Re: [PATCH 13/13] ext4: Support for synchronous DAX faults

[Christoph Hellwig <hch@infradead.org>]

> +	pfn_t pfn;
>  
>  	if (write) {
>  		sb_start_pagefault(sb);
> @@ -287,16 +288,39 @@ static int ext4_dax_huge_fault(struct vm_fault *vmf,
>  		down_read(&EXT4_I(inode)->i_mmap_sem);
>  		handle = ext4_journal_start_sb(sb, EXT4_HT_WRITE_PAGE,
>  					       EXT4_DATA_TRANS_BLOCKS(sb));
> +		if (IS_ERR(handle)) {
> +			up_read(&EXT4_I(inode)->i_mmap_sem);
> +			sb_end_pagefault(sb);
> +			return VM_FAULT_SIGBUS;
> +		}
>  	} else {
>  		down_read(&EXT4_I(inode)->i_mmap_sem);
>  	}
> -	if (!IS_ERR(handle))
> -		result = dax_iomap_fault(vmf, pe_size, &ext4_iomap_ops, NULL);
> -	else
> -		result = VM_FAULT_SIGBUS;
> +	result = dax_iomap_fault(vmf, pe_size, &ext4_iomap_ops, &pfn);

Maybe split the error handling refactor into a simple prep patch to
make this one more readable?

> +		/* Write fault but PFN mapped only RO? */
> +		if (result & VM_FAULT_NEEDDSYNC) {
> +			int err;
> +			loff_t start = ((loff_t)vmf->pgoff) << PAGE_SHIFT;
> +			size_t len = 0;
> +
> +			if (pe_size == PE_SIZE_PTE)
> +				len = PAGE_SIZE;
> +#ifdef CONFIG_FS_DAX_PMD
> +			else if (pe_size == PE_SIZE_PMD)
> +				len = HPAGE_PMD_SIZE;
> +#endif
> +			else
> +				WARN_ON_ONCE(1);
> +			err = vfs_fsync_range(vmf->vma->vm_file, start,
> +					      start + len - 1, 1);
> +			if (err)
> +				result = VM_FAULT_SIGBUS;
> +			else
> +				result = dax_insert_pfn_mkwrite(vmf, pe_size,
> +								pfn);
> +		}

I think this needs to become a helper exported from the DAX code,
way too much magic inside the file system as-is.