linux-kernel - Re: [PATCHv7 02/19] mm/zsmalloc: Prepare to variable MAX_PHYSMEM

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Date:   Thu, 28 Sep 2017 10:10:34 +0200
From:   Ingo Molnar <mingo@...nel.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>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andy Lutomirski <luto@...capital.net>,
        Cyrill Gorcunov <gorcunov@...nvz.org>,
        Borislav Petkov <bp@...e.de>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Minchan Kim <minchan@...nel.org>,
        Nitin Gupta <ngupta@...are.org>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Subject: Re: [PATCHv7 02/19] mm/zsmalloc: Prepare to variable MAX_PHYSMEM_BITS


* 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.
> 
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> Cc: Minchan Kim <minchan@...nel.org>
> Cc: Nitin Gupta <ngupta@...are.org>
> Cc: Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
> ---
>  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 ...

Thanks,

	Ingo