[PATCH V11 3/9] famfs_fuse: Plumb the GET_FMAP message/response

[John Groves]

From: John Groves <john@xxxxxxxxxx>

On completion of an OPEN in famfs mode, issue a GET_FMAP request to the
server to retrieve the file's file-offset-to-dax map (fmap) and cache it
on the fuse_inode (fi->famfs_meta). Once the map is cached, read, write
and mmap are resolved directly to dax with no further upcalls.

- uapi: add the FUSE_GET_FMAP opcode.
- famfs.c: add fuse_get_fmap(), which retrieves the fmap into a
  kvmalloc'd buffer. The fmap size is not known in advance, so it uses a
  size probe: it starts with a PAGE_SIZE buffer and passes that size to
  the server (via fuse_getxattr_in.size). If the whole fmap does not
  fit, the server replies with just the header, whose fmap_size field
  reports the required size, and the kernel reallocates exactly that and
  retries once. The reply is bounded by FMAP_BUFSIZE_MAX (16 MiB); a
  larger fmap is rejected with -EFBIG. A famfs file is fixed-size, so a
  reply that reports a different size on the retry is rejected as a
  server bug.
- file.c: hook the OPEN path to fetch the fmap for regular files on a
  famfs connection; failure is fatal to the open.
- fuse_i.h/inode.c: add the fi->famfs_meta pointer and its init/free
  helpers.

The retrieved map is parsed into its in-memory form in the following
patch.

Signed-off-by: John Groves <john@xxxxxxxxxx>
---
 MAINTAINERS               |   7 ++
 fs/fuse/Makefile          |   1 +
 fs/fuse/famfs.c           | 134 ++++++++++++++++++++++++++++++++++++++
 fs/fuse/file.c            |  14 +++-
 fs/fuse/fuse_i.h          |  70 ++++++++++++++++++--
 fs/fuse/inode.c           |   8 ++-
 fs/fuse/iomode.c          |   2 +-
 include/uapi/linux/fuse.h |   3 +
 8 files changed, 231 insertions(+), 8 deletions(-)
 create mode 100644 fs/fuse/famfs.c

diff --git a/MAINTAINERS b/MAINTAINERS
index 806bd2d80d15..0d0fded4fddb 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -10713,6 +10713,13 @@ F:	fs/fuse/backing.c
 F:	fs/fuse/iomode.c
 F:	fs/fuse/passthrough.c
 
+FUSE FILESYSTEM [FAMFS Fabric-Attached Memory File System]
+M:	John Groves <John@xxxxxxxxxx>
+L:	linux-cxl@xxxxxxxxxxxxxxx
+L:	linux-fsdevel@xxxxxxxxxxxxxxx
+S:	Supported
+F:	fs/fuse/famfs.c
+
 FUTEX SUBSYSTEM
 M:	Thomas Gleixner <tglx@xxxxxxxxxx>
 M:	Ingo Molnar <mingo@xxxxxxxxxx>
diff --git a/fs/fuse/Makefile b/fs/fuse/Makefile
index 245e67852b03..66507b9cfe1f 100644
--- a/fs/fuse/Makefile
+++ b/fs/fuse/Makefile
@@ -18,5 +18,6 @@ fuse-$(CONFIG_FUSE_DAX) += dax.o
 fuse-$(CONFIG_FUSE_PASSTHROUGH) += passthrough.o backing.o
 fuse-$(CONFIG_SYSCTL) += sysctl.o
 fuse-$(CONFIG_FUSE_IO_URING) += dev_uring.o
+fuse-$(CONFIG_FUSE_FAMFS_DAX) += famfs.o
 
