xfs: implement cgroup aware writeback

Link every newly allocated writeback bio to cgroup pointed to by the
writeback control structure, and charge every byte written back to it.

Tested-by: Stefan Priebe - Profihost AG <[e-mail]>
Signed-off-by: Christoph Hellwig <[e-mail]>
Reviewed-by: Darrick J. Wong <[e-mail]>
Signed-off-by: Darrick J. Wong <[e-mail]>

fs/xfs/xfs_aops.c

···
 	bio->bi_iter.bi_sector = sector;
 	bio->bi_opf = REQ_OP_WRITE | wbc_to_write_flags(wbc);
 	bio->bi_write_hint = inode->i_write_hint;
+	wbc_init_bio(wbc, bio);
 
 	ioend = container_of(bio, struct xfs_ioend, io_inline_bio);
 	INIT_LIST_HEAD(&ioend->io_list);
···
 	struct bio *new;
 
 	new = bio_alloc(GFP_NOFS, BIO_MAX_PAGES);
-	bio_copy_dev(new, prev);
+	bio_copy_dev(new, prev);/* also copies over blkcg information */
 	new->bi_iter.bi_sector = bio_end_sector(prev);
 	new->bi_opf = prev->bi_opf;
 	new->bi_write_hint = prev->bi_write_hint;
···
 	}
 
 	wpc->ioend->io_size += len;
+	wbc_account_io(wbc, page, len);
 }
 
 STATIC void

fs/xfs/xfs_super.c

···
 	sb->s_maxbytes = xfs_max_file_offset(sb->s_blocksize_bits);
 	sb->s_max_links = XFS_MAXLINK;
 	sb->s_time_gran = 1;
+	sb->s_iflags |= SB_I_CGROUPWB;
+
 	set_posix_acl_flag(sb);
 
 	/* version 5 superblocks support inode version counters. */