[PATCH V7 11/19] famfs_fuse: Basic fuse kernel ABI enablement for famfs

John Groves
2026-01-18T22:32:56Z

From: John Groves <john@groves.net>

This patch starts the kernel ABI enablement of famfs in fuse.

- Kconfig: Add FUSE_FAMFS_DAX config parameter, to control
compilation of famfs within fuse.
- FUSE_DAX_FMAP flag in INIT request/reply
- fuse_conn->famfs_iomap (enable famfs-mapped files) to denote a
famfs-enabled connection

Reviewed-by: Joanne Koong <joannelkoong@gmail.com>
Signed-off-by: John Groves <john@groves.net>
---
fs/fuse/Kconfig | 14 ++++++++++++++
fs/fuse/fuse_i.h | 3 +++
fs/fuse/inode.c | 6 ++++++
include/uapi/linux/fuse.h | 5 +++++
4 files changed, 28 insertions(+)

diff --git a/fs/fuse/Kconfig b/fs/fuse/Kconfig
index 3a4ae632c94a..5ca9fae62c7b 100644
--- a/fs/fuse/Kconfig
+++ b/fs/fuse/Kconfig
@@ -76,3 +76,17 @@ config FUSE_IO_URING

If you want to allow fuse server/client communication through io-uring,
answer Y
+
+config FUSE_FAMFS_DAX
+ bool "FUSE support for fs-dax filesystems backed by devdax"
+ depends on FUSE_FS
+ depends on DEV_DAX
+ depends on FS_DAX
+ default FUSE_FS
+ help
+ This enables the fabric-attached memory file system (famfs),
+ which enables formatting devdax memory as a file system. Famfs
+ is primarily intended for scale-out shared access to
+ disaggregated memory.
+
+ To enable famfs or other fuse/fs-dax file systems, answer Y
diff --git a/fs/fuse/fuse_i.h b/fs/fuse/fuse_i.h
index 45e108dec771..2839efb219a9 100644
--- a/fs/fuse/fuse_i.h
+++ b/fs/fuse/fuse_i.h
@@ -921,6 +921,9 @@ struct fuse_conn {
/* Is synchronous FUSE_INIT allowed? */
unsigned int sync_init:1;

+ /* dev_dax_iomap support for famfs */
+ unsigned int famfs_iomap:1;
+
/* Use io_uring for communication */
unsigned int io_uring;

diff --git a/fs/fuse/inode.c b/fs/fuse/inode.c
index ed667920997f..acabf92a11f8 100644
--- a/fs/fuse/inode.c
+++ b/fs/fuse/inode.c
@@ -1456,6 +1456,10 @@ static void process_init_reply(struct fuse_mount *fm, struct fuse_args *args,

if (flags & FUSE_REQUEST_TIMEOUT)
timeout = arg->request_timeout;
+
+ if (IS_ENABLED(CONFIG_FUSE_FAMFS_DAX) &&
+ flags & FUSE_DAX_FMAP)
+ fc->famfs_iomap = 1;
} else {
ra_pages = fc->max_read / PAGE_SIZE;
fc->no_lock = 1;
@@ -1517,6 +1521,8 @@ static struct fuse_init_args *fuse_new_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 c13e1f9a2f12..25686f088e6a 100644
--- a/include/uapi/linux/fuse.h
+++ b/include/uapi/linux/fuse.h
@@ -240,6 +240,9 @@
* - add FUSE_COPY_FILE_RANGE_64
* - add struct fuse_copy_file_range_out
* - add FUSE_NOTIFY_PRUNE
+ *
+ * 7.46
+ * - Add FUSE_DAX_FMAP capability - ability to handle in-kernel fsdax maps
*/

#ifndef _LINUX_FUSE_H
@@ -448,6 +451,7 @@ struct fuse_file_lock {
* FUSE_OVER_IO_URING: Indicate that client supports io-uring
* FUSE_REQUEST_TIMEOUT: kernel supports timing out requests.
* init_out.request_timeout contains the timeout (in secs)
+ * FUSE_DAX_FMAP: kernel supports dev_dax_iomap (aka famfs) fmaps
*/
#define FUSE_ASYNC_READ (1 << 0)
#define FUSE_POSIX_LOCKS (1 << 1)
@@ -495,6 +499,7 @@ struct fuse_file_lock {
#define FUSE_ALLOW_IDMAP (1ULL << 40)
#define FUSE_OVER_IO_URING (1ULL << 41)
#define FUSE_REQUEST_TIMEOUT (1ULL << 42)
+#define FUSE_DAX_FMAP (1ULL << 43)

/**
* CUSE INIT request/reply flags
--
2.52.0