 virtiofs-y := virtio_fs.o
diff --git a/fs/fuse/famfs.c b/fs/fuse/famfs.c
new file mode 100644
index 000000000000..80e6640ac970
--- /dev/null
+++ b/fs/fuse/famfs.c
@@ -0,0 +1,134 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * famfs - dax file system for shared fabric-attached memory
+ *
+ * Copyright 2023-2026 Micron Technology, Inc.
+ *
+ * This file system, originally based on ramfs the dax support from xfs,
+ * is intended to allow multiple host systems to mount a common file system
+ * view of dax files that map to shared memory.
+ */
+
+#include <linux/cleanup.h>
+#include <linux/fs.h>
+#include <linux/mm.h>
+#include <linux/dax.h>
+#include <linux/iomap.h>
+#include <linux/path.h>
+#include <linux/namei.h>
+#include <linux/string.h>
+
+#include "fuse_i.h"
+
+
+#define FMAP_BUFSIZE_INIT PAGE_SIZE
+/*
+ * Largest GET_FMAP reply buffer we will kvmalloc. Any fmap whose whole message
+ * fits in this buffer is handled; there is no separate extent-count cap, so the
+ * effective extent limit is just this size / sizeof(simple_ext) (~699k extents
+ * => ~1.3 TiB per striped file at a 2 MiB chunk). kvmalloc-backed, so it may
+ * exceed the contiguous kmalloc limit. Matches the server's reply-buffer cap.
+ */
+#define FMAP_BUFSIZE_MAX (16 * 1024 * 1024)
+
+int fuse_get_fmap(struct fuse_mount *fm, struct inode *inode)
+{
+	struct fuse_inode *fi = get_fuse_inode(inode);
+	u64 nodeid = get_node_id(inode);
+	size_t bufsize = FMAP_BUFSIZE_INIT;
+	void *fmap_buf = NULL;
+	ssize_t fmap_size;
+	int attempt;
+	int rc;
+
+	/* Don't retrieve if we already have the famfs metadata */
+	if (fi->famfs_meta)
+		return 0;
+
+	/*
+	 * The fmap size is not known in advance. Start with a modest buffer and,
+	 * if the server reports (via the returned header's fmap_size) that the
+	 * whole fmap did not fit, reallocate exactly that size and retry once.
+	 * The server learns our buffer size from the request's
+	 * fuse_getxattr_in.size (GETXATTR-style size probe).
+	 */
+	for (attempt = 0; ; attempt++) {
+		struct fuse_getxattr_in in = { .size = bufsize };
+		struct fuse_famfs_fmap_header *fmh;
+		u32 required;
+
+		FUSE_ARGS(args);
+
+		fmap_buf = kvmalloc(bufsize, GFP_KERNEL);
+		if (!fmap_buf)
+			return -ENOMEM;
+
+		args.opcode = FUSE_GET_FMAP;
+		args.nodeid = nodeid;
+		args.in_numargs = 1;
+		args.in_args[0].size = sizeof(in);
+		args.in_args[0].value = &in;
+		/*
+		 * Variable-sized output buffer; fuse_simple_request() returns
+		 * the size of the output payload.
+		 */
+		args.out_argvar = true;
+		args.out_numargs = 1;
+		args.out_args[0].size = bufsize;
+		args.out_args[0].value = fmap_buf;
+
+		rc = fuse_simple_request(fm, &args);
+		if (rc < 0) {
+			pr_err("%s: err=%d from fuse_simple_request()\n",
+			       __func__, rc);
+			kvfree(fmap_buf);
+			return rc;
+		}
+		fmap_size = rc;
+
+		/* Need at least a header to learn the required size */
+		if (fmap_size < (ssize_t)sizeof(*fmh)) {
+			pr_err("%s: short fmap reply %zd\n", __func__, fmap_size);
+			kvfree(fmap_buf);
+			return -EIO;
+		}
+
+		fmh = fmap_buf;
+		required = fmh->fmap_size;
+
+		/* Whole fmap fit in the buffer -> parse it */
+		if (required <= bufsize)
+			break;
+
+		/* Too small: server sent only the header. Grow and retry once. */
+		kvfree(fmap_buf);
+		fmap_buf = NULL;
+
+		if (required > FMAP_BUFSIZE_MAX) {
+			pr_err("%s: fmap size %u exceeds max %zu\n",
+			       __func__, required, (size_t)FMAP_BUFSIZE_MAX);
+			return -EFBIG;
+		}
+		if (attempt >= 1) {
+			/*
+			 * A famfs file is fixed-size, so the server must report
+			 * the same fmap_size on the retry as on the first
+			 * request. A larger value means the file's size/fmap
+			 * changed between the two GET_FMAPs -- a server bug.
+			 */
+			pr_err("%s: fmap grew %zu -> %u across GET_FMAP retries; famfs file size must not change (server bug)\n",
+			       __func__, bufsize, required);
+			return -EINVAL;
+		}
+		bufsize = required;
+	}
+
+	/* We retrieved the "fmap" (the file's map to memory), but
+	 * we haven't used it yet. A call to famfs_file_init_dax() will be added
+	 * here in a subsequent patch, when we add the ability to attach
+	 * fmaps to files.
+	 */
+
+	kvfree(fmap_buf);
+	return 0;
+}
diff --git a/fs/fuse/file.c b/fs/fuse/file.c
index 995e37c935c6..b4e7b6a64587 100644
--- a/fs/fuse/file.c
+++ b/fs/fuse/file.c
@@ -282,6 +282,16 @@ static int fuse_open(struct inode *inode, struct file *file)
 	err = fuse_do_open(fm, get_node_id(inode), file, false);
 	if (!err) {
 		ff = file->private_data;
+
+		if ((fm->fc->famfs_iomap) && (S_ISREG(inode->i_mode))) {
+			/* Get the famfs fmap - failure is fatal */
+			err = fuse_get_fmap(fm, inode);
+			if (err) {
+				fuse_sync_release(fi, ff, file->f_flags);
+				goto out_nowrite;
+			}
+		}
+
 		err = fuse_finish_open(inode, file);
 		if (err)
 			fuse_sync_release(fi, ff, file->f_flags);
@@ -289,12 +299,14 @@ static int fuse_open(struct inode *inode, struct file *file)
 			fuse_truncate_update_attr(inode, file);
 	}
 
