On Thu, Jul 3, 2025 at 8:51 PM John Groves <John@xxxxxxxxxx> 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.
>
> GET_FMAP has a variable-size response payload, and the allocated size
> is sent in the in_args[0].size field. If the fmap would overflow the
> message, the fuse server sends a reply of size 'sizeof(uint32_t)' which
> specifies the size of the fmap message. Then the kernel can realloc a
> large enough buffer and try again.
>
> Signed-off-by: John Groves <john@xxxxxxxxxx>
> ---
> fs/fuse/file.c | 84 +++++++++++++++++++++++++++++++++++++++
> fs/fuse/fuse_i.h | 36 ++++++++++++++++-
> fs/fuse/inode.c | 19 +++++++--
> fs/fuse/iomode.c | 2 +-
> include/uapi/linux/fuse.h | 18 +++++++++
> 5 files changed, 154 insertions(+), 5 deletions(-)
>
> diff --git a/fs/fuse/file.c b/fs/fuse/file.c
> index 93b82660f0c8..8616fb0a6d61 100644
> --- a/fs/fuse/file.c
> +++ b/fs/fuse/file.c
> @@ -230,6 +230,77 @@ static void fuse_truncate_update_attr(struct inode *inode, struct file *file)
> fuse_invalidate_attr_mask(inode, FUSE_STATX_MODSIZE);
> }
>
> +#if IS_ENABLED(CONFIG_FUSE_FAMFS_DAX)
We generally try to avoid #ifdef blocks in c files
keep them mostly in h files and use in c files
if (IS_ENABLED(CONFIG_FUSE_FAMFS_DAX))
also #if IS_ENABLED(CONFIG_FUSE_FAMFS_DAX)
it a bit strange for a bool Kconfig because it looks too
much like the c code, so I prefer
#ifdef CONFIG_FUSE_FAMFS_DAX
when you have to use it
If you need entire functions compiled out, why not put them in famfs.c?
> +
> +#define FMAP_BUFSIZE 4096
> +
> +static int
> +fuse_get_fmap(struct fuse_mount *fm, struct inode *inode, u64 nodeid)
> +{
> + struct fuse_get_fmap_in inarg = { 0 };
> + size_t fmap_bufsize = FMAP_BUFSIZE;
> + ssize_t fmap_size;
> + int retries = 1;
> + void *fmap_buf;
> + int rc;
> +
> + FUSE_ARGS(args);
> +
> + fmap_buf = kcalloc(1, FMAP_BUFSIZE, GFP_KERNEL);
> + if (!fmap_buf)
> + return -EIO;
> +
> + retry_once:
> + inarg.size = fmap_bufsize;
> +
> + args.opcode = FUSE_GET_FMAP;
> + args.nodeid = nodeid;
> +
> + args.in_numargs = 1;
> + args.in_args[0].size = sizeof(inarg);
> + args.in_args[0].value = &inarg;
> +
> + /* 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;
> +
> + if (retries && fmap_size == sizeof(uint32_t)) {
> + /* fmap size exceeded fmap_bufsize;
> + * actual fmap size returned in fmap_buf;
> + * realloc and retry once
> + */
> + fmap_bufsize = *((uint32_t *)fmap_buf);
> +
> + --retries;
> + kfree(fmap_buf);
> + fmap_buf = kcalloc(1, fmap_bufsize, GFP_KERNEL);
> + if (!fmap_buf)
> + return -EIO;
> +
> + goto retry_once;
> + }
> +
> + /* Will call famfs_file_init_dax() when that gets added */
> +
> + kfree(fmap_buf);
> + return 0;
> +}
> +#endif
> +
> static int fuse_open(struct inode *inode, struct file *file)
> {
> struct fuse_mount *fm = get_fuse_mount(inode);
> @@ -263,6 +334,19 @@ static int fuse_open(struct inode *inode, struct file *file)
>
> err = fuse_do_open(fm, get_node_id(inode), file, false);
> if (!err) {
> +#if IS_ENABLED(CONFIG_FUSE_FAMFS_DAX)
> + if (fm->fc->famfs_iomap) {
That should be
> + if (IS_ENABLED(CONFIG_FUSE_FAMFS_DAX) &&
> + fm->fc->famfs_iomap) {
> + if (S_ISREG(inode->i_mode)) {
> + int rc;
> + /* Get the famfs fmap */
> + rc = fuse_get_fmap(fm, inode,
> + get_node_id(inode));
> + if (rc)
> + pr_err("%s: fuse_get_fmap err=%d\n",
> + __func__, rc);
> + }
> + }
> +#endif
