[PATCH v13 11/12] famfs: Report device capacity via statfs so df works

From: John Groves <[email protected]>

Replace simple_statfs(), which reports zero blocks (so df omits the mount),
with famfs_statfs() reporting real capacity and usage.

Add dax_fsdev_size() in drivers/dax/fsdev.c, returning the size fsdev
caches at probe (dev_dax->cached_size - the sum of the device's ranges,
stable while bound), exported. It lives in fsdev.c because cached_size is
set only by the fsdev driver, and famfs only ever holds fsdev-mode daxdevs
(fs_dax_get() enforces DAXDRV_FSDEV_TYPE); famfs.ko therefore depends on
fsdev_dax.ko.

famfs tracks two byte counters under a new stats_sem:
 - total_capacity: summed in famfs_install_daxdev() from dax_fsdev_size(),
   covering the mount primary and every DAXDEV_OPEN secondary, counted once
   per daxdev (on the valid 0->1 transition).
 - used_capacity: summed in famfs_file_init_dax() from the fmap's mapped
   device bytes (superblock + log + data files).

famfs_statfs() reports total and free (total - used). Free is an
approximation of the userspace allocator's free space (it ignores allocator
gaps and reserved regions), which is adequate for df.

(Side note: I am the maintainer of drivers/dax/fsdev.c)

Signed-off-by: John Groves <[email protected]>
---
 drivers/dax/fsdev.c       | 19 +++++++++++++++
 fs/famfs/famfs_file.c     |  5 ++++
 fs/famfs/famfs_inode.c    | 49 ++++++++++++++++++++++++++++++++++-----
 fs/famfs/famfs_internal.h |  8 +++++++
 include/linux/dax.h       |  1 +
 5 files changed, 76 insertions(+), 6 deletions(-)

diff --git a/drivers/dax/fsdev.c b/drivers/dax/fsdev.c
index 188b2526bee4..a5b4b2d79428 100644
--- a/drivers/dax/fsdev.c
+++ b/drivers/dax/fsdev.c
@@ -104,6 +104,25 @@ static size_t fsdev_dax_recovery_write(struct dax_device *dax_dev, pgoff_t pgoff
 	return _copy_from_iter_flushcache(addr, bytes, i);
 }
 
+/**
+ * dax_fsdev_size() - total size in bytes of an fsdev dax device
+ * @dax_dev: the dax device (must be bound to this driver)
+ *
+ * Returns the size cached at probe time (sum of all ranges); it cannot change
+ * while the driver is bound. Only valid for fsdev dax devices - callers
+ * ensure that (e.g. fs_dax_get() enforces DAXDRV_FSDEV_TYPE). Returns 0 if the
+ * device is not alive.
+ */
+u64 dax_fsdev_size(struct dax_device *dax_dev)
+{
+	struct dev_dax *dev_dax = dax_get_private(dax_dev);
+
+	if (!dev_dax)
+		return 0;
+	return dev_dax->cached_size;
+}
+EXPORT_SYMBOL_GPL(dax_fsdev_size);
+
 static const struct dax_operations dev_dax_ops = {
 	.direct_access = fsdev_dax_direct_access,
 	.zero_page_range = fsdev_dax_zero_page_range,
diff --git a/fs/famfs/famfs_file.c b/fs/famfs/famfs_file.c
index e7fa7324c374..17359fa4007b 100644
--- a/fs/famfs/famfs_file.c
+++ b/fs/famfs/famfs_file.c
@@ -282,6 +282,11 @@ famfs_file_init_dax(struct file *file, void __user *arg)
 	}
 	inode_unlock(inode);
 
+	/* Account the mapped device bytes for statfs (only on success) */
+	if (!rc) {
+		scoped_guard(rwsem_write, &fsi->stats_sem)
+			fsi->used_capacity += extent_total;
+	}
 out:
 	kvfree(fmap_buf);
 	if (meta)
diff --git a/fs/famfs/famfs_inode.c b/fs/famfs/famfs_inode.c
index 2f1938adbfbc..61b668b299ef 100644
--- a/fs/famfs/famfs_inode.c
+++ b/fs/famfs/famfs_inode.c
@@ -24,6 +24,7 @@
 #include <linux/iomap.h>
 #include <linux/path.h>
 #include <linux/namei.h>
+#include <linux/statfs.h>
 
 #include "famfs_internal.h"
 
@@ -348,8 +349,38 @@ famfs_evict_inode(struct inode *inode)
 	clear_inode(inode);
 }
 