+out_nowrite:
 	if (is_wb_truncate || dax_truncate)
 		fuse_release_nowrite(inode);
 	if (!err) {
 		if (is_truncate)
 			truncate_pagecache(inode, 0);
-		else if (!(ff->open_flags & FOPEN_KEEP_CACHE))
+		else if (!(ff->open_flags & FOPEN_KEEP_CACHE) &&
+			 !fuse_file_famfs(fi))
 			invalidate_inode_pages2(inode->i_mapping);
 	}
 	if (dax_truncate)
diff --git a/fs/fuse/fuse_i.h b/fs/fuse/fuse_i.h
index 9c354118c931..5bacc5098620 100644
--- a/fs/fuse/fuse_i.h
+++ b/fs/fuse/fuse_i.h
@@ -236,6 +236,14 @@ struct fuse_inode {
 	 * be modified, so preserve the blocksize specified by the server.
 	 */
 	u8 cached_i_blkbits;
+
+#if IS_ENABLED(CONFIG_FUSE_FAMFS_DAX)
+	/* Pointer to the file's famfs metadata. Primary content is the
+	 * in-memory version of the fmap - the map from file's offset range
+	 * to DAX memory
+	 */
+	void *famfs_meta;
+#endif
 };
 
 /** FUSE inode state bits */
@@ -1218,11 +1226,8 @@ void fuse_free_conn(struct fuse_conn *fc);
 
 /* dax.c */
 
-static inline bool fuse_file_famfs(struct fuse_inode *fuse_inode) /* Will be superseded */
-{
-	(void)fuse_inode;
-	return false;
-}
+static inline int fuse_file_famfs(struct fuse_inode *fi); /* forward */
+
 #define FUSE_IS_VIRTIO_DAX(fuse_inode) (IS_ENABLED(CONFIG_FUSE_DAX)	\
 					&& IS_DAX(&(fuse_inode)->inode)  \
 					&& !fuse_file_famfs(fuse_inode))
@@ -1339,4 +1344,59 @@ extern void fuse_sysctl_unregister(void);
 #define fuse_sysctl_unregister()	do { } while (0)
 #endif /* CONFIG_SYSCTL */
 
