Re: [PATCH] mm: Define ARCH_HAS_FIRST_USER_ADDRESS

[Anshuman Khandual <anshuman.khandual@arm.com>]

On 4/14/21 10:52 AM, Christophe Leroy wrote:
> 
> 
> Le 14/04/2021 à 04:54, Anshuman Khandual a écrit :
>> Currently most platforms define FIRST_USER_ADDRESS as 0UL duplicating the
>> same code all over. Instead define a new option ARCH_HAS_FIRST_USER_ADDRESS
>> for those platforms which would override generic default FIRST_USER_ADDRESS
>> value 0UL. This makes it much cleaner with reduced code.
>>
>> Cc: linux-alpha@vger.kernel.org
>> Cc: linux-snps-arc@lists.infradead.org
>> Cc: linux-arm-kernel@lists.infradead.org
>> Cc: linux-csky@vger.kernel.org
>> Cc: linux-hexagon@vger.kernel.org
>> Cc: linux-ia64@vger.kernel.org
>> Cc: linux-m68k@lists.linux-m68k.org
>> Cc: linux-mips@vger.kernel.org
>> Cc: openrisc@lists.librecores.org
>> Cc: linux-parisc@vger.kernel.org
>> Cc: linuxppc-dev@lists.ozlabs.org
>> Cc: linux-riscv@lists.infradead.org
>> Cc: linux-s390@vger.kernel.org
>> Cc: linux-sh@vger.kernel.org
>> Cc: sparclinux@vger.kernel.org
>> Cc: linux-um@lists.infradead.org
>> Cc: linux-xtensa@linux-xtensa.org
>> Cc: x86@kernel.org
>> Cc: linux-mm@kvack.org
>> Cc: linux-kernel@vger.kernel.org
>> Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
>> ---
>>   arch/alpha/include/asm/pgtable.h             | 1 -
>>   arch/arc/include/asm/pgtable.h               | 6 ------
>>   arch/arm/Kconfig                             | 1 +
>>   arch/arm64/include/asm/pgtable.h             | 2 --
>>   arch/csky/include/asm/pgtable.h              | 1 -
>>   arch/hexagon/include/asm/pgtable.h           | 3 ---
>>   arch/ia64/include/asm/pgtable.h              | 1 -
>>   arch/m68k/include/asm/pgtable_mm.h           | 1 -
>>   arch/microblaze/include/asm/pgtable.h        | 2 --
>>   arch/mips/include/asm/pgtable-32.h           | 1 -
>>   arch/mips/include/asm/pgtable-64.h           | 1 -
>>   arch/nds32/Kconfig                           | 1 +
>>   arch/nios2/include/asm/pgtable.h             | 2 --
>>   arch/openrisc/include/asm/pgtable.h          | 1 -
>>   arch/parisc/include/asm/pgtable.h            | 2 --
>>   arch/powerpc/include/asm/book3s/pgtable.h    | 1 -
>>   arch/powerpc/include/asm/nohash/32/pgtable.h | 1 -
>>   arch/powerpc/include/asm/nohash/64/pgtable.h | 2 --
>>   arch/riscv/include/asm/pgtable.h             | 2 --
>>   arch/s390/include/asm/pgtable.h              | 2 --
>>   arch/sh/include/asm/pgtable.h                | 2 --
>>   arch/sparc/include/asm/pgtable_32.h          | 1 -
>>   arch/sparc/include/asm/pgtable_64.h          | 3 ---
>>   arch/um/include/asm/pgtable-2level.h         | 1 -
>>   arch/um/include/asm/pgtable-3level.h         | 1 -
>>   arch/x86/include/asm/pgtable_types.h         | 2 --
>>   arch/xtensa/include/asm/pgtable.h            | 1 -
>>   include/linux/mm.h                           | 4 ++++
>>   mm/Kconfig                                   | 4 ++++
>>   29 files changed, 10 insertions(+), 43 deletions(-)
>>
>> diff --git a/include/linux/mm.h b/include/linux/mm.h
>> index 8ba434287387..47098ccd715e 100644
>> --- a/include/linux/mm.h
>> +++ b/include/linux/mm.h
>> @@ -46,6 +46,10 @@ extern int sysctl_page_lock_unfairness;
>>     void init_mm_internals(void);
>>   +#ifndef ARCH_HAS_FIRST_USER_ADDRESS
> 
> I guess you didn't test it ..... :)

In fact I did :) Though just booted it on arm64 and cross compiled on
multiple others platforms.

> 
> should be #ifndef CONFIG_ARCH_HAS_FIRST_USER_ADDRESS

Right, meant that instead.

> 
>> +#define FIRST_USER_ADDRESS    0UL
>> +#endif
> 
> But why do we need a config option at all for that ?
> 
> Why not just:
> 
> #ifndef FIRST_USER_ADDRESS
> #define FIRST_USER_ADDRESS    0UL
> #endif

This sounds simpler. But just wondering, would not there be any possibility
of build problems due to compilation sequence between arch and generic code ?