linux-kernel - Re: [PATCH v11 19/21] dax: Add dax_zero_page

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Date:	Thu, 16 Oct 2014 18:01:26 -0400
From:	Matthew Wilcox <willy@...ux.intel.com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:	Matthew Wilcox <matthew.r.wilcox@...el.com>,
	linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	Ross Zwisler <ross.zwisler@...ux.intel.com>
Subject: Re: [PATCH v11 19/21] dax: Add dax_zero_page_range

On Thu, Oct 16, 2014 at 02:38:24PM +0200, Mathieu Desnoyers wrote:
> > +int dax_zero_page_range(struct inode *inode, loff_t from, unsigned length,
> 
> nit: unsigned -> unsigned int ?
> 
> Do we want a unsigned int or unsigned long here ?

It's supposed to be for a fragment of a page, so until we see a machine
with PAGE_SIZE > 4GB, we're good to use an unsigned int.

> >  	if (!length)
> >  		return 0;
> > +	BUG_ON((offset + length) > PAGE_CACHE_SIZE);
> 
> Isn't it a bit extreme to BUG_ON this condition ? We could return an
> error to the caller, and perhaps WARN_ON_ONCE(), but BUG_ON() appears to
> be slightly too strong here.

Dave Chinner asked for it :-)  The filesystem is supposed to be doing
this clamping (until the last version, I had this function doing the
clamping, and I was told off for "leaving landmines lying around".

> > +static inline int dax_zero_page_range(struct inode *i, loff_t frm,
> > +						unsigned len, get_block_t gb)
> > +{
> > +	return 0;
> 
> Should we return 0 or -ENOSYS here ?

I kind of wonder if we shouldn't just declare the function.  It's called
like this:

        if (IS_DAX(inode))
                return dax_zero_page_range(inode, from, length, ext4_get_block);
        return __ext4_block_zero_page_range(handle, mapping, from, length);

and if CONFIG_DAX is not set, IS_DAX evaluates to 0 at compile time, so
the compiler will optimise out the call to dax_zero_page_range() anyway.
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