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

[Dave Jiang <dave.jiang@intel.com>]

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 */