[PATCH] shmem: skip empty unmap ranges when punching holes

[Hungyu Lin <dennylin0707@gmail.com>]

When a hole ends within the first page, shmem_fallocate() calculates
unmap_end as -1. Casting it to u64 makes the range check succeed even
though the hole contains no complete pages.

For example, punching one byte at offset zero calls unmap_mapping_range()
with a zero length, which means unmap to the end of the file. The file
contents are preserved outside the hole, but unrelated mappings are
removed and must fault back in. This can be reproduced with fallocate -p
-o 0 -l 1 on a mapped tmpfs file.

Compare the exclusive rounded end against the rounded start to skip
empty ranges. Keep the unsigned comparison so rounding the start near
the maximum file offset continues to be handled correctly.

Fixes: 83e4fa9c16e4 ("tmpfs: support fallocate FALLOC_FL_PUNCH_HOLE")
Signed-off-by: Hungyu Lin <dennylin0707@gmail.com>
---
 mm/shmem.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/shmem.c b/mm/shmem.c
index d97fc32e7c94..08d5336c27a0 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -3864,7 +3864,7 @@ static long shmem_fallocate(struct file *file, int mode, loff_t offset,
 		WRITE_ONCE(inode->i_private, &shmem_falloc);
 		spin_unlock(&inode->i_lock);
 
-		if ((u64)unmap_end > (u64)unmap_start)
+		if ((u64)(unmap_end + 1) > (u64)unmap_start)
 			unmap_mapping_range(mapping, unmap_start,
 					    1 + unmap_end - unmap_start, 0);
 		shmem_truncate_range(inode, offset, offset + len - 1);
-- 
2.43.0