Re: [PATCH] mm: Define ARCH_HAS_FIRST_USER_ADDRESS

[Christophe Leroy <christophe.leroy@csgroup.eu>]

Le 14/04/2021 à 07:59, Anshuman Khandual a écrit :
> 
> 
> 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 ?
> 

For sure it has to be addresses carefully, but there are already a lot of stuff like that around 
pgtables.h

For instance, pte_offset_kernel() has a generic definition in linux/pgtables.h based on whether it 
is already defined or not.

Taking into account that FIRST_USER_ADDRESS is today in the architectures's asm/pgtables.h, I think 
putting the fallback definition in linux/pgtable.h would do the trick.