> ff = file->private_data;
> err = fuse_finish_open(inode, file);
> if (err)
> diff --git a/fs/fuse/fuse_i.h b/fs/fuse/fuse_i.h
> index f4ee61046578..e01d6e5c6e93 100644
> --- a/fs/fuse/fuse_i.h
> +++ b/fs/fuse/fuse_i.h
> @@ -193,6 +193,10 @@ struct fuse_inode {
> /** Reference to backing file in passthrough mode */
> struct fuse_backing *fb;
> #endif
> +
> +#if IS_ENABLED(CONFIG_FUSE_FAMFS_DAX)
> + void *famfs_meta;
> +#endif
> };
>
> /** FUSE inode state bits */
> @@ -945,6 +949,8 @@ struct fuse_conn {
> #endif
>
> #if IS_ENABLED(CONFIG_FUSE_FAMFS_DAX)
> + struct rw_semaphore famfs_devlist_sem;
> + struct famfs_dax_devlist *dax_devlist;
> char *shadow;
> #endif
> };
> @@ -1435,11 +1441,14 @@ void fuse_free_conn(struct fuse_conn *fc);
>
> /* dax.c */
>
> +static inline int fuse_file_famfs(struct fuse_inode *fi); /* forward */
> +
> /* This macro is used by virtio_fs, but now it also needs to filter for
> * "not famfs"
> */
> #define FUSE_IS_VIRTIO_DAX(fuse_inode) (IS_ENABLED(CONFIG_FUSE_DAX) \
> - && IS_DAX(&fuse_inode->inode))
> + && IS_DAX(&fuse_inode->inode) \
> + && !fuse_file_famfs(fuse_inode))
>
> ssize_t fuse_dax_read_iter(struct kiocb *iocb, struct iov_iter *to);
> ssize_t fuse_dax_write_iter(struct kiocb *iocb, struct iov_iter *from);
> @@ -1550,4 +1559,29 @@ extern void fuse_sysctl_unregister(void);
> #define fuse_sysctl_unregister() do { } while (0)
> #endif /* CONFIG_SYSCTL */
>
> +/* famfs.c */
> +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
> +}
> +
> #endif /* _FS_FUSE_I_H */
> diff --git a/fs/fuse/inode.c b/fs/fuse/inode.c
> index a7e1cf8257b0..b071d16f7d04 100644
> --- a/fs/fuse/inode.c
> +++ b/fs/fuse/inode.c
> @@ -117,6 +117,9 @@ 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:
> @@ -138,6 +141,13 @@ static void fuse_free_inode(struct inode *inode)
> if (IS_ENABLED(CONFIG_FUSE_PASSTHROUGH))
> fuse_backing_put(fuse_inode_backing(fi));
>
> +#if IS_ENABLED(CONFIG_FUSE_FAMFS_DAX)
> + if (S_ISREG(inode->i_mode) && fi->famfs_meta) {
> + famfs_meta_free(fi);
> + famfs_meta_set(fi, NULL);
> + }
> +#endif
> +
> kmem_cache_free(fuse_inode_cachep, fi);
> }
>
> @@ -1002,6 +1012,9 @@ void fuse_conn_init(struct fuse_conn *fc, struct fuse_mount *fm,
> if (IS_ENABLED(CONFIG_FUSE_PASSTHROUGH))
> fuse_backing_files_init(fc);
>
> + if (IS_ENABLED(CONFIG_FUSE_FAMFS_DAX))
> + pr_notice("%s: Kernel is FUSE_FAMFS_DAX capable\n", __func__);
> +
> INIT_LIST_HEAD(&fc->mounts);
> list_add(&fm->fc_entry, &fc->mounts);
> fm->fc = fc;
> @@ -1036,9 +1049,8 @@ void fuse_conn_put(struct fuse_conn *fc)
> }
> if (IS_ENABLED(CONFIG_FUSE_PASSTHROUGH))
> fuse_backing_files_free(fc);
> -#if IS_ENABLED(CONFIG_FUSE_FAMFS_DAX)
> - kfree(fc->shadow);
> -#endif
> + if (IS_ENABLED(CONFIG_FUSE_FAMFS_DAX))
> + kfree(fc->shadow);
> call_rcu(&fc->rcu, delayed_release);
> }
> }
> @@ -1425,6 +1437,7 @@ static void process_init_reply(struct fuse_mount *fm, struct fuse_args *args,
> * those capabilities, they are held here).
> */
> fc->famfs_iomap = 1;
> + init_rwsem(&fc->famfs_devlist_sem);
> }
> } else {
> ra_pages = fc->max_read / PAGE_SIZE;
> diff --git a/fs/fuse/iomode.c b/fs/fuse/iomode.c
> index aec4aecb5d79..443b337b0c05 100644
> --- a/fs/fuse/iomode.c
> +++ b/fs/fuse/iomode.c
> @@ -204,7 +204,7 @@ 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;
This is where fuse_is_dax() helper would be handy.
Thanks,
Amir.
