[PATCH 3/4] VFS/btrfs: add STATX_TREE_ID

[NeilBrown <neilb@suse.de>]

A new 64bit field is added to the data that can be returned by statx() -
the "tree id".
The tree id serves two needs.
1/ it extends the available inode number space.  While a filesystem
   SHOULD ensure the inode number is unique across the filesystem,
   this is sometimes impractical.  In such situations, 'tree id'
   may be used to guarantee uniqueness.  It can identify a separate
   allocation domain.
   A particular case when separate allocation domains is useful
   is when a directory tree can be effectively "reflink"ed.
   Updating all inode numbers in such a tree is prohibitively expensive.
2/ it can identify a collection of objects that provide a coherent
   "tree" in some locally-defined sense.

This patch uses STATX_TREE_ID to export the subvol id for btrfs.

samples/vfs/test_statx.c is extended to report the treeid.

Also: a new superblock field is added: s_tree_id_bits.  This can store
  the number of significant bits in the reported treeid.  It is
  currently unused, but could be used by overlayfs to determine how
  to add a filesystem number to the treeid to differentiate files in
  different underlying filesystems.

Signed-off-by: NeilBrown <neilb@suse.de>
---
 fs/btrfs/inode.c          |    4 ++++
 fs/btrfs/super.c          |    1 +
 fs/stat.c                 |    1 +
 include/linux/fs.h        |    2 +-
 include/linux/stat.h      |   13 +++++++++++++
 include/uapi/linux/stat.h |    3 ++-
 samples/vfs/test-statx.c  |    4 +++-
 7 files changed, 25 insertions(+), 3 deletions(-)

diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index 30fa64cbe6dc..c878726d090c 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -9215,6 +9215,10 @@ static int btrfs_getattr(struct user_namespace *mnt_userns,
 	case BTRFS_MANY_DEVS:
 		stat->dev = BTRFS_I(inode)->root->anon_dev;
 	}
+	if (request_mask & STATX_TREE_ID) {
+		stat->tree_id = BTRFS_I(inode)->root->root_key.objectid;
+		stat->result_mask |= STATX_TREE_ID;
+	}
 
 	spin_lock(&BTRFS_I(inode)->lock);
 	delalloc_bytes = BTRFS_I(inode)->new_delalloc_bytes;
diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
index b1aecb834234..e6d166150660 100644
--- a/fs/btrfs/super.c
+++ b/fs/btrfs/super.c
@@ -1410,6 +1410,7 @@ static int btrfs_fill_super(struct super_block *sb,
 #endif
 	sb->s_flags |= SB_I_VERSION;
 	sb->s_iflags |= SB_I_CGROUPWB;
+	sb->s_tree_id_bits = 48;
 
 	err = super_setup_bdi(sb);
 	if (err) {
diff --git a/fs/stat.c b/fs/stat.c
index 1fa38bdec1a6..2dd5d3d67793 100644
--- a/fs/stat.c
+++ b/fs/stat.c
@@ -580,6 +580,7 @@ cp_statx(const struct kstat *stat, struct statx __user *buffer)
 	tmp.stx_dev_major = MAJOR(stat->dev);
 	tmp.stx_dev_minor = MINOR(stat->dev);
 	tmp.stx_mnt_id = stat->mnt_id;
+	tmp.stx_tree_id = stat->tree_id;
 
 	return copy_to_user(buffer, &tmp, sizeof(tmp)) ? -EFAULT : 0;
 }
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 640574294216..a777c1b1706a 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1482,7 +1482,7 @@ struct super_block {
 
 	unsigned int		s_max_links;
 	fmode_t			s_mode;
-
+	short			s_tree_id_bits;
 	/*
 	 * The next field is for VFS *only*. No filesystems have any business
 	 * even looking at it. You had been warned.
diff --git a/include/linux/stat.h b/include/linux/stat.h
index fff27e603814..08ee409786b3 100644
--- a/include/linux/stat.h
+++ b/include/linux/stat.h
@@ -46,6 +46,19 @@ struct kstat {
 	struct timespec64 btime;			/* File creation time */
 	u64		blocks;
 	u64		mnt_id;
+	/* Treeid can be used to extend the inode number space.  Two inodes
+	 * with different 'tree_id' are different, even if 'ino' is the same
+	 * (though fs should make ino different as often as possible).
+	 * When tree_id is requested and STATX_TREE_ID is set in result_mask,
+	 * 'ino' MUST be unique across the filesystem.  Specifically, two
+	 * open files that report the same dev, ino, and tree_id MUST be
+	 * the same.
+	 * If a directory and an object in that directory have the same dev
+	 * and tree_id, they can be assumed to be in a meaningful tree, though
+	 * the meaning is subject to local interpretation.  The set of inodes
+	 * with a common tree_id is not required to be contiguous.
+	 */
+	u64		tree_id;
 };
 
 #endif
diff --git a/include/uapi/linux/stat.h b/include/uapi/linux/stat.h
index 1500a0f58041..725cf3f8e873 100644
--- a/include/uapi/linux/stat.h
+++ b/include/uapi/linux/stat.h
@@ -124,7 +124,7 @@ struct statx {
 	__u32	stx_dev_minor;
 	/* 0x90 */
 	__u64	stx_mnt_id;
-	__u64	__spare2;
+	__u64	stx_tree_id;
 	/* 0xa0 */
 	__u64	__spare3[12];	/* Spare space for future expansion */
 	/* 0x100 */
@@ -152,6 +152,7 @@ struct statx {
 #define STATX_BASIC_STATS	0x000007ffU	/* The stuff in the normal stat struct */
 #define STATX_BTIME		0x00000800U	/* Want/got stx_btime */
 #define STATX_MNT_ID		0x00001000U	/* Got stx_mnt_id */
+#define STATX_TREE_ID		0x00002000U	/* Want/got stx_treeid and clean stX_ino */
 
 #define STATX__RESERVED		0x80000000U	/* Reserved for future struct statx expansion */
 
diff --git a/samples/vfs/test-statx.c b/samples/vfs/test-statx.c
index 49c7a46cee07..c1141764fa2e 100644
--- a/samples/vfs/test-statx.c
+++ b/samples/vfs/test-statx.c
@@ -118,6 +118,8 @@ static void dump_statx(struct statx *stx)
 			break;
 		}
 	}
+	if (stx->stx_mask & STATX_TREE_ID)
+		printf(" Tree: %-12llu", (unsigned long long) stx->stx_tree_id);
 	printf("\n");
 
 	if (stx->stx_mask & STATX_MODE)
@@ -218,7 +220,7 @@ int main(int argc, char **argv)
 	struct statx stx;
 	int ret, raw = 0, atflag = AT_SYMLINK_NOFOLLOW;
 
-	unsigned int mask = STATX_BASIC_STATS | STATX_BTIME;
+	unsigned int mask = STATX_BASIC_STATS | STATX_BTIME | STATX_TREE_ID;
 
 	for (argv++; *argv; argv++) {
 		if (strcmp(*argv, "-F") == 0) {