Re: [PATCH V7 14/19] famfs_fuse: GET_DAXDEV message and daxdev_table

Dave Jiang
2026-02-19T18:51:18Z

On 1/18/26 3:33 PM, John Groves wrote:
> From: John Groves <john@groves.net>
>
> - The new GET_DAXDEV message/response is added
> - The famfs.c:famfs_teardown() function is added as a primary teardown
> function for famfs.
> - The command it triggered by the update_daxdev_table() call, if there
> are any daxdevs in the subject fmap that are not represented in the
> daxdev_table yet.
> - fs/namei.c: export may_open_dev()
>
> Signed-off-by: John Groves <john@groves.net>
> ---
> fs/fuse/famfs.c | 230 +++++++++++++++++++++++++++++++++++++-
> fs/fuse/famfs_kfmap.h | 26 +++++
> fs/fuse/fuse_i.h | 19 ++++
> fs/fuse/inode.c | 7 +-
> fs/namei.c | 1 +
> include/uapi/linux/fuse.h | 20 ++++
> 6 files changed, 301 insertions(+), 2 deletions(-)
>
> diff --git a/fs/fuse/famfs.c b/fs/fuse/famfs.c
> index a9728e11f1dd..7aa2eb2e99bf 100644
> --- a/fs/fuse/famfs.c
> +++ b/fs/fuse/famfs.c
> @@ -21,6 +21,231 @@
> #include "famfs_kfmap.h"
> #include "fuse_i.h"
>
> +/*
> + * famfs_teardown()
> + *
> + * Deallocate famfs metadata for a fuse_conn
> + */
> +void
> +famfs_teardown(struct fuse_conn *fc)
> +{
> + struct famfs_dax_devlist *devlist = fc->dax_devlist;
> + int i;
> +
> + fc->dax_devlist = NULL;
> +
> + if (!devlist)
> + return;
> +
> + if (!devlist->devlist)
> + goto out;

I think if you declare devlist with __free(), you can just return instead of having a goto.

DJ

> +
> + /* Close & release all the daxdevs in our table */
> + for (i = 0; i < devlist->nslots; i++) {
> + struct famfs_daxdev *dd = &devlist->devlist[i];
> +
> + if (!dd->valid)
> + continue;
> +
> + /* Release reference from dax_dev_get() */
> + if (dd->devp)
> + put_dax(dd->devp);
> +
> + kfree(dd->name);
> + }
> + kfree(devlist->devlist);
> +
> +out:
> + kfree(devlist);
> +}
> +
> +static int
> +famfs_verify_daxdev(const char *pathname, dev_t *devno)
> +{
> + struct inode *inode;
> + struct path path;
> + int err;
> +
> + if (!pathname || !*pathname)
> + return -EINVAL;
> +
> + err = kern_path(pathname, LOOKUP_FOLLOW, &path);
> + if (err)
> + return err;
> +
> + inode = d_backing_inode(path.dentry);
> + if (!S_ISCHR(inode->i_mode)) {
> + err = -EINVAL;
> + goto out_path_put;
> + }
> +
> + if (!may_open_dev(&path)) { /* had to export this */
> + err = -EACCES;
> + goto out_path_put;
> + }
> +
> + *devno = inode->i_rdev;
> +
> +out_path_put:
> + path_put(&path);
> + return err;
> +}
> +
> +/**
> + * famfs_fuse_get_daxdev() - Retrieve info for a DAX device from fuse server
> + *
> + * Send a GET_DAXDEV message to the fuse server to retrieve info on a
> + * dax device.
