linux-ext4 - Re: [PATCH 17/19] ext4: Add support for MAP

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Message-ID: <20171017113052.GC24136@quack2.suse.cz>
Date:   Tue, 17 Oct 2017 13:30:52 +0200
From:   Jan Kara <jack@...e.cz>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Christoph Hellwig <hch@...radead.org>, Jan Kara <jack@...e.cz>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-ext4 <linux-ext4@...r.kernel.org>,
        linux-xfs@...r.kernel.org,
        Ross Zwisler <ross.zwisler@...ux.intel.com>,
        Ted Tso <tytso@....edu>,
        "Darrick J. Wong" <darrick.wong@...cle.com>
Subject: Re: [PATCH 17/19] ext4: Add support for MAP_SYNC flag

On Fri 13-10-17 08:52:28, Dan Williams wrote:
> On Fri, Oct 13, 2017 at 12:22 AM, Christoph Hellwig <hch@...radead.org> wrote:
> > 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.
> 
> We have this general and perennial problem of parsing why the kernel
> is throwing an error. It saves a debug step if the error code is
> unique, and in this case would indicate that the filesystem supports
> MAP_SYNC but the administrator failed to arrange for the device to be
> in DAX mode.

So I understand this wish however I think the error codes should also
reflect the nature of the problem. And EPERM has something to do with access
permissions, not whether file supports DAX access or not.

								Honza

-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR