[PATCH 4.19 004/139] mm/khugepaged: collapse_shmem() stop if punched or truncated

[Greg Kroah-Hartman]

4.19-stable review patch.  If anyone has any objections, please let me know.

------------------

commit 701270fa193aadf00bdcf607738f64997275d4c7 upstream.

Huge tmpfs testing showed that although collapse_shmem() recognizes a
concurrently truncated or hole-punched page correctly, its handling of
holes was liable to refill an emptied extent.  Add check to stop that.

Link: http://lkml.kernel.org/r/alpine.LSU.2.11.1811261522040.2275@xxxxxxxxxxxx
Fixes: f3f0e1d2150b2 ("khugepaged: add support of collapse for tmpfs/shmem pages")
Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
Reviewed-by: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Cc: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
Cc: Jerome Glisse <jglisse@xxxxxxxxxx>
Cc: Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>	[4.8+]
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
 mm/khugepaged.c | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index a31d740e6cd1..0378f758b065 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1349,6 +1349,16 @@ static void collapse_shmem(struct mm_struct *mm,
 	radix_tree_for_each_slot(slot, &mapping->i_pages, &iter, start) {
 		int n = min(iter.index, end) - index;
 
+		/*
+		 * Stop if extent has been hole-punched, and is now completely
+		 * empty (the more obvious i_size_read() check would take an
+		 * irq-unsafe seqlock on 32-bit).
+		 */
+		if (n >= HPAGE_PMD_NR) {
+			result = SCAN_TRUNCATED;
+			goto tree_locked;
+		}
+
 		/*
 		 * Handle holes in the radix tree: charge it from shmem and
 		 * insert relevant subpage of new_page into the radix-tree.
@@ -1459,6 +1469,11 @@ static void collapse_shmem(struct mm_struct *mm,
 	if (result == SCAN_SUCCEED && index < end) {
 		int n = end - index;
 
+		/* Stop if extent has been truncated, and is now empty */
+		if (n >= HPAGE_PMD_NR) {
+			result = SCAN_TRUNCATED;
+			goto tree_locked;
+		}
 		if (!shmem_charge(mapping->host, n)) {
 			result = SCAN_FAIL;
 			goto tree_locked;
-- 
2.17.1