[PATCH] mm: skip empty finite ranges in truncate_pagecache_range

[Hungyu Lin]

For a finite range ending before the first page boundary,
truncate_pagecache_range() calculates unmap_end as -1. The unsigned
comparison then treats the empty full-page interval as nonempty.

For example, lstart == 0 and lend == 0 cause unmap_mapping_range() to be
called with a zero length, meaning unmap to EOF. Punching one byte at
offset zero in a mapped FUSE file can therefore remove mappings outside
the hole, causing unnecessary page-table work and subsequent faults.

Compare the exclusive rounded end against the rounded start, while
explicitly preserving lend == -1 as the EOF sentinel. Perform the
addition after converting to u64 so the largest inclusive end does not
introduce a signed overflow in the comparison.

Fixes: 5a7203947a1d ("mm: teach truncate_inode_pages_range() to handle non page aligned ranges")
Signed-off-by: Hungyu Lin <dennylin0707@xxxxxxxxx>
---
 mm/truncate.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/truncate.c b/mm/truncate.c
index b58ba940be47..43c6cf99027f 100644
--- a/mm/truncate.c
+++ b/mm/truncate.c
@@ -919,7 +919,7 @@ void truncate_pagecache_range(struct inode *inode, loff_t lstart, loff_t lend)
 	 * once (before truncating pagecache), and without "even_cows" flag:
 	 * hole-punching should not remove private COWed pages from the hole.
 	 */
-	if ((u64)unmap_end > (u64)unmap_start)
+	if (lend == -1 || (u64)unmap_end + 1 > (u64)unmap_start)
 		unmap_mapping_range(mapping, unmap_start,
 				    1 + unmap_end - unmap_start, 0);
 	truncate_inode_pages_range(mapping, lstart, lend);
-- 
2.43.0