Re: [RFC PATCH 1/3] fs: dax.c: move fs hole signifier from DAX_ZERO_PAGE to XA_ZERO_ENTRY

[Jan Kara <jack@suse.cz>]

On Mon 30-11-20 06:22:42, Amy Parker wrote:
> > > +/*
> > > + * A zero entry, XA_ZERO_ENTRY, is used to represent a zero page. This
> > > + * definition helps with checking if an entry is a PMD size.
> > > + */
> > > +#define XA_ZERO_PMD_ENTRY DAX_PMD | (unsigned long)XA_ZERO_ENTRY
> > > +
> >
> > Firstly, if you define a macro, we usually wrap it inside braces like:
> >
> > #define XA_ZERO_PMD_ENTRY (DAX_PMD | (unsigned long)XA_ZERO_ENTRY)
> >
> > to avoid unexpected issues when macro expands and surrounding operators
> > have higher priority.
> 
> Oops! Must've missed that - I'll make sure to get on that when
> revising this patch.
> 
> > Secondly, I don't think you can combine XA_ZERO_ENTRY with DAX_PMD (or any
> > other bits for that matter). XA_ZERO_ENTRY is defined as
> > xa_mk_internal(257) which is ((257 << 2) | 2) - DAX bits will overlap with
> > the bits xarray internal entries are using and things will break.
> 
> Could you provide an example of this overlap? I can't seem to find any.

Well XA_ZERO_ENTRY | DAX_PMD == ((257 << 2) | 2) | (1 << 1). So the way
you've defined XA_ZERO_PMD_ENTRY the DAX_PMD will just get lost. AFAIU (but
Matthew might correct me here), for internal entries (and XA_ZERO_ENTRY is
one instance of such entry) low 10-bits of the of the entry values are
reserved for internal xarray usage so DAX could use only higher bits. For
classical value entries, only the lowest bit is reserved for xarray usage,
all the rest is available for the user (and so DAX uses it).
 
								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR