ext4: use ext4_get_block_write in buffer write

Allocate uninitialized extent before ext4 buffer write and
convert the extent to initialized after io completes.
The purpose is to make sure an extent can only be marked
initialized after it has been written with new data so
we can safely drop the i_mutex lock in ext4 DIO read without
exposing stale data. This helps to improve multi-thread DIO
read performance on high-speed disks.

Skip the nobh and data=journal mount cases to make things simple for now.

Signed-off-by: Jiaying Zhang <jiayingz@google.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
int ext4_convert_unwritten_extents(struct inode *inode, loff_t offset,
				    ssize_t len)
{
	handle_t *handle;
	ext4_lblk_t block;
	unsigned int max_blocks;
	int ret = 0;
	int ret2 = 0;
	struct buffer_head map_bh;
	unsigned int credits, blkbits = inode->i_blkbits;

	block = offset >> blkbits;
	/*
	 * We can't just convert len to max_blocks because
	 * If blocksize = 4096 offset = 3072 and len = 2048
	 */
	max_blocks = (EXT4_BLOCK_ALIGN(len + offset, blkbits) >> blkbits)
							- block;
	/*
	 * credits to insert 1 extent into extent tree
	 */
	credits = ext4_chunk_trans_blocks(inode, max_blocks);
	while (ret >= 0 && ret < max_blocks) {
		block = block + ret;
		max_blocks = max_blocks - ret;
		handle = ext4_journal_start(inode, credits);
		if (IS_ERR(handle)) {
			ret = PTR_ERR(handle);
			break;
		}
		map_bh.b_state = 0;
		ret = ext4_get_blocks(handle, inode, block,
				      max_blocks, &map_bh,
				      EXT4_GET_BLOCKS_IO_CONVERT_EXT);
		if (ret <= 0) {
			WARN_ON(ret <= 0);
			printk(KERN_ERR "%s: ext4_ext_get_blocks "
				    "returned error inode#%lu, block=%u, "
				    "max_blocks=%u", __func__,
				    inode->i_ino, block, max_blocks);
		}
		ext4_mark_inode_dirty(handle, inode);
		ret2 = ext4_journal_stop(handle);
		if (ret <= 0 || ret2 )
			break;
	}
	return ret > 0 ? ret2 : ret;
}

int ext4_convert_unwritten_extents(struct inode *inode, loff_t offset,
				    ssize_t len)
{
	handle_t *handle;
	ext4_lblk_t block;
	unsigned int max_blocks;
	int ret = 0;
	int ret2 = 0;
	struct buffer_head map_bh;
	unsigned int credits, blkbits = inode->i_blkbits;

	block = offset >> blkbits;
	/*
	 * We can't just convert len to max_blocks because
	 * If blocksize = 4096 offset = 3072 and len = 2048
	 */
	max_blocks = (EXT4_BLOCK_ALIGN(len + offset, blkbits) >> blkbits)
							- block;
	/*
	 * credits to insert 1 extent into extent tree
	 */
	credits = ext4_chunk_trans_blocks(inode, max_blocks);
	while (ret >= 0 && ret < max_blocks) {
		block = block + ret;
		max_blocks = max_blocks - ret;
		handle = ext4_journal_start(inode, credits);
		if (IS_ERR(handle)) {
			ret = PTR_ERR(handle);
			break;
		}
		map_bh.b_state = 0;
		ret = ext4_get_blocks(handle, inode, block,
				      max_blocks, &map_bh,
				      EXT4_GET_BLOCKS_IO_CONVERT_EXT);
		if (ret <= 0) {
			WARN_ON(ret <= 0);
			printk(KERN_ERR "%s: ext4_ext_get_blocks "
				    "returned error inode#%lu, block=%u, "
				    "max_blocks=%u", __func__,
				    inode->i_ino, block, max_blocks);
		}
		ext4_mark_inode_dirty(handle, inode);
		ret2 = ext4_journal_stop(handle);
		if (ret <= 0 || ret2 )
			break;
	}
	return ret > 0 ? ret2 : ret;
}
