Re: [PATCHv7 02/19] mm/zsmalloc: Prepare to variable MAX_PHYSMEM_BITS

[Kirill A. Shutemov]

On Thu, Sep 28, 2017 at 10:10:34AM +0200, Ingo Molnar wrote:
> 
> * Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx> wrote:
> 
> > With boot-time switching between paging mode we will have variable
> > MAX_PHYSMEM_BITS.
> > 
> > Let's use the maximum variable possible for CONFIG_X86_5LEVEL=y
> > configuration to define zsmalloc data structures.
> > 
> > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> > Cc: Minchan Kim <minchan@xxxxxxxxxx>
> > Cc: Nitin Gupta <ngupta@xxxxxxxxxx>
> > Cc: Sergey Senozhatsky <sergey.senozhatsky.work@xxxxxxxxx>
> > ---
> >  mm/zsmalloc.c | 6 ++++++
> >  1 file changed, 6 insertions(+)
> > 
> > diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
> > index 7c38e850a8fc..fe22661f2fe5 100644
> > --- a/mm/zsmalloc.c
> > +++ b/mm/zsmalloc.c
> > @@ -93,7 +93,13 @@
> >  #define MAX_PHYSMEM_BITS BITS_PER_LONG
> >  #endif
> >  #endif
> > +
> > +#ifdef CONFIG_X86_5LEVEL
> > +/* MAX_PHYSMEM_BITS is variable, use maximum value here */
> > +#define _PFN_BITS		(52 - PAGE_SHIFT)
> > +#else
> >  #define _PFN_BITS		(MAX_PHYSMEM_BITS - PAGE_SHIFT)
> > +#endif
> 
> This is a totally ugly hack, polluting generic MM code with an x86-ism and an 
> arbitrary hard-coded constant that would silently lose validity when x86 paging 
> gets extended again ...

Well, yes it's ugly. And I would be glad to find better solution. But I
don't see one.

And it won't break silently on x86 paging expanding as it won't use
CONFIG_X86_5LEVEL, so we would fallback to MAX_PHYSMEM_BITS - PAGE_SHIFT.

I worth noting that the code already has x86 hack. See PAE special case
for MAX_PHYSMEM_BITS.

-- 
 Kirill A. Shutemov