Re: [PATCH v3] ext4: Rewrite ext4_page_mkwrite() to use generic helpers

[Christoph Hellwig <hch@infradead.org>]

>  	loff_t size;
>  	unsigned long len;
> +	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;
>  
>  	/*
> +	 * This check is racy but catches the common case. We rely on
> +	 * __block_page_mkwrite() to do a reliable check.
>  	 */
> +	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;

Is there any way to simply provide a different vm_operations_struct
and thus ->fault implementation for the delalloc vs non-delalloc case?

I think splitting those two cases completely would make the code a lot
more readable, even if there is a tiny amount of code duplication.