linux-ext4 - Re: [PATCH 14/19] dax: Implement dax_finish_sync

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Date:   Fri, 13 Oct 2017 00:21:01 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Jan Kara <jack@...e.cz>
Cc:     linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org,
        linux-xfs@...r.kernel.org, Christoph Hellwig <hch@...radead.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Ross Zwisler <ross.zwisler@...ux.intel.com>,
        Ted Tso <tytso@....edu>,
        "Darrick J. Wong" <darrick.wong@...cle.com>
Subject: Re: [PATCH 14/19] dax: Implement dax_finish_sync_fault()

On Wed, Oct 11, 2017 at 10:05:58PM +0200, Jan Kara wrote:
> Implement a function that filesystems can call to finish handling of
> synchronous page faults. It takes care of syncing appropriare file range
> and insertion of page table entry.
> 
> Signed-off-by: Jan Kara <jack@...e.cz>

Looks fine except for a few nitpicks below:

Reviewed-by: Christoph Hellwig <hch@....de>

> +	spin_lock_irq(&mapping->tree_lock);
> +	entry = get_unlocked_mapping_entry(mapping, index, &slot);
> +	/* Did we race with someone splitting entry or so? */
> +	if (!entry || (pe_size == PE_SIZE_PTE && !dax_is_pte_entry(entry)) ||
> +	    (pe_size == PE_SIZE_PMD && !dax_is_pmd_entry(entry))) {

Minor nitpick:

Maybe keep each of the three conditions on a separate line to make it a
little easier to read.

> + * This function calls fdatasync() on the range of file touched by the page
> + * fault and handles inserting of appropriate page table entry.

Nitpick: we don't call fdatasync, as that doesn't even exist in the
kernel, but vfs_fsync_range.  But documenting which functions we call
doesn't seem all that useful to start with, so if you really want to
document it I'd say something like:

"Ensures that the file range touched by the page fault is stored
 persistently on the media"

or similar.

> +	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 = PMD_SIZE;
> +#endif

Is there really any point to ifdef out this single branch?

> +	else
> +		WARN_ON_ONCE(1);
> +	err = vfs_fsync_range(vmf->vma->vm_file, start, start + len - 1, 1);

(this just reminds me that I hate the vfs_fsync_range /->fsync calling
conventions.., offset + len would be way to easy)