This fix is analogous to Peter Xu's fix for hugetlb [0]. If we don't
put_page() after getting the page out of the page cache, we leak the
reference.

The fix can be verified by checking /proc/meminfo and running the
userfaultfd selftest in shmem mode. Without the fix, we see MemFree /
MemAvailable steadily decreasing with each run of the test. With the
fix, memory is correctly freed after the test program exits.

Fixes: 00da60b9d0a0 ("userfaultfd: support minor fault handling for shmem")
Signed-off-by: Axel Rasmussen <axelrasmussen@google.com>
---
 mm/shmem.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/mm/shmem.c b/mm/shmem.c
index ef8c9f5e92fc..d2e0e81b7d2e 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c

@@ -1831,6 +1831,7 @@ static int shmem_getpage_gfp(struct inode *inode, pgoff_t index,
 
 	if (page && vma && userfaultfd_minor(vma)) {
 		unlock_page(page);
+		put_page(page);
 		*fault_type = handle_userfault(vmf, VM_UFFD_MINOR);
 		return 0;
 	}

-- 
2.31.0.rc2.261.g7f71774620-goog