[PATCH 11/16] tmpfs: fcntl(fd, F_MEM_LOCKED) to test if memlocked

[Hugh Dickins <hughd@google.com>]

Though we have not yet found a compelling need to make the locked status
of a tmpfs file visible, and offer no tool to show it, the kernel ought
to be able to support such a tool: add the F_MEM_LOCKED fcntl, returning
-1 on failure (not tmpfs), 0 when not F_MEM_LOCKED, 1 when F_MEM_LOCKED.

Signed-off-by: Hugh Dickins <hughd@google.com>
---
 fs/fcntl.c                 | 1 +
 include/uapi/linux/fcntl.h | 1 +
 mm/shmem.c                 | 4 ++++
 3 files changed, 6 insertions(+)

diff --git a/fs/fcntl.c b/fs/fcntl.c
index a3534764b50e..0d8dc723732d 100644
--- a/fs/fcntl.c
+++ b/fs/fcntl.c
@@ -439,6 +439,7 @@ static long do_fcntl(int fd, unsigned int cmd, unsigned long arg,
 	case F_NOHUGEPAGE:
 	case F_MEM_LOCK:
 	case F_MEM_UNLOCK:
+	case F_MEM_LOCKED:
 		err = shmem_fcntl(filp, cmd, arg);
 		break;
 	default:
diff --git a/include/uapi/linux/fcntl.h b/include/uapi/linux/fcntl.h
index 21dc969df0fd..012585e8c9ab 100644
--- a/include/uapi/linux/fcntl.h
+++ b/include/uapi/linux/fcntl.h
@@ -88,6 +88,7 @@
  */
 #define F_MEM_LOCK		(F_LINUX_SPECIFIC_BASE + 17)
 #define F_MEM_UNLOCK		(F_LINUX_SPECIFIC_BASE + 18)
+#define F_MEM_LOCKED		(F_LINUX_SPECIFIC_BASE + 19)
 
 /*
  * Types of directory notifications that may be requested.
diff --git a/mm/shmem.c b/mm/shmem.c
index ba9b9900287b..6e53dabe658b 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -2299,6 +2299,9 @@ static int shmem_memlock_fcntl(struct file *file, unsigned int cmd)
 	bool cleanup_mapping = false;
 	int retval = 0;
 
+	if (cmd == F_MEM_LOCKED)
+		return !!info->mlock_ucounts;
+
 	inode_lock(inode);
 	if (cmd == F_MEM_LOCK) {
 		if (!info->mlock_ucounts) {
@@ -2762,6 +2765,7 @@ long shmem_fcntl(struct file *file, unsigned int cmd, unsigned long arg)
 		break;
 	case F_MEM_LOCK:
 	case F_MEM_UNLOCK:
+	case F_MEM_LOCKED:
 		error = shmem_memlock_fcntl(file, cmd);
 		break;
 	}
-- 
2.26.2