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Message-ID: <CAOQ4uxgeZQYEY8WsthebjV9f4qZd+nWJnCGWog6D1=wu+MFpSQ@mail.gmail.com>
Date: Thu, 6 Nov 2025 19:50:15 +0100
From: Amir Goldstein <amir73il@...il.com>
To: "Darrick J. Wong" <djwong@...nel.org>
Cc: miklos@...redi.hu, joannelkoong@...il.com, bernd@...ernd.com,
neal@...pa.dev, linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 04/31] fuse: adapt FUSE_DEV_IOC_BACKING_{OPEN,CLOSE} to
add new iomap devices
On Wed, Oct 29, 2025 at 1:56 AM Darrick J. Wong <djwong@...nel.org> wrote:
>
> From: Darrick J. Wong <djwong@...nel.org>
>
> Enable the use of the backing file open/close ioctls so that fuse
> servers can register block devices for use with iomap.
>
> Signed-off-by: "Darrick J. Wong" <djwong@...nel.org>
> ---
Reviewed-by: Amir Goldstein <amir73il@...il.com>
> fs/fuse/fuse_i.h | 5 ++
> include/uapi/linux/fuse.h | 3 +
> fs/fuse/Kconfig | 1
> fs/fuse/backing.c | 12 +++++
> fs/fuse/file_iomap.c | 101 +++++++++++++++++++++++++++++++++++++++++----
> fs/fuse/trace.c | 1
> 6 files changed, 113 insertions(+), 10 deletions(-)
>
>
> diff --git a/fs/fuse/fuse_i.h b/fs/fuse/fuse_i.h
> index 61fb65f3604d61..274de907257d94 100644
> --- a/fs/fuse/fuse_i.h
> +++ b/fs/fuse/fuse_i.h
> @@ -97,12 +97,14 @@ struct fuse_submount_lookup {
> };
>
> struct fuse_conn;
> +struct fuse_backing;
>
> /** Operations for subsystems that want to use a backing file */
> struct fuse_backing_ops {
> int (*may_admin)(struct fuse_conn *fc, uint32_t flags);
> int (*may_open)(struct fuse_conn *fc, struct file *file);
> int (*may_close)(struct fuse_conn *fc, struct file *file);
> + int (*post_open)(struct fuse_conn *fc, struct fuse_backing *fb);
> unsigned int type;
> int id_start;
> int id_end;
> @@ -112,6 +114,7 @@ struct fuse_backing_ops {
> struct fuse_backing {
> struct file *file;
> struct cred *cred;
> + struct block_device *bdev;
> const struct fuse_backing_ops *ops;
>
> /** refcount */
> @@ -1706,6 +1709,8 @@ static inline bool fuse_has_iomap(const struct inode *inode)
> {
> return get_fuse_conn(inode)->iomap;
> }
> +
> +extern const struct fuse_backing_ops fuse_iomap_backing_ops;
> #else
> # define fuse_iomap_enabled(...) (false)
> # define fuse_has_iomap(...) (false)
> diff --git a/include/uapi/linux/fuse.h b/include/uapi/linux/fuse.h
> index 7d709cf12b41a7..e571f8ceecbfad 100644
> --- a/include/uapi/linux/fuse.h
> +++ b/include/uapi/linux/fuse.h
> @@ -1136,7 +1136,8 @@ struct fuse_notify_prune_out {
>
> #define FUSE_BACKING_TYPE_MASK (0xFF)
> #define FUSE_BACKING_TYPE_PASSTHROUGH (0)
> -#define FUSE_BACKING_MAX_TYPE (FUSE_BACKING_TYPE_PASSTHROUGH)
> +#define FUSE_BACKING_TYPE_IOMAP (1)
> +#define FUSE_BACKING_MAX_TYPE (FUSE_BACKING_TYPE_IOMAP)
>
> #define FUSE_BACKING_FLAGS_ALL (FUSE_BACKING_TYPE_MASK)
>
> diff --git a/fs/fuse/Kconfig b/fs/fuse/Kconfig
> index bb867afe6e867c..52803c533f47f9 100644
> --- a/fs/fuse/Kconfig
> +++ b/fs/fuse/Kconfig
> @@ -75,6 +75,7 @@ config FUSE_IOMAP
> depends on FUSE_FS
> depends on BLOCK
> select FS_IOMAP
> + select FUSE_BACKING
> help
> Enable fuse servers to operate the regular file I/O path through
> the fs-iomap library in the kernel. This enables higher performance
> diff --git a/fs/fuse/backing.c b/fs/fuse/backing.c
> index b83a3c1b2dff7a..7786f6e5fd02f2 100644
> --- a/fs/fuse/backing.c
> +++ b/fs/fuse/backing.c
> @@ -90,6 +90,10 @@ fuse_backing_ops_from_map(const struct fuse_backing_map *map)
> #ifdef CONFIG_FUSE_PASSTHROUGH
> case FUSE_BACKING_TYPE_PASSTHROUGH:
> return &fuse_passthrough_backing_ops;
> +#endif
> +#ifdef CONFIG_FUSE_IOMAP
> + case FUSE_BACKING_TYPE_IOMAP:
> + return &fuse_iomap_backing_ops;