+/*
+ * famfs_statfs() - report device capacity and consumption so 'df' works.
+ * @total_capacity is the sum of installed daxdev sizes; @used_capacity is the
+ * sum of device bytes mapped by fmaps (superblock + log + data files). Free is
+ * the difference - an approximation of the userspace allocator's free space
+ * (it ignores allocator gaps / reserved regions), which is fine for df.
+ */
+static int
+famfs_statfs(struct dentry *dentry, struct kstatfs *buf)
+{
+	struct famfs_fs_info *fsi = dentry->d_sb->s_fs_info;
+	u64 total, used, free;
+
+	scoped_guard(rwsem_read, &fsi->stats_sem) {
+		total = fsi->total_capacity;
+		used  = fsi->used_capacity;
+	}
+	free = total > used ? total - used : 0;
+
+	buf->f_type    = FAMFS_SUPER_MAGIC;
+	buf->f_bsize   = PAGE_SIZE;
+	buf->f_frsize  = PAGE_SIZE;
+	buf->f_blocks  = total >> PAGE_SHIFT;
+	buf->f_bfree   = free  >> PAGE_SHIFT;
+	buf->f_bavail  = free  >> PAGE_SHIFT;	/* no root reservation */
+	buf->f_namelen = NAME_MAX;
+	buf->f_fsid    = u64_to_fsid(huge_encode_dev(dentry->d_sb->s_dev));
+	return 0;
+}
+
 static const struct super_operations famfs_super_ops = {
-	.statfs		= simple_statfs,
+	.statfs		= famfs_statfs,
 	.drop_inode	= inode_just_drop,
 	.show_options	= famfs_show_options,
 	.evict_inode    = famfs_evict_inode,
@@ -444,6 +475,7 @@ famfs_install_daxdev(
 	const char *name)
 {
 	struct famfs_daxdev *daxdev;
+	struct dax_device *devp = NULL;
 	int rc = 0;
 
 	if (index >= fsi->dax_devlist->nslots) {
@@ -507,6 +539,15 @@ famfs_install_daxdev(
 
 		wmb(); /* All other fields must be visible before valid */
 		daxdev->valid = 1;
+		devp = daxdev->devp;
+	}
+
+	/* Freshly installed: add its capacity to the statfs accounting */
+	if (devp) {
+		u64 sz = dax_fsdev_size(devp);
+
+		scoped_guard(rwsem_write, &fsi->stats_sem)
+			fsi->total_capacity += sz;
 	}
 
 	return 0;
@@ -769,6 +810,7 @@ famfs_init_fs_context(struct fs_context *fc)
 		return -ENOMEM;
 
 	init_rwsem(&fsi->devlist_sem);
+	init_rwsem(&fsi->stats_sem);
 	atomic64_set(&fsi->opts, FAMFS_OPT_DEFAULT);
 	fsi->mount_opts.mode = FAMFS_DEFAULT_MODE;
 	fc->s_fs_info        = fsi;
@@ -802,16 +844,11 @@ static struct file_system_type famfs_fs_type = {
 /******************************************************************************
  * Module stuff
  */
-#define FAMFS_MODULE_INCOMPLETE 1
-
 static int __init
 init_famfs_fs(void)
 {
 	int rc;
 
-	if (FAMFS_MODULE_INCOMPLETE)
-		return -ENODEV;
-
 	rc = register_filesystem(&famfs_fs_type);
 
 	return rc;
diff --git a/fs/famfs/famfs_internal.h b/fs/famfs/famfs_internal.h
index 5e043b5a0ddd..8765be41e4a7 100644
--- a/fs/famfs/famfs_internal.h
+++ b/fs/famfs/famfs_internal.h
@@ -123,6 +123,11 @@ struct famfs_dax_devlist {
  *               point, or if other "shutdown" conditions exist
  * @dax_devlist: Table of backing daxdevs (slot 0 is the mount primary)
  * @devlist_sem: Serializes installs into, and teardown of, @dax_devlist
+ * @stats_sem:   Protects the statfs accounting counters below
+ * @total_capacity: Sum of installed daxdev sizes, in bytes (grows as daxdevs
+ *               are added)
+ * @used_capacity:  Sum of installed fmap sizes, in bytes (grows as MAP_CREATE
+ *               attaches fmaps; this is device bytes consumed, not file size)
  */
 struct famfs_fs_info {
 	struct famfs_mount_opts   mount_opts;
@@ -130,6 +135,9 @@ struct famfs_fs_info {
 	bool                      deverror;
 	struct famfs_dax_devlist *dax_devlist;
 	struct rw_semaphore       devlist_sem;
+	struct rw_semaphore       stats_sem;
+	u64                       total_capacity;
+	u64                       used_capacity;
 };
 
 /*
diff --git a/include/linux/dax.h b/include/linux/dax.h
index 29113eb95e72..c25ef499d04f 100644
--- a/include/linux/dax.h
+++ b/include/linux/dax.h
@@ -256,6 +256,7 @@ static inline void dax_break_layout_final(struct inode *inode)
 
 bool dax_alive(struct dax_device *dax_dev);
 void *dax_get_private(struct dax_device *dax_dev);
+u64 dax_fsdev_size(struct dax_device *dax_dev);
 int dax_set_ops(struct dax_device *dax_dev, const struct dax_operations *ops);
 long dax_direct_access(struct dax_device *dax_dev, pgoff_t pgoff, long nr_pages,
 		enum dax_access_mode mode, void **kaddr, unsigned long *pfn);
-- 
2.53.0