[RFC V2 10/18] famfs_fuse: Basic fuse kernel ABI enablement for famfs

[John Groves]

* FUSE_DAX_FMAP flag in INIT request/reply

* fuse_conn->famfs_iomap (enable famfs-mapped files) to denote a
  famfs-enabled connection

Signed-off-by: John Groves <john@xxxxxxxxxx>
---
 fs/fuse/fuse_i.h          |  3 +++
 fs/fuse/inode.c           | 14 ++++++++++++++
 include/uapi/linux/fuse.h |  4 ++++
 3 files changed, 21 insertions(+)

diff --git a/fs/fuse/fuse_i.h b/fs/fuse/fuse_i.h
index 9d87ac48d724..a592c1002861 100644
--- a/fs/fuse/fuse_i.h
+++ b/fs/fuse/fuse_i.h
@@ -873,6 +873,9 @@ struct fuse_conn {
 	/* Use io_uring for communication */
 	unsigned int io_uring;
 
+	/* dev_dax_iomap support for famfs */
+	unsigned int famfs_iomap:1;
+
 	/** Maximum stack depth for passthrough backing files */
 	int max_stack_depth;
 
diff --git a/fs/fuse/inode.c b/fs/fuse/inode.c
index 29147657a99f..e48e11c3f9f3 100644
--- a/fs/fuse/inode.c
+++ b/fs/fuse/inode.c
@@ -1392,6 +1392,18 @@ static void process_init_reply(struct fuse_mount *fm, struct fuse_args *args,
 			}
 			if (flags & FUSE_OVER_IO_URING && fuse_uring_enabled())
 				fc->io_uring = 1;
+			if (IS_ENABLED(CONFIG_FUSE_FAMFS_DAX) &&
+			    flags & FUSE_DAX_FMAP) {
+				/* XXX: Should also check that fuse server
+				 * has CAP_SYS_RAWIO and/or CAP_SYS_ADMIN,
+				 * since it is directing the kernel to access
+				 * dax memory directly - but this function
+				 * appears not to be called in fuse server
+				 * process context (b/c even if it drops
+				 * those capabilities, they are held here).
+				 */
+				fc->famfs_iomap = 1;
+			}
 		} else {
 			ra_pages = fc->max_read / PAGE_SIZE;
 			fc->no_lock = 1;
@@ -1450,6 +1462,8 @@ void fuse_send_init(struct fuse_mount *fm)
 		flags |= FUSE_SUBMOUNTS;
 	if (IS_ENABLED(CONFIG_FUSE_PASSTHROUGH))
 		flags |= FUSE_PASSTHROUGH;
+	if (IS_ENABLED(CONFIG_FUSE_FAMFS_DAX))
+		flags |= FUSE_DAX_FMAP;
 
 	/*
 	 * This is just an information flag for fuse server. No need to check
diff --git a/include/uapi/linux/fuse.h b/include/uapi/linux/fuse.h
index 5e0eb41d967e..6c384640c79b 100644
--- a/include/uapi/linux/fuse.h
+++ b/include/uapi/linux/fuse.h
@@ -229,6 +229,8 @@
  *    - FUSE_URING_IN_OUT_HEADER_SZ
  *    - FUSE_URING_OP_IN_OUT_SZ
  *    - enum fuse_uring_cmd
+ *  7.43
+ *    - Add FUSE_DAX_FMAP capability - ability to handle in-kernel fsdax maps
  */
 
 #ifndef _LINUX_FUSE_H
@@ -435,6 +437,7 @@ struct fuse_file_lock {
  *		    of the request ID indicates resend requests
  * FUSE_ALLOW_IDMAP: allow creation of idmapped mounts
  * FUSE_OVER_IO_URING: Indicate that client supports io-uring
+ * FUSE_DAX_FMAP: kernel supports dev_dax_iomap (aka famfs) fmaps
  */
 #define FUSE_ASYNC_READ		(1 << 0)
 #define FUSE_POSIX_LOCKS	(1 << 1)
@@ -482,6 +485,7 @@ struct fuse_file_lock {
 #define FUSE_DIRECT_IO_RELAX	FUSE_DIRECT_IO_ALLOW_MMAP
 #define FUSE_ALLOW_IDMAP	(1ULL << 40)
 #define FUSE_OVER_IO_URING	(1ULL << 41)
+#define FUSE_DAX_FMAP		(1ULL << 42)
 
 /**
  * CUSE INIT request/reply flags
-- 
2.49.0