Re: [PATCH 1/4] mm/zsmalloc: Prepare to variable MAX_PHYSMEM_BITS

[Nitin Gupta]

On Fri, Oct 20, 2017 at 12:59 PM, 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.
>
> The patch introduces MAX_POSSIBLE_PHYSMEM_BITS to cover such case.
> It also suits well to handle PAE special case.
>


I see that with your upcoming patch, MAX_PHYSMEM_BITS is turned into a
variable for x86_64 case as: (pgtable_l5_enabled ? 52 : 46).

Even with this change, I don't see a need for this new
MAX_POSSIBLE_PHYSMEM_BITS constant.


> -#ifndef MAX_PHYSMEM_BITS
> -#ifdef CONFIG_HIGHMEM64G
> -#define MAX_PHYSMEM_BITS 36
> -#else /* !CONFIG_HIGHMEM64G */
> +#ifndef MAX_POSSIBLE_PHYSMEM_BITS
> +#ifdef MAX_PHYSMEM_BITS
> +#define MAX_POSSIBLE_PHYSMEM_BITS MAX_PHYSMEM_BITS
> +#else


This ifdef on HIGHMEM64G is redundant, as x86 already defines
MAX_PHYSMEM_BITS = 36 in PAE case. So, all that zsmalloc should do is:

#ifndef MAX_PHYSMEM_BITS
#define MAX_PHYSMEM_BITS BITS_PER_LONG
#endif

.. and then no change is needed for rest of derived constants like _PFN_BITS.

It is upto every arch to define correct MAX_PHYSMEM_BITS (variable or constant)
based on whatever configurations the arch supports. If not defined,
zsmalloc picks
a reasonable default of BITS_PER_LONG.

I will send a patch which makes the change to remove ifdef on CONFIG_HIGHMEM64G.

Thanks,
Nitin