xfs: track the iunlink list pointer in the xfs_inode

Having direct access to the i_next_unlinked pointer in unlinked
inodes greatly simplifies the processing of inodes on the unlinked
list. We no longer need to look up the inode buffer just to find
next inode in the list if the xfs_inode is in memory. These
improvements will be realised over upcoming patches as other
dependencies on the inode buffer for unlinked list processing are
removed.

Signed-off-by: Dave Chinner <[e-mail]>
Reviewed-by: Darrick J. Wong <[e-mail]>
Reviewed-by: Christoph Hellwig <[e-mail]>

fs/xfs/libxfs/xfs_inode_buf.c

···
 	ip->i_nblocks = be64_to_cpu(from->di_nblocks);
 	ip->i_extsize = be32_to_cpu(from->di_extsize);
 	ip->i_forkoff = from->di_forkoff;
-	ip->i_diflags	= be16_to_cpu(from->di_flags);
+	ip->i_diflags = be16_to_cpu(from->di_flags);
+	ip->i_next_unlinked = be32_to_cpu(from->di_next_unlinked);
 
 	if (from->di_dmevmask || from->di_dmstate)
 		xfs_iflags_set(ip, XFS_IPRESERVE_DM_FIELDS);

fs/xfs/xfs_inode.c

···
 
 	/* Make sure the old pointer isn't garbage. */
 	old_value = be32_to_cpu(dip->di_next_unlinked);
-	if (!xfs_verify_agino_or_null(pag, old_value)) {
+	if (old_value != ip->i_next_unlinked ||
+	    !xfs_verify_agino_or_null(pag, old_value)) {
 		xfs_inode_verifier_error(ip, -EFSCORRUPTED, __func__, dip,
 				sizeof(*dip), __this_address);
 		error = -EFSCORRUPTED;
···
 		if (error)
 			return error;
 		ASSERT(old_agino == NULLAGINO);
+		ip->i_next_unlinked = next_agino;
 
 		/*
 		 * agino has been unlinked, add a backref from the next inode
···
 	error = xfs_iunlink_update_inode(tp, ip, pag, NULLAGINO, &next_agino);
 	if (error)
 		return error;
+	ip->i_next_unlinked = NULLAGINO;
 
 	/*
 	 * If there was a backref pointing from the next inode back to this

fs/xfs/xfs_inode.h

···
 	uint64_t		i_diflags2;	/* XFS_DIFLAG2_... */
 	struct timespec64	i_crtime;	/* time created */
 
+	/* unlinked list pointers */
+	xfs_agino_t		i_next_unlinked;
+
 	/* VFS inode */
 	struct inode		i_vnode;	/* embedded VFS inode */
 

fs/xfs/xfs_log_recover.c

···
 	xfs_agino_t			agino,
 	int				bucket)
 {
-	struct xfs_buf			*ibp;
-	struct xfs_dinode		*dip;
 	struct xfs_inode		*ip;
 	xfs_ino_t			ino;
 	int				error;
···
 	if (error)
 		goto fail;
 
-	/*
-	 * Get the on disk inode to find the next inode in the bucket.
-	 */
-	error = xfs_imap_to_bp(pag->pag_mount, NULL, &ip->i_imap, &ibp);
-	if (error)
-		goto fail_iput;
-	dip = xfs_buf_offset(ibp, ip->i_imap.im_boffset);
-
 	xfs_iflags_clear(ip, XFS_IRECOVERY);
 	ASSERT(VFS_I(ip)->i_nlink == 0);
 	ASSERT(VFS_I(ip)->i_mode != 0);
 
-	/* setup for the next pass */
-	agino = be32_to_cpu(dip->di_next_unlinked);
-	xfs_buf_relse(ibp);
-
+	agino = ip->i_next_unlinked;
 	xfs_irele(ip);
 	return agino;
 
- fail_iput:
-	xfs_irele(ip);
  fail:
 	/*
 	 * We can't read in the inode this bucket points to, or this inode