linux-kernel - Re: [PATCH 1/4] mm/zsmalloc: Prepare to variable MAX_PHYSMEM

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Date:   Fri, 20 Oct 2017 18:43:55 -0700
From:   Nitin Gupta <ngupta@...are.org>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>, x86@...nel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Andy Lutomirski <luto@...capital.net>,
        Cyrill Gorcunov <gorcunov@...nvz.org>,
        Borislav Petkov <bp@...e.de>, Andi Kleen <ak@...ux.intel.com>,
        linux-mm <linux-mm@...ck.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Minchan Kim <minchan@...nel.org>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Subject: Re: [PATCH 1/4] mm/zsmalloc: Prepare to variable MAX_PHYSMEM_BITS

On Fri, Oct 20, 2017 at 12:59 PM, Kirill A. Shutemov
<kirill.shutemov@...ux.intel.com> 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