Re: [PATCH V3 14/21] famfs_fuse: Plumb the GET_FMAP message/response

[Jonathan Cameron <jonathan.cameron@huawei.com>]

On Wed,  7 Jan 2026 09:33:23 -0600
John Groves <John@Groves.net> wrote:

> 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>
A few things inline.

J

> diff --git a/fs/fuse/famfs.c b/fs/fuse/famfs.c
> new file mode 100644
> index 000000000000..0f7e3f00e1e7
> --- /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 2023-2025 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/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)
> +{
> +	struct fuse_inode *fi = get_fuse_inode(inode);
> +	size_t fmap_bufsize = FMAP_BUFSIZE;
> +	u64 nodeid = get_node_id(inode);
> +	ssize_t fmap_size;
> +	void *fmap_buf;
> +	int rc;
> +
> +	FUSE_ARGS(args);
> +
> +	/* Don't retrieve if we already have the famfs metadata */
> +	if (fi->famfs_meta)
> +		return 0;
> +
> +	fmap_buf = kcalloc(1, FMAP_BUFSIZE, GFP_KERNEL);

If there is only ever 1, does kcalloc() make sense over kzalloc()?

> +	if (!fmap_buf)
> +		return -EIO;
> +
> +	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);

Leaks the fmap_buf?  Maybe use a __free() so no need to keep track of htat.


> +		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.
> +	 */
> +
> +	kfree(fmap_buf);
> +	return 0;
> +}

> diff --git a/fs/fuse/fuse_i.h b/fs/fuse/fuse_i.h
> index 84d0ee2a501d..691c7850cf4e 100644
> --- a/fs/fuse/fuse_i.h
> +++ b/fs/fuse/fuse_i.h
> @@ -223,6 +223,14 @@ struct fuse_inode {

>  
> +static inline struct fuse_backing *famfs_meta_set(struct fuse_inode *fi,
> +						       void *meta)
> +{
> +#if IS_ENABLED(CONFIG_FUSE_FAMFS_DAX)
> +	return xchg(&fi->famfs_meta, meta);
> +#else
> +	return NULL;
> +#endif
> +}
> +
> +static inline void famfs_meta_free(struct fuse_inode *fi)
> +{
> +	/* Stub wil be connected in a subsequent commit */
> +}
> +
> +static inline int fuse_file_famfs(struct fuse_inode *fi)
> +{
> +#if IS_ENABLED(CONFIG_FUSE_FAMFS_DAX)
> +	return (READ_ONCE(fi->famfs_meta) != NULL);
> +#else
> +	return 0;
> +#endif
> +}
> +
> +#if IS_ENABLED(CONFIG_FUSE_FAMFS_DAX)
> +int fuse_get_fmap(struct fuse_mount *fm, struct inode *inode);
> +#else
> +static inline int
> +fuse_get_fmap(struct fuse_mount *fm, struct inode *inode)
> +{
> +	return 0;
> +}
> +#endif
I'd do a single block under one if IS_ENABLED() and then use an else
for the stubs.   Should end up more readable.

Jonathan