Re: [PATCH V7 12/19] famfs_fuse: Plumb the GET_FMAP message/response

Dave Jiang
2026-02-19T17:12:23Z

On 1/18/26 3:33 PM, John Groves wrote:
> From: John Groves <john@groves.net>
>
> Upon completion of an OPEN, if we're in famfs-mode we do a GET_FMAP to
> retrieve and cache up the file-to-dax map in the kernel. If this
> succeeds, read/write/mmap are resolved direct-to-dax with no upcalls.
>
> Signed-off-by: John Groves <john@groves.net>
> ---
> MAINTAINERS | 8 +++++
> fs/fuse/Makefile | 1 +
> fs/fuse/famfs.c | 74 +++++++++++++++++++++++++++++++++++++++
> 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 | 7 ++++
> 8 files changed, 176 insertions(+), 8 deletions(-)
> create mode 100644 fs/fuse/famfs.c
>
> diff --git a/MAINTAINERS b/MAINTAINERS
> index 10aa5120d93f..e3d0aa5eb361 100644
> --- a/MAINTAINERS
> +++ b/MAINTAINERS
> @@ -10379,6 +10379,14 @@ F: fs/fuse/
> F: include/uapi/linux/fuse.h
> F: tools/testing/selftests/filesystems/fuse/
>
> +FUSE [FAMFS Fabric-Attached Memory File System]
> +M: John Groves <jgroves@micron.com>
> +M: John Groves <John@Groves.net>
> +L: linux-cxl@vger.kernel.org
> +L: linux-fsdevel@vger.kernel.org
> +S: Supported
> +F: fs/fuse/famfs.c
> +
> FUTEX SUBSYSTEM
> M: Thomas Gleixner <tglx@kernel.org>
> M: Ingo Molnar <mingo@redhat.com>
> diff --git a/fs/fuse/Makefile b/fs/fuse/Makefile
> index 22ad9538dfc4..3f8dcc8cbbd0 100644
> --- a/fs/fuse/Makefile
> +++ b/fs/fuse/Makefile
> @@ -17,5 +17,6 @@ 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..615819cc922d
> --- /dev/null
> +++ b/fs/fuse/famfs.c
> @@ -0,0 +1,74 @@
> +// 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 PAGE_SIZE
> +
> +int
> +fuse_get_fmap(struct fuse_mount *fm, struct inode *inode)

keep the return int on the same line?

> +{
> + void *fmap_buf __free(kfree) = NULL;

Should do the variable declaration when you do the kzalloc(). That way you can avoid any potential use before check issues.

> + struct fuse_inode *fi = get_fuse_inode(inode);
> + size_t fmap_bufsize = FMAP_BUFSIZE;
> + u64 nodeid = get_node_id(inode);
> + ssize_t fmap_size;
> + int rc;
> +
> + FUSE_ARGS(args);
> +
> + /* Don't retrieve if we already have the famfs metadata */
> + if (fi->famfs_meta)
> + return 0;
> +
> + fmap_buf = kzalloc(FMAP_BUFSIZE, GFP_KERNEL);
> + if (!fmap_buf)
> + return -EIO;

-ENOMEM?

DJ
> +
> + args.opcode = FUSE_GET_FMAP;
> + args.nodeid = nodeid;
> +
> + /* Variable-sized output buffer
> + * this causes fuse_simple_request() to return the size of the
> + * output payload
> + */
> + args.out_argvar = true;
> + args.out_numargs = 1;
> + args.out_args[0].size = fmap_bufsize;
> + args.out_args[0].value = fmap_buf;
> +
> + /* Send GET_FMAP command */
> + rc = fuse_simple_request(fm, &args);
> + if (rc < 0) {
> + pr_err("%s: err=%d from fuse_simple_request()\n",
> + __func__, rc);
> + return rc;
> + }
> + fmap_size = rc;
> +
> + /* 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.
> + */
> +
> + return 0;
> +}
> diff --git a/fs/fuse/file.c b/fs/fuse/file.c
> index 093569033ed1..1f64bf68b5ee 100644
> --- a/fs/fuse/file.c
> +++ b/fs/fuse/file.c
> @@ -277,6 +277,16 @@ 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);
> @@ -284,12 +294,14 @@ 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 2839efb219a9..b66b5ca0bc11 100644
> --- a/fs/fuse/fuse_i.h
> +++ b/fs/fuse/fuse_i.h
> @@ -223,6 +223,14 @@ struct fuse_inode {
> * 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 */
> @@ -1511,11 +1519,8 @@ 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))
> @@ -1634,4 +1639,59 @@ 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 acabf92a11f8..f2d742d723dc 100644
> --- a/fs/fuse/inode.c
> +++ b/fs/fuse/inode.c
> @@ -120,6 +120,9 @@ 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:
> @@ -141,6 +144,9 @@ 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);
> }
>
> @@ -162,7 +168,7 @@ 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..9eff9083d3b5 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,
>
> @@ -1313,4 +1316,8 @@ struct fuse_uring_cmd_req {
> uint8_t padding[6];
> };
>
> +/* Famfs fmap message components */
> +
> +#define FAMFS_FMAP_MAX 32768 /* Largest supported fmap message */
> +
> #endif /* _LINUX_FUSE_H */