> #endif
> default:
> break;
> @@ -138,8 +142,16 @@ int fuse_backing_open(struct fuse_conn *fc, struct fuse_backing_map *map)
> fb->file = file;
> fb->cred = prepare_creds();
> fb->ops = ops;
> + fb->bdev = NULL;
> refcount_set(&fb->count, 1);
>
> + res = ops->post_open ? ops->post_open(fc, fb) : 0;
> + if (res) {
> + fuse_backing_free(fb);
> + fb = NULL;
> + goto out;
> + }
> +
> res = fuse_backing_id_alloc(fc, fb);
> if (res < 0) {
> fuse_backing_free(fb);
> diff --git a/fs/fuse/file_iomap.c b/fs/fuse/file_iomap.c
> index b6fc70068c5542..e4fea3bdc0c2ce 100644
> --- a/fs/fuse/file_iomap.c
> +++ b/fs/fuse/file_iomap.c
> @@ -319,10 +319,6 @@ static inline bool fuse_iomap_check_mapping(const struct inode *inode,
> return false;
> }
>
> - /* XXX: we don't support devices yet */
> - if (BAD_DATA(map->dev != FUSE_IOMAP_DEV_NULL))
> - return false;
> -
> /* No overflows in the device range, if supplied */
> if (map->addr != FUSE_IOMAP_NULL_ADDR &&
> BAD_DATA(check_add_overflow(map->addr, map->length, &end)))
> @@ -334,6 +330,7 @@ static inline bool fuse_iomap_check_mapping(const struct inode *inode,
> /* Convert a mapping from the server into something the kernel can use */
> static inline void fuse_iomap_from_server(struct inode *inode,
> struct iomap *iomap,
> + const struct fuse_backing *fb,
> const struct fuse_iomap_io *fmap)
> {
> iomap->addr = fmap->addr;
> @@ -341,7 +338,9 @@ static inline void fuse_iomap_from_server(struct inode *inode,
> iomap->length = fmap->length;
> iomap->type = fuse_iomap_type_from_server(fmap->type);
> iomap->flags = fuse_iomap_flags_from_server(fmap->flags);
> - iomap->bdev = inode->i_sb->s_bdev; /* XXX */
> +
> + iomap->bdev = fb ? fb->bdev : NULL;
> + iomap->dax_dev = NULL;
> }
>
> /* Convert a mapping from the kernel into something the server can use */
> @@ -392,6 +391,27 @@ static inline bool fuse_is_iomap_file_write(unsigned int opflags)
> return opflags & (IOMAP_WRITE | IOMAP_ZERO | IOMAP_UNSHARE);
> }
>
> +static inline struct fuse_backing *
> +fuse_iomap_find_dev(struct fuse_conn *fc, const struct fuse_iomap_io *map)
> +{
> + struct fuse_backing *ret = NULL;
> +
> + if (map->dev != FUSE_IOMAP_DEV_NULL && map->dev < INT_MAX)
> + ret = fuse_backing_lookup(fc, &fuse_iomap_backing_ops,
> + map->dev);
> +
> + switch (map->type) {
> + case FUSE_IOMAP_TYPE_MAPPED:
> + case FUSE_IOMAP_TYPE_UNWRITTEN:
> + /* Mappings backed by space must have a device/addr */
> + if (BAD_DATA(ret == NULL))
> + return ERR_PTR(-EFSCORRUPTED);
> + break;
> + }
> +
> + return ret;
> +}
> +
> static int fuse_iomap_begin(struct inode *inode, loff_t pos, loff_t count,
> unsigned opflags, struct iomap *iomap,
> struct iomap *srcmap)
> @@ -405,6 +425,8 @@ static int fuse_iomap_begin(struct inode *inode, loff_t pos, loff_t count,
> };
> struct fuse_iomap_begin_out outarg = { };
> struct fuse_mount *fm = get_fuse_mount(inode);
> + struct fuse_backing *read_dev = NULL;
> + struct fuse_backing *write_dev = NULL;
> FUSE_ARGS(args);
> int err;
>
> @@ -431,24 +453,44 @@ static int fuse_iomap_begin(struct inode *inode, loff_t pos, loff_t count,
> if (err)
> return err;
>
> + read_dev = fuse_iomap_find_dev(fm->fc, &outarg.read);
> + if (IS_ERR(read_dev))
> + return PTR_ERR(read_dev);
> +
> if (fuse_is_iomap_file_write(opflags) &&
> outarg.write.type != FUSE_IOMAP_TYPE_PURE_OVERWRITE) {
> + /* open the write device */
> + write_dev = fuse_iomap_find_dev(fm->fc, &outarg.write);
> + if (IS_ERR(write_dev)) {
> + err = PTR_ERR(write_dev);
> + goto out_read_dev;
> + }
> +
> /*
> * For an out of place write, we must supply the write mapping
> * via @iomap, and the read mapping via @srcmap.
