Date: Wed, 13 Apr 2016 14:22:49 -0400
From: Matthew Wilcox <willy@linux.intel.com>
To: Toshi Kani <toshi.kani@hpe.com>
Cc: akpm@linux-foundation.org, dan.j.williams@intel.com,
        linux-nvdimm@ml01.01.org, linux-fsdevel@vger.kernel.org,
        linux-kernel@vger.kernel.org, "Theodore Ts'o" <tytso@mit.edu>,
        Andreas Dilger <adilger.kernel@dilger.ca>, Jan Kara <jack@suse.cz>,
        Ross Zwisler <ross.zwisler@linux.intel.com>,
        linux-ext4@vger.kernel.org
Subject: Re: [PATCH v2 2/5] ext4: call dax_get_unmapped_area() for DAX pmd
 mappings
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On Wed, Apr 13, 2016 at 09:08:36AM -0600, Toshi Kani wrote:
> > Could you do something like:
> > 
> > ?#ifdef CONFIG_FS_DAX
> > ?struct page *read_dax_sector(struct block_device *bdev, sector_t n);
> > +unsigned long dax_get_unmapped_area(struct file *filp, unsigned long
> > addr,
> > +???????????????unsigned long len, unsigned long pgoff, unsigned long
> > flags);
> > ?#else
> > ?static inline struct page *read_dax_sector(struct block_device *bdev,
> > ?????????????????sector_t n)
> > ?{
> > ?????????return ERR_PTR(-ENXIO);
> > ?}
> > +#define dax_get_unmapped_area	NULL
> > ?#endif
> > 
> > in patch 1/5.??Then there's no need for the ifdefs in each filesystem.
> 
> I thought about it, but I do not think we can use an inline function to an
> entry point.

That's not an inline function.  It's just NULL.  So after the preprocessor
is done with it, it just looks like:

	.get_unmapped_area = NULL,

and it won't be called by get_unmapped_area().