> + *
> + * @fm: fuse_mount
> + * @index: the index of the dax device; daxdevs are referred to by index
> + * in fmaps, and the server resolves the index to a particular daxdev
> + *
> + * Returns: 0=success
> + * -errno=failure
> + */
> +static int
> +famfs_fuse_get_daxdev(struct fuse_mount *fm, const u64 index)
> +{
> + struct fuse_daxdev_out daxdev_out = { 0 };
> + struct fuse_conn *fc = fm->fc;
> + struct famfs_daxdev *daxdev;
> + int rc;
> +
> + FUSE_ARGS(args);
> +
> + /* Store the daxdev in our table */
> + if (index >= fc->dax_devlist->nslots) {
> + pr_err("%s: index(%lld) > nslots(%d)\n",
> + __func__, index, fc->dax_devlist->nslots);
> + return -EINVAL;
> + }
> +
> + args.opcode = FUSE_GET_DAXDEV;
> + args.nodeid = index;
> +
> + args.in_numargs = 0;
> +
> + args.out_numargs = 1;
> + args.out_args[0].size = sizeof(daxdev_out);
> + args.out_args[0].value = &daxdev_out;
> +
> + /* Send GET_DAXDEV command */
> + rc = fuse_simple_request(fm, &args);
> + if (rc) {
> + pr_err("%s: rc=%d from fuse_simple_request()\n",
> + __func__, rc);
> + /* Error will be that the payload is smaller than FMAP_BUFSIZE,
> + * which is the max we can handle. Empty payload handled below.
> + */
> + return rc;
> + }
> +
> + scoped_guard(rwsem_write, &fc->famfs_devlist_sem) {
> + daxdev = &fc->dax_devlist->devlist[index];
> +
> + /* Abort if daxdev is now valid (races are possible here) */
> + if (daxdev->valid) {
> + pr_debug("%s: daxdev already known\n", __func__);
> + return 0;
> + }
> +
> + /* Verify dev is valid and can be opened and gets the devno */
> + rc = famfs_verify_daxdev(daxdev_out.name, &daxdev->devno);
> + if (rc) {
> + pr_err("%s: rc=%d from famfs_verify_daxdev()\n",
> + __func__, rc);
> + return rc;
> + }
> +
> + daxdev->name = kstrdup(daxdev_out.name, GFP_KERNEL);
> + if (!daxdev->name)
> + return -ENOMEM;
> +
> + /* This will fail if it's not a dax device */
> + daxdev->devp = dax_dev_get(daxdev->devno);
> + if (!daxdev->devp) {
> + pr_warn("%s: device %s not found or not dax\n",
> + __func__, daxdev_out.name);
> + kfree(daxdev->name);
> + daxdev->name = NULL;
> + return -ENODEV;
> + }
> +
> + wmb(); /* All other fields must be visible before valid */
> + daxdev->valid = 1;
> + }
> +
> + return 0;
> +}
> +
> +/**
> + * famfs_update_daxdev_table() - Update the daxdev table
> + * @fm: fuse_mount
> + * @meta: famfs_file_meta, in-memory format, built from a GET_FMAP response
> + *
> + * This function is called for each new file fmap, to verify whether all
> + * referenced daxdevs are already known (i.e. in the table). Any daxdev
> + * indices referenced in @meta but not in the table will be retrieved via
> + * famfs_fuse_get_daxdev() and added to the table
> + *
> + * Return: 0=success
> + * -errno=failure
> + */
> +static int
> +famfs_update_daxdev_table(
> + struct fuse_mount *fm,
> + const struct famfs_file_meta *meta)
> +{
> + struct famfs_dax_devlist *local_devlist;
> + struct fuse_conn *fc = fm->fc;
> + int indices_to_fetch[MAX_DAXDEVS];
> + int n_to_fetch = 0;
> + int err;
> +
> + /* First time through we will need to allocate the dax_devlist */
> + if (!fc->dax_devlist) {
> + local_devlist = kcalloc(1, sizeof(*fc->dax_devlist), GFP_KERNEL);
> + if (!local_devlist)
> + return -ENOMEM;
> +
> + local_devlist->nslots = MAX_DAXDEVS;
> +
> + local_devlist->devlist = kcalloc(MAX_DAXDEVS,
