From mboxrd@z Thu Jan  1 00:00:00 1970
From: Christoph Hellwig <hch@infradead.org>
Subject: Re: [PATCH 17/19] ext4: Add support for MAP_SYNC flag
Date: Fri, 13 Oct 2017 00:22:24 -0700
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Cc: Jan Kara <jack@suse.cz>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        linux-ext4 <linux-ext4@vger.kernel.org>,
        linux-xfs@vger.kernel.org, Christoph Hellwig <hch@infradead.org>,
        Ross Zwisler <ross.zwisler@linux.intel.com>,
        Ted Tso <tytso@mit.edu>,
        "Darrick J. Wong" <darrick.wong@oracle.com>
To: Dan Williams <dan.j.williams@intel.com>
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On Wed, Oct 11, 2017 at 03:11:21PM -0700, Dan Williams wrote:
> > +       /*
> > +        * We don't support synchronous mappings for non-DAX files. At least
> > +        * until someone comes with a sensible use case.
> > +        */
> > +       if (!IS_DAX(file_inode(file)) && (map_flags & MAP_SYNC))
> > +               return -EOPNOTSUPP;
> 
> Perhaps EPERM instead to differentiate the unsupported flags case?
> There's precedent for mmap returning EPERM for reasons other than
> incompatible PROT flags.

Why would we want to voluntarily use arcane errors for a entirely
new interface under our control?

-EOPNOTSUPP is nice and explicit about what is going on.