btrfs: remove location key from struct btrfs_inode

Currently struct btrfs_inode has a key member, named "location", that is
either:

1) The key of the inode's item. In this case the objectid is the number
   of the inode;

2) A key stored in a dir entry with a type of BTRFS_ROOT_ITEM_KEY, for
   the case where we have a root that is a snapshot of a subvolume that
   points to other subvolumes. In this case the objectid is the ID of
   a subvolume inside the snapshotted parent subvolume.

The key is only used to lookup the inode item for the first case, while
for the second it's never used since it corresponds to directory stubs
created with new_simple_dir() and which are marked as dummy, so there's
no actual inode item to ever update. In the second case we only check
the key type at btrfs_ino() for 32 bits platforms and its objectid is
only needed for unlink.

Instead of using a key we can do fine with just the objectid, since we
can generate the key whenever we need it having only the objectid, as
in all use cases the type is always BTRFS_INODE_ITEM_KEY and the offset
is always 0.

So use only an objectid instead of a full key. This reduces the size of
struct btrfs_inode from 1048 bytes down to 1040 bytes on a release kernel.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

1 files changed, 41 insertions, 6 deletions

diff --git a/fs/btrfs/btrfs_inode.h b/fs/btrfs/btrfs_inode.h
index a1c2e922a5dd..754671a73333 100644
--- a/fs/btrfs/btrfs_inode.h
+++ b/fs/btrfs/btrfs_inode.h
@@ -99,6 +99,29 @@ enum {
 	 * range).
 	 */
 	BTRFS_INODE_COW_WRITE_ERROR,
+	/*
+	 * Indicate this is a directory that points to a subvolume for which
+	 * there is no root reference item. That's a case like the following:
+	 *
+	 *   $ btrfs subvolume create /mnt/parent
+	 *   $ btrfs subvolume create /mnt/parent/child
+	 *   $ btrfs subvolume snapshot /mnt/parent /mnt/snap
+	 *
+	 * If subvolume "parent" is root 256, subvolume "child" is root 257 and
+	 * snapshot "snap" is root 258, then there's no root reference item (key
+	 * BTRFS_ROOT_REF_KEY in the root tree) for the subvolume "child"
+	 * associated to root 258 (the snapshot) - there's only for the root
+	 * of the "parent" subvolume (root 256). In the chunk root we have a
+	 * (256 BTRFS_ROOT_REF_KEY 257) key but we don't have a
+	 * (258 BTRFS_ROOT_REF_KEY 257) key - the sames goes for backrefs, we
+	 * have a (257 BTRFS_ROOT_BACKREF_KEY 256) but we don't have a
+	 * (257 BTRFS_ROOT_BACKREF_KEY 258) key.
+	 *
+	 * So when opening the "child" dentry from the snapshot's directory,
+	 * we don't find a root ref item and we create a stub inode. This is
+	 * done at new_simple_dir(), called from btrfs_lookup_dentry().
+	 */
+	BTRFS_INODE_ROOT_STUB,
 };
 
 /* in memory btrfs inode */
@@ -106,10 +129,15 @@ struct btrfs_inode {
 	/* which subvolume this inode belongs to */
 	struct btrfs_root *root;
 
-	/* key used to find this inode on disk.  This is used by the code
-	 * to read in roots of subvolumes
+	/*
+	 * This is either:
+	 *
+	 * 1) The objectid of the corresponding BTRFS_INODE_ITEM_KEY;
+	 *
+	 * 2) In case this a root stub inode (BTRFS_INODE_ROOT_STUB flag set),
+	 *    the ID of that root.
 	 */
-	struct btrfs_key location;
+	u64 objectid;
 
 	/* Cached value of inode property 'compression'. */
 	u8 prop_compress;
@@ -340,10 +368,9 @@ static inline unsigned long btrfs_inode_hash(u64 objectid,
  */
 static inline u64 btrfs_ino(const struct btrfs_inode *inode)
 {
-	u64 ino = inode->location.objectid;
+	u64 ino = inode->objectid;
 
-	/* type == BTRFS_ROOT_ITEM_KEY: subvol dir */
-	if (inode->location.type == BTRFS_ROOT_ITEM_KEY)
+	if (test_bit(BTRFS_INODE_ROOT_STUB, &inode->runtime_flags))
 		ino = inode->vfs_inode.i_ino;
 	return ino;
 }
@@ -357,6 +384,14 @@ static inline u64 btrfs_ino(const struct btrfs_inode *inode)
 
 #endif
 
+static inline void btrfs_get_inode_key(const struct btrfs_inode *inode,
+				       struct btrfs_key *key)
+{
+	key->objectid = inode->objectid;
+	key->type = BTRFS_INODE_ITEM_KEY;
+	key->offset = 0;
+}
+
 static inline void btrfs_i_size_write(struct btrfs_inode *inode, u64 size)
 {
 	i_size_write(&inode->vfs_inode, size);