> + sizeof(struct famfs_daxdev),
> + GFP_KERNEL);
> + if (!local_devlist->devlist) {
> + kfree(local_devlist);
> + return -ENOMEM;
> + }
> +
> + /* We don't need famfs_devlist_sem here because we use cmpxchg */
> + if (cmpxchg(&fc->dax_devlist, NULL, local_devlist) != NULL) {
> + kfree(local_devlist->devlist);
> + kfree(local_devlist); /* another thread beat us to it */
> + }
> + }
> +
> + /* Collect indices that need fetching while holding read lock */
> + scoped_guard(rwsem_read, &fc->famfs_devlist_sem) {
> + unsigned long i;
> +
> + for_each_set_bit(i, (unsigned long *)&meta->dev_bitmap, MAX_DAXDEVS) {
> + if (!(fc->dax_devlist->devlist[i].valid))
> + indices_to_fetch[n_to_fetch++] = i;
> + }
> + }
> +
> + /* Fetch needed daxdevs outside the read lock */
> + for (int j = 0; j < n_to_fetch; j++) {
> + err = famfs_fuse_get_daxdev(fm, indices_to_fetch[j]);
> + if (err)
> + pr_err("%s: failed to get daxdev=%d\n",
> + __func__, indices_to_fetch[j]);
> + }
> +
> + return 0;
> +}
>
> /***************************************************************************/
>
> @@ -184,7 +409,7 @@ famfs_fuse_meta_alloc(
> /* ie_in = one interleaved extent in fmap_buf */
> ie_in = fmap_buf + next_offset;
>
> - /* Move past one interleaved extent header in fmap_buf */
> + /* Move past 1 interleaved extent header in fmap_buf */
> next_offset += sizeof(*ie_in);
> if (next_offset > fmap_buf_size) {
> pr_err("%s:%d: fmap_buf underflow offset/size %ld/%ld\n",
> @@ -329,6 +554,9 @@ famfs_file_init_dax(
> if (rc)
> goto errout;
>
> + /* Make sure this fmap doesn't reference any unknown daxdevs */
> + famfs_update_daxdev_table(fm, meta);
> +
> /* Publish the famfs metadata on fi->famfs_meta */
> inode_lock(inode);
>
> diff --git a/fs/fuse/famfs_kfmap.h b/fs/fuse/famfs_kfmap.h
> index 18ab22bcc5a1..eb9f70b5cb81 100644
> --- a/fs/fuse/famfs_kfmap.h
> +++ b/fs/fuse/famfs_kfmap.h
> @@ -64,4 +64,30 @@ struct famfs_file_meta {
> };
> };
>
> +/*
> + * famfs_daxdev - tracking struct for a daxdev within a famfs file system
> + *
> + * This is the in-memory daxdev metadata that is populated by parsing
> + * the responses to GET_FMAP messages
> + */
> +struct famfs_daxdev {
> + /* Include dev uuid? */
> + bool valid;
> + bool error;
> + dev_t devno;
> + struct dax_device *devp;
> + char *name;
> +};
> +
> +#define MAX_DAXDEVS 24
> +
> +/*
> + * famfs_dax_devlist - list of famfs_daxdev's
> + */
> +struct famfs_dax_devlist {
> + int nslots;
> + int ndevs;
> + struct famfs_daxdev *devlist;
> +};
> +
> #endif /* FAMFS_KFMAP_H */
> diff --git a/fs/fuse/fuse_i.h b/fs/fuse/fuse_i.h
> index dbfec5b9c6e1..83e24cee994b 100644
> --- a/fs/fuse/fuse_i.h
> +++ b/fs/fuse/fuse_i.h
> @@ -1006,6 +1006,11 @@ struct fuse_conn {
> /* Request timeout (in jiffies). 0 = no timeout */
> unsigned int req_timeout;
> } timeout;
> +
> +#if IS_ENABLED(CONFIG_FUSE_FAMFS_DAX)
> + struct rw_semaphore famfs_devlist_sem;
> + struct famfs_dax_devlist *dax_devlist;
> +#endif
> };
>
> /*
> @@ -1647,6 +1652,8 @@ int famfs_file_init_dax(struct fuse_mount *fm,
> size_t fmap_size);
> void __famfs_meta_free(void *map);
>
> +void famfs_teardown(struct fuse_conn *fc);
> +
> /* Set fi->famfs_meta = NULL regardless of prior value */
> static inline void famfs_meta_init(struct fuse_inode *fi)
> {
