On Sun, Nov 08, 2020 at 05:33:22PM -0800, Darrick J. Wong wrote:
> On Sun, Nov 08, 2020 at 05:15:55PM -0800, Amy Parker wrote:
> > I've been writing a patch to migrate the defined DAX_ZERO_PAGE
> > to XA_ZERO_ENTRY for representing holes in files.
>
> Why? IIRC XA_ZERO_ENTRY ("no mapping in the address space") isn't the
> same as DAX_ZERO_PAGE ("the zero page is mapped into the address space
> because we took a read fault on a sparse file hole").
There's no current user of XA_ZERO_ENTRY in i_pages, whether it be
DAX or non-DAX.
> > XA_ZERO_ENTRY
> > is defined in include/linux/xarray.h, where it's defined using
> > xa_mk_internal(257). This function returns a void pointer, which
> > is incompatible with the bitwise arithmetic it is performed on with.
We don't really perform bitwise arithmetic on it, outside of:
static int dax_is_zero_entry(void *entry)
{
return xa_to_value(entry) & DAX_ZERO_PAGE;
}
> > Currently, DAX_ZERO_PAGE is defined as an unsigned long,
> > so I considered typecasting it. Typecasting every time would be
> > repetitive and inefficient. I thought about making a new definition
> > for it which has the typecast, but this breaks the original point of
> > using already defined terms.
> >
> > Should we go the route of adding a new definition, we might as
> > well just change the definition of DAX_ZERO_PAGE. This would
> > break the simplicity of the current DAX bit definitions:
> >
> > #define DAX_LOCKED (1UL << 0)
> > #define DAX_PMD (1UL << 1)
> > #define DAX_ZERO_PAGE (1UL << 2)
> > #define DAX_EMPTY (1UL << 3)
I was proposing deleting the entire bit and shifting DAX_EMPTY down.
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