ext4: Rewrite ext4_page_mkwrite() to use generic helpers

	struct page *page = vmf->page;

	loff_t size;

	unsigned long len;

	int ret = -EINVAL;

	void *fsdata;

	int ret;

	struct file *file = vma->vm_file;

	struct inode *inode = file->f_path.dentry->d_inode;

	struct address_space *mapping = inode->i_mapping;

	handle_t *handle;

	get_block_t *get_block;

	int retries = 0;

	/*

	 * Get i_alloc_sem to stop truncates messing with the inode. We cannot

	 * get i_mutex because we are already holding mmap_sem.

	 * This check is racy but catches the common case. We rely on

	 * __block_page_mkwrite() to do a reliable check.

	 */

	down_read(&inode->i_alloc_sem);

	size = i_size_read(inode);

	if (page->mapping != mapping || size <= page_offset(page)

	    || !PageUptodate(page)) {

		/* page got truncated from under us? */

		goto out_unlock;

	vfs_check_frozen(inode->i_sb, SB_FREEZE_WRITE);

	/* Delalloc case is easy... */

	if (test_opt(inode->i_sb, DELALLOC) &&

	    !ext4_should_journal_data(inode) &&

	    !ext4_nonda_switch(inode->i_sb)) {

		do {

			ret = __block_page_mkwrite(vma, vmf,

						   ext4_da_get_block_prep);

		} while (ret == -ENOSPC &&

		       ext4_should_retry_alloc(inode->i_sb, &retries));

		goto out_ret;

	}

	ret = 0;

	lock_page(page);

	wait_on_page_writeback(page);

	if (PageMappedToDisk(page)) {

		up_read(&inode->i_alloc_sem);

		return VM_FAULT_LOCKED;

	size = i_size_read(inode);

	/* Page got truncated from under us? */

	if (page->mapping != mapping || page_offset(page) > size) {

		unlock_page(page);

		ret = VM_FAULT_NOPAGE;

		goto out;

	}

	if (page->index == size >> PAGE_CACHE_SHIFT)

		len = size & ~PAGE_CACHE_MASK;

	else

		len = PAGE_CACHE_SIZE;

	/*

	 * return if we have all the buffers mapped. This avoid

	 * the need to call write_begin/write_end which does a

	 * journal_start/journal_stop which can block and take

	 * long time

	 * Return if we have all the buffers mapped. This avoids the need to do

	 * journal_start/journal_stop which can block and take a long time

	 */

	if (page_has_buffers(page)) {

		if (!walk_page_buffers(NULL, page_buffers(page), 0, len, NULL,

					ext4_bh_unmapped)) {

			up_read(&inode->i_alloc_sem);

			return VM_FAULT_LOCKED;

			/* Wait so that we don't change page under IO */

			wait_on_page_writeback(page);

			ret = VM_FAULT_LOCKED;

			goto out;

		}

	}

	unlock_page(page);

	/*

	 * OK, we need to fill the hole... Do write_begin write_end

	 * to do block allocation/reservation.We are not holding

	 * inode.i__mutex here. That allow * parallel write_begin,

	 * write_end call. lock_page prevent this from happening

	 * on the same page though

	 */

	ret = mapping->a_ops->write_begin(file, mapping, page_offset(page),

			len, AOP_FLAG_UNINTERRUPTIBLE, &page, &fsdata);

	if (ret < 0)

		goto out_unlock;

	ret = mapping->a_ops->write_end(file, mapping, page_offset(page),

			len, len, page, fsdata);

	if (ret < 0)

		goto out_unlock;

	ret = 0;

	/*

	 * write_begin/end might have created a dirty page and someone

	 * could wander in and start the IO.  Make sure that hasn't

	 * happened.

	 */

	lock_page(page);

	wait_on_page_writeback(page);

	up_read(&inode->i_alloc_sem);

	return VM_FAULT_LOCKED;

out_unlock:

	if (ret)

	/* OK, we need to fill the hole... */

	if (ext4_should_dioread_nolock(inode))

		get_block = ext4_get_block_write;

	else

		get_block = ext4_get_block;

retry_alloc:

	handle = ext4_journal_start(inode, ext4_writepage_trans_blocks(inode));

	if (IS_ERR(handle)) {

		ret = VM_FAULT_SIGBUS;

		goto out;

	}

	ret = __block_page_mkwrite(vma, vmf, get_block);

	if (!ret && ext4_should_journal_data(inode)) {

		if (walk_page_buffers(handle, page_buffers(page), 0,

			  PAGE_CACHE_SIZE, NULL, do_journal_get_write_access)) {

			unlock_page(page);

			ret = VM_FAULT_SIGBUS;

	up_read(&inode->i_alloc_sem);

			goto out;

		}

		ext4_set_inode_state(inode, EXT4_STATE_JDATA);

	}

	ext4_journal_stop(handle);

	if (ret == -ENOSPC && ext4_should_retry_alloc(inode->i_sb, &retries))

		goto retry_alloc;

out_ret:

	ret = block_page_mkwrite_return(ret);

out:

	return ret;

}