			clear_highpage(new_page + (index % HPAGE_PMD_NR));
+			index++;
 		}
 
 		local_irq_disable();
-- 
2.20.0.rc0.387.gc7a69e6b6c-goog