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

[Ingo Molnar]

* Kirill A. Shutemov <kirill@xxxxxxxxxxxxx> wrote:

> 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.

Old mistakes don't justify new ones.

It's possible to do better: for example if we provide a MAX_POSSIBLE_PHYSMEM_BITS 
define that is the higher value then code which needs this for sizing can use it?

That could eliminate the PAE dependency as well perhaps.

Thanks,

	Ingo