statx: stx_subvol

Add a new statx field for (sub)volume identifiers, as implemented by
btrfs and bcachefs.

This includes bcachefs support; we'll definitely want btrfs support as
well.

Link: https://lore.kernel.org/linux-fsdevel/2uvhm6gweyl7iyyp2xpfryvcu2g3padagaeqcbiavjyiis6prl@yjm725bizncq/
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>
Cc: Josef Bacik <josef@toxicpanda.com>
Cc: Miklos Szeredi <mszeredi@redhat.com>
Cc: Christian Brauner <brauner@kernel.org>
Cc: David Howells <dhowells@redhat.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>
Link: https://lore.kernel.org/r/20240308022914.196982-1-kent.overstreet@linux.dev
Signed-off-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: Kent Overstreet

fs/bcachefs/fs.c

@@ -841,6 +841,9 @@ static int bch2_getattr(struct mnt_idmap *idmap,
 	stat->blksize	= block_bytes(c);
 	stat->blocks	= inode->v.i_blocks;
 
+	stat->subvol	= inode->ei_subvol;
+	stat->result_mask |= STATX_SUBVOL;
+
 	if (request_mask & STATX_BTIME) {
 		stat->result_mask |= STATX_BTIME;
 		stat->btime = bch2_time_to_timespec(c, inode->ei_inode.bi_otime);

fs/btrfs/inode.c

@@ -8779,6 +8779,9 @@ static int btrfs_getattr(struct mnt_idmap *idmap,
 	generic_fillattr(idmap, request_mask, inode, stat);
 	stat->dev = BTRFS_I(inode)->root->anon_dev;
 
+	stat->subvol = BTRFS_I(inode)->root->root_key.objectid;
+	stat->result_mask |= STATX_SUBVOL;
+
 	spin_lock(&BTRFS_I(inode)->lock);
 	delalloc_bytes = BTRFS_I(inode)->new_delalloc_bytes;
 	inode_bytes = inode_get_bytes(inode);

fs/stat.c

@@ -658,6 +658,7 @@ cp_statx(const struct kstat *stat, struct statx __user *buffer)
 	tmp.stx_mnt_id = stat->mnt_id;
 	tmp.stx_dio_mem_align = stat->dio_mem_align;
 	tmp.stx_dio_offset_align = stat->dio_offset_align;
+	tmp.stx_subvol = stat->subvol;
 
 	return copy_to_user(buffer, &tmp, sizeof(tmp)) ? -EFAULT : 0;
 }

include/linux/stat.h

@@ -53,6 +53,7 @@ struct kstat {
 	u32		dio_mem_align;
 	u32		dio_offset_align;
 	u64		change_cookie;
+	u64		subvol;
 };
 
 /* These definitions are internal to the kernel for now. Mainly used by nfsd. */

include/uapi/linux/stat.h

@@ -126,8 +126,9 @@ struct statx {
 	__u64	stx_mnt_id;
 	__u32	stx_dio_mem_align;	/* Memory buffer alignment for direct I/O */
 	__u32	stx_dio_offset_align;	/* File offset alignment for direct I/O */
+	__u64	stx_subvol;	/* Subvolume identifier */
 	/* 0xa0 */
-	__u64	__spare3[12];	/* Spare space for future expansion */
+	__u64	__spare3[11];	/* Spare space for future expansion */
 	/* 0x100 */
 };
 
@@ -155,6 +156,7 @@ struct statx {
 #define STATX_MNT_ID		0x00001000U	/* Got stx_mnt_id */
 #define STATX_DIOALIGN		0x00002000U	/* Want/got direct I/O alignment info */
 #define STATX_MNT_ID_UNIQUE	0x00004000U	/* Want/got extended stx_mount_id */
+#define STATX_SUBVOL		0x00008000U	/* Want/got stx_subvol */
 
 #define STATX__RESERVED		0x80000000U	/* Reserved for future struct statx expansion */