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.
> 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.