> */
> - fuse_iomap_from_server(inode, iomap, &outarg.write);
> - fuse_iomap_from_server(inode, srcmap, &outarg.read);
> + fuse_iomap_from_server(inode, iomap, write_dev, &outarg.write);
> + fuse_iomap_from_server(inode, srcmap, read_dev, &outarg.read);
> } else {
> /*
> * For everything else (reads, reporting, and pure overwrites),
> * we can return the sole mapping through @iomap and leave
> * @srcmap unchanged from its default (HOLE).
> */
> - fuse_iomap_from_server(inode, iomap, &outarg.read);
> + fuse_iomap_from_server(inode, iomap, read_dev, &outarg.read);
> }
>
> - return 0;
> + /*
> + * XXX: if we ever want to support closing devices, we need a way to
> + * track the fuse_backing refcount all the way through bio endios.
> + * For now we put the refcount here because you can't remove an iomap
> + * device until unmount time.
> + */
> + fuse_backing_put(write_dev);
> +out_read_dev:
> + fuse_backing_put(read_dev);
> + return err;
> }
>
> /* Decide if we send FUSE_IOMAP_END to the fuse server */
> @@ -523,3 +565,44 @@ const struct iomap_ops fuse_iomap_ops = {
> .iomap_begin = fuse_iomap_begin,
> .iomap_end = fuse_iomap_end,
> };
> +
> +static int fuse_iomap_may_admin(struct fuse_conn *fc, unsigned int flags)
> +{
> + if (!fc->iomap)
> + return -EPERM;
> +
> + if (flags)
> + return -EINVAL;
> +
> + return 0;
> +}
> +
> +static int fuse_iomap_may_open(struct fuse_conn *fc, struct file *file)
> +{
> + if (!S_ISBLK(file_inode(file)->i_mode))
> + return -ENODEV;
> +
> + return 0;
> +}
> +
> +static int fuse_iomap_post_open(struct fuse_conn *fc, struct fuse_backing *fb)
> +{
> + fb->bdev = I_BDEV(fb->file->f_mapping->host);
> + return 0;
> +}
> +
> +static int fuse_iomap_may_close(struct fuse_conn *fc, struct file *file)
> +{
> + /* We only support closing iomap block devices at unmount */
> + return -EBUSY;
> +}
> +
> +const struct fuse_backing_ops fuse_iomap_backing_ops = {
> + .type = FUSE_BACKING_TYPE_IOMAP,
> + .id_start = 1,
> + .id_end = 1025, /* maximum 1024 block devices */
> + .may_admin = fuse_iomap_may_admin,
> + .may_open = fuse_iomap_may_open,
> + .may_close = fuse_iomap_may_close,
> + .post_open = fuse_iomap_post_open,
> +};
> diff --git a/fs/fuse/trace.c b/fs/fuse/trace.c
> index 93bd72efc98cd0..68d2eecb8559a5 100644
> --- a/fs/fuse/trace.c
> +++ b/fs/fuse/trace.c
> @@ -6,6 +6,7 @@
> #include "dev_uring_i.h"
> #include "fuse_i.h"
> #include "fuse_dev_i.h"
> +#include "iomap_i.h"
>
> #include <linux/pagemap.h>
>
>
>
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