Re: [PATCH V12] mm/debug: Add tests validating architecture page table helpers

[Gerald Schaefer <gerald.schaefer@de.ibm.com>]

On Wed, 12 Feb 2020 15:12:54 +0530
Anshuman Khandual <anshuman.khandual@arm.com> wrote:

> >> +/*
> >> + * On s390 platform, the lower 12 bits are used to identify given page table
> >> + * entry type and for other arch specific requirements. But these bits might
> >> + * affect the ability to clear entries with pxx_clear(). So while loading up
> >> + * the entries skip all lower 12 bits in order to accommodate s390 platform.
> >> + * It does not have affect any other platform.
> >> + */
> >> +#define RANDOM_ORVALUE	(0xfffffffffffff000UL)  
> > 
> > I'd suggest you generate this mask with something like
> > GENMASK(BITS_PER_LONG, PAGE_SHIFT).  
> 
> IIRC the lower 12 bits constrains on s390 platform might not be really related
> to it's PAGE_SHIFT which can be a variable, but instead just a constant number.
> But can definitely use GENMASK or it's variants here.
> 
> https://lkml.org/lkml/2019/9/5/862

PAGE_SHIFT would be fine, it is 12 on s390. However, in order to be
more precise, we do not really need all 12 bits, only the last 4 bits.
So, something like this would work:

#define RANDOM_ORVALUE GENMASK(BITS_PER_LONG - 1, 4)

The text in the comment could then also be changed from 12 to 4, and
be a bit more specific on the fact that the impact on pxx_clear()
results from the dynamic page table folding logic on s390:

/*
 * On s390 platform, the lower 4 bits are used to identify given page table
 * entry type. But these bits might affect the ability to clear entries with
 * pxx_clear() because of how dynamic page table folding works on s390. So
 * while loading up the entries do not change the lower 4 bits.
 * It does not have affect any other platform.
 */