> @@ -1668,6 +1675,11 @@ static inline void famfs_meta_free(struct fuse_inode *fi)
> }
> }
>
> +static inline void famfs_init_devlist_sem(struct fuse_conn *fc)
> +{
> + init_rwsem(&fc->famfs_devlist_sem);
> +}
> +
> static inline int fuse_file_famfs(struct fuse_inode *fi)
> {
> return (READ_ONCE(fi->famfs_meta) != NULL);
> @@ -1677,6 +1689,9 @@ int fuse_get_fmap(struct fuse_mount *fm, struct inode *inode);
>
> #else /* !CONFIG_FUSE_FAMFS_DAX */
>
> +static inline void famfs_teardown(struct fuse_conn *fc)
> +{
> +}
> static inline struct fuse_backing *famfs_meta_set(struct fuse_inode *fi,
> void *meta)
> {
> @@ -1687,6 +1702,10 @@ static inline void famfs_meta_free(struct fuse_inode *fi)
> {
> }
>
> +static inline void famfs_init_devlist_sem(struct fuse_conn *fc)
> +{
> +}
> +
> static inline int fuse_file_famfs(struct fuse_inode *fi)
> {
> return 0;
> diff --git a/fs/fuse/inode.c b/fs/fuse/inode.c
> index b9933d0fbb9f..c5c7f2aeda3f 100644
> --- a/fs/fuse/inode.c
> +++ b/fs/fuse/inode.c
> @@ -1047,6 +1047,9 @@ void fuse_conn_put(struct fuse_conn *fc)
> WARN_ON(atomic_read(&bucket->count) != 1);
> kfree(bucket);
> }
> + if (IS_ENABLED(CONFIG_FUSE_FAMFS_DAX))
> + famfs_teardown(fc);
> +
> if (IS_ENABLED(CONFIG_FUSE_PASSTHROUGH))
> fuse_backing_files_free(fc);
> call_rcu(&fc->rcu, delayed_release);
> @@ -1476,8 +1479,10 @@ static void process_init_reply(struct fuse_mount *fm, struct fuse_args *args,
> u64 in_flags = ((u64)ia->in.flags2 << 32)
> | ia->in.flags;
>
> - if (in_flags & FUSE_DAX_FMAP)
> + if (in_flags & FUSE_DAX_FMAP) {
> + famfs_init_devlist_sem(fc);
> fc->famfs_iomap = 1;
> + }
> }
> } else {
> ra_pages = fc->max_read / PAGE_SIZE;
> diff --git a/fs/namei.c b/fs/namei.c
> index cf16b6822dd3..99ac58975394 100644
> --- a/fs/namei.c
> +++ b/fs/namei.c
> @@ -4171,6 +4171,7 @@ bool may_open_dev(const struct path *path)
> return !(path->mnt->mnt_flags & MNT_NODEV) &&
> !(path->mnt->mnt_sb->s_iflags & SB_I_NODEV);
> }
> +EXPORT_SYMBOL(may_open_dev);
>
> static int may_open(struct mnt_idmap *idmap, const struct path *path,
> int acc_mode, int flag)
> diff --git a/include/uapi/linux/fuse.h b/include/uapi/linux/fuse.h
> index cf678bebbfe0..1b82895108be 100644
> --- a/include/uapi/linux/fuse.h
> +++ b/include/uapi/linux/fuse.h
> @@ -247,6 +247,9 @@
> * - struct fuse_famfs_simple_ext
> * - struct fuse_famfs_iext
> * - struct fuse_famfs_fmap_header
> + * - Add the following structs for the GET_DAXDEV message and reply
> + * - struct fuse_get_daxdev_in
> + * - struct fuse_get_daxdev_out
> * - Add the following enumerated types
> * - enum fuse_famfs_file_type
> * - enum famfs_ext_type
> @@ -678,6 +681,7 @@ enum fuse_opcode {
>
> /* Famfs / devdax opcodes */
> FUSE_GET_FMAP = 54,
> + FUSE_GET_DAXDEV = 55,
>
> /* CUSE specific operations */
> CUSE_INIT = 4096,
> @@ -1369,6 +1373,22 @@ struct fuse_famfs_fmap_header {
> uint64_t reserved1;
> };
>
> +struct fuse_get_daxdev_in {
> + uint32_t daxdev_num;
> +};
> +
> +#define DAXDEV_NAME_MAX 256
> +
> +/* fuse_daxdev_out has enough space for a uuid if we need it */
> +struct fuse_daxdev_out {
> + uint16_t index;
> + uint16_t reserved;
> + uint32_t reserved2;
> + uint64_t reserved3;
> + uint64_t reserved4;
> + char name[DAXDEV_NAME_MAX];
> +};
> +
> static inline int32_t fmap_msg_min_size(void)
> {
> /* Smallest fmap message is a header plus one simple extent */