linux-kernel - Re: [PATCH V2 1/7] m68k/mm: Change pmd

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Message-ID: <a35f99b6-1510-443c-bb6f-7e312cbd4f79@arm.com>
Date: Tue, 17 Sep 2024 11:27:49 +0100
From: Ryan Roberts <ryan.roberts@....com>
To: David Hildenbrand <david@...hat.com>,
 Anshuman Khandual <anshuman.khandual@....com>, linux-mm@...ck.org
Cc: Andrew Morton <akpm@...ux-foundation.org>,
 "Mike Rapoport (IBM)" <rppt@...nel.org>, Arnd Bergmann <arnd@...db.de>,
 x86@...nel.org, linux-m68k@...ts.linux-m68k.org,
 linux-fsdevel@...r.kernel.org, kasan-dev@...glegroups.com,
 linux-kernel@...r.kernel.org, linux-perf-users@...r.kernel.org,
 Geert Uytterhoeven <geert@...ux-m68k.org>, Guo Ren <guoren@...nel.org>,
 Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH V2 1/7] m68k/mm: Change pmd_val()

On 17/09/2024 11:20, David Hildenbrand wrote:
> On 17.09.24 09:31, Anshuman Khandual wrote:
>> This changes platform's pmd_val() to access the pmd_t element directly like
>> other architectures rather than current pointer address based dereferencing
>> that prevents transition into pmdp_get().
>>
>> Cc: Geert Uytterhoeven <geert@...ux-m68k.org>
>> Cc: Guo Ren <guoren@...nel.org>
>> Cc: Arnd Bergmann <arnd@...db.de>
>> Cc: linux-m68k@...ts.linux-m68k.org
>> Cc: linux-kernel@...r.kernel.org
>> Signed-off-by: Anshuman Khandual <anshuman.khandual@....com>
>> ---
>>   arch/m68k/include/asm/page.h | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/arch/m68k/include/asm/page.h b/arch/m68k/include/asm/page.h
>> index 8cfb84b49975..be3f2c2a656c 100644
>> --- a/arch/m68k/include/asm/page.h
>> +++ b/arch/m68k/include/asm/page.h
>> @@ -19,7 +19,7 @@
>>    */
>>   #if !defined(CONFIG_MMU) || CONFIG_PGTABLE_LEVELS == 3
>>   typedef struct { unsigned long pmd; } pmd_t;
>> -#define pmd_val(x)    ((&x)->pmd)
>> +#define pmd_val(x)    ((x).pmd)
>>   #define __pmd(x)    ((pmd_t) { (x) } )
>>   #endif
>>   
> 
> Trying to understand what's happening here, I stumbled over
> 
> commit ef22d8abd876e805b604e8f655127de2beee2869
> Author: Peter Zijlstra <peterz@...radead.org>
> Date:   Fri Jan 31 13:45:36 2020 +0100
> 
>     m68k: mm: Restructure Motorola MMU page-table layout
>         The Motorola 68xxx MMUs, 040 (and later) have a fixed 7,7,{5,6}
>     page-table setup, where the last depends on the page-size selected (8k
>     vs 4k resp.), and head.S selects 4K pages. For 030 (and earlier) we
>     explicitly program 7,7,6 and 4K pages in %tc.
>         However, the current code implements this mightily weird. What it does
>     is group 16 of those (6 bit) pte tables into one 4k page to not waste
>     space. The down-side is that that forces pmd_t to be a 16-tuple
>     pointing to consecutive pte tables.
>         This breaks the generic code which assumes READ_ONCE(*pmd) will be
>     word sized.
> 
> Where we did
> 
>  #if !defined(CONFIG_MMU) || CONFIG_PGTABLE_LEVELS == 3
> -typedef struct { unsigned long pmd[16]; } pmd_t;
> -#define pmd_val(x)     ((&x)->pmd[0])
> -#define __pmd(x)       ((pmd_t) { { (x) }, })
> +typedef struct { unsigned long pmd; } pmd_t;
> +#define pmd_val(x)     ((&x)->pmd)
> +#define __pmd(x)       ((pmd_t) { (x) } )
>  #endif
> 
> So I assume this should be fine

I think you're implying that taking the address then using arrow operator was
needed when pmd was an array? I don't really understand that if so? Surely:

  ((x).pmd[0])

would have worked too? I traced back further, and a version of that macro exists
with the "address of" and arrow operator since the beginning of (git) time.

> 
> Acked-by: David Hildenbrand <david@...hat.com>
>