smb3: enable swap on SMB3 mounts

Add experimental support for allowing a swap file to be on an SMB3
mount.  There are use cases where swapping over a secure network
filesystem is preferable. In some cases there are no local
block devices large enough, and network block devices can be
hard to setup and secure.  And in some cases there are no
local block devices at all (e.g. with the recent addition of
remote boot over SMB3 mounts).

There are various enhancements that can be added later e.g.:
- doing a mandatory byte range lock over the swapfile (until
the Linux VFS is modified to notify the file system that an open
is for a swapfile, when the file can be opened "DENY_ALL" to prevent
others from opening it).
- pinning more buffers in the underlying transport to minimize memory
allocations in the TCP stack under the fs
- documenting how to create ACLs (on the server) to secure the
swapfile (or adding additional tools to cifs-utils to make it easier)

Signed-off-by: Steve French <stfrench@microsoft.com>
Acked-by: Pavel Shilovsky <pshilov@microsoft.com>
Reviewed-by: Ronnie Sahlberg <lsahlber@redhat.com>

1 files changed, 1 insertions, 0 deletions

diff --git a/fs/cifs/cifsglob.h b/fs/cifs/cifsglob.h
index 7448e7202e7a..05dd3dea684b 100644
--- a/fs/cifs/cifsglob.h
+++ b/fs/cifs/cifsglob.h
@@ -1313,6 +1313,7 @@ struct cifsFileInfo {
 	struct tcon_link *tlink;
 	unsigned int f_flags;
 	bool invalidHandle:1;	/* file closed via session abend */
+	bool swapfile:1;
 	bool oplock_break_cancelled:1;
 	unsigned int oplock_epoch; /* epoch from the lease break */
 	__u32 oplock_level; /* oplock/lease level from the lease break */