Message-ID: <0100019bc8346445-aa717c06-5252-4bcf-9cd5-91748c2d932d-000000@email.amazonses.com>
Date: Fri, 16 Jan 2026 19:07:15 +0000
From: John Groves <john@...alactic.com>
To: John Groves <John@...ves.net>, 
	Miklos Szeredi <miklos@...redi.hu>, 
	Dan Williams <dan.j.williams@...el.com>, 
	Bernd Schubert <bschubert@....com>, 
	Alison Schofield <alison.schofield@...el.com>
Cc: John Groves <jgroves@...ron.com>, 
	John Groves <jgroves@...tmail.com>, 
	Jonathan Corbet <corbet@....net>, 
	Vishal Verma <vishal.l.verma@...el.com>, 
	Dave Jiang <dave.jiang@...el.com>, 
	Matthew Wilcox <willy@...radead.org>, 
	Jan Kara <jack@...e.cz>, 
	Alexander Viro <viro@...iv.linux.org.uk>, 
	David Hildenbrand <david@...nel.org>, 
	Christian Brauner <brauner@...nel.org>, 
	Darrick J . Wong <djwong@...nel.org>, 
	Randy Dunlap <rdunlap@...radead.org>, 
	Jeff Layton <jlayton@...nel.org>, 
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	linux-fsdevel@...r.kernel.org <linux-fsdevel@...r.kernel.org>
Subject: [PATCH V5 14/19] famfs_fuse: GET_DAXDEV message and daxdev_table

From: John Groves <john@...ves.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@...ves.net>
---
 fs/fuse/famfs.c           | 228 +++++++++++++++++++++++++++++++++++++-
 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, 299 insertions(+), 2 deletions(-)

diff --git a/fs/fuse/famfs.c b/fs/fuse/famfs.c
index e3b5f204de82..438fc286d1a6 100644
--- a/fs/fuse/famfs.c
+++ b/fs/fuse/famfs.c
@@ -21,6 +21,229 @@
 #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;
+
+	/* 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;
+		}
+
+		/* 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);
+			return -ENODEV;
+		}
+
+		daxdev->name = kstrdup(daxdev_out.name, GFP_KERNEL);
+		if (!daxdev->name)
+			return -ENOMEM;
+
+		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;
+}
 
 /***************************************************************************/
 
@@ -183,7 +406,7 @@ 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",
@@ -322,6 +545,9 @@ 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);
 	if (fi->famfs_meta) {
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 */
-- 
2.52.0

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