+/* famfs.c */
+
+#if IS_ENABLED(CONFIG_FUSE_FAMFS_DAX)
+void __famfs_meta_free(void *map);
+
+/* Set fi->famfs_meta = NULL regardless of prior value */
+static inline void famfs_meta_init(struct fuse_inode *fi)
+{
+	fi->famfs_meta = NULL;
+}
+
+/* Set fi->famfs_meta iff the current value is NULL */
+static inline struct fuse_backing *famfs_meta_set(struct fuse_inode *fi,
+						  void *meta)
+{
+	return cmpxchg(&fi->famfs_meta, NULL, meta);
+}
+
+static inline void famfs_meta_free(struct fuse_inode *fi)
+{
+	famfs_meta_set(fi, NULL);
+}
+
+static inline int fuse_file_famfs(struct fuse_inode *fi)
+{
+	return (READ_ONCE(fi->famfs_meta) != NULL);
+}
+
+int fuse_get_fmap(struct fuse_mount *fm, struct inode *inode);
+
+#else /* !CONFIG_FUSE_FAMFS_DAX */
+
+static inline struct fuse_backing *famfs_meta_set(struct fuse_inode *fi,
+						  void *meta)
+{
+	return NULL;
+}
+
+static inline void famfs_meta_free(struct fuse_inode *fi)
+{
+}
+
+static inline int fuse_file_famfs(struct fuse_inode *fi)
+{
+	return 0;
+}
+
+static inline int
+fuse_get_fmap(struct fuse_mount *fm, struct inode *inode)
+{
+	return 0;
+}
+
+#endif /* CONFIG_FUSE_FAMFS_DAX */
+
 #endif /* _FS_FUSE_I_H */
diff --git a/fs/fuse/inode.c b/fs/fuse/inode.c
index c347471d04b6..e030a302120f 100644
--- a/fs/fuse/inode.c
+++ b/fs/fuse/inode.c
@@ -106,6 +106,9 @@ static struct inode *fuse_alloc_inode(struct super_block *sb)
 	if (IS_ENABLED(CONFIG_FUSE_PASSTHROUGH))
 		fuse_inode_backing_set(fi, NULL);
 
+	if (IS_ENABLED(CONFIG_FUSE_FAMFS_DAX))
+		famfs_meta_set(fi, NULL);
+
 	return &fi->inode;
 
 out_free_forget:
@@ -127,6 +130,9 @@ static void fuse_free_inode(struct inode *inode)
 	if (IS_ENABLED(CONFIG_FUSE_PASSTHROUGH))
 		fuse_backing_put(fuse_inode_backing(fi));
 
+	if (S_ISREG(inode->i_mode) && fuse_file_famfs(fi))
+		famfs_meta_free(fi);
+
 	kmem_cache_free(fuse_inode_cachep, fi);
 }
 
@@ -148,7 +154,7 @@ static void fuse_evict_inode(struct inode *inode)
 	/* Will write inode on close/munmap and in all other dirtiers */
 	WARN_ON(inode_state_read_once(inode) & I_DIRTY_INODE);
 
-	if (FUSE_IS_VIRTIO_DAX(fi))
+	if (FUSE_IS_VIRTIO_DAX(fi) || fuse_file_famfs(fi))
 		dax_break_layout_final(inode);
 
 	truncate_inode_pages_final(&inode->i_data);
diff --git a/fs/fuse/iomode.c b/fs/fuse/iomode.c
index 31ee7f3304c6..948148316ef0 100644
--- a/fs/fuse/iomode.c
+++ b/fs/fuse/iomode.c
@@ -203,7 +203,7 @@ int fuse_file_io_open(struct file *file, struct inode *inode)
 	 * io modes are not relevant with DAX and with server that does not
 	 * implement open.
 	 */
-	if (FUSE_IS_VIRTIO_DAX(fi) || !ff->args)
+	if (FUSE_IS_VIRTIO_DAX(fi) || fuse_file_famfs(fi) || !ff->args)
 		return 0;
 
 	/*
diff --git a/include/uapi/linux/fuse.h b/include/uapi/linux/fuse.h
index 25686f088e6a..d323c20e79bd 100644
--- a/include/uapi/linux/fuse.h
+++ b/include/uapi/linux/fuse.h
@@ -669,6 +669,9 @@ enum fuse_opcode {
 	FUSE_STATX		= 52,
 	FUSE_COPY_FILE_RANGE_64	= 53,
 
+	/* Famfs / devdax opcodes */
+	FUSE_GET_FMAP           = 54,
+
 	/* CUSE specific operations */
 	CUSE_INIT		= 4096,
 
-- 
2.53.0