Re: [PATCH 17/19] ext4: Add support for MAP_SYNC flag

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On Fri 13-10-17 08:52:28, Dan Williams wrote:
> On Fri, Oct 13, 2017 at 12:22 AM, Christoph Hellwig <hch@xxxxxxxxxxxxx> 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@xxxxxxxx>
SUSE Labs, CR



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