linux-kernel - Re: [PATCH v2 01/23] asm-generic: add generic MMU versions of mmu context functions

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Date:   Tue, 01 Sep 2020 17:14:49 +1000
From:   Nicholas Piggin <npiggin@...il.com>
To:     Mike Rapoport <rppt@...nel.org>
Cc:     Arnd Bergmann <arnd@...db.de>, linux-arch@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v2 01/23] asm-generic: add generic MMU versions of mmu
 context functions

Excerpts from Mike Rapoport's message of September 1, 2020 4:49 pm:
> On Thu, Aug 27, 2020 at 12:52:27AM +1000, Nicholas Piggin wrote:
>> Many of these are no-ops on many architectures, so extend mmu_context.h
>> to cover MMU and NOMMU, and split the NOMMU bits out to nommu_context.h
>> 
>> Cc: Arnd Bergmann <arnd@...db.de>
>> Cc: linux-arch@...r.kernel.org
>> Signed-off-by: Nicholas Piggin <npiggin@...il.com>
>> ---
>>  arch/microblaze/include/asm/mmu_context.h |  2 +-
>>  arch/sh/include/asm/mmu_context.h         |  2 +-
>>  include/asm-generic/mmu_context.h         | 57 +++++++++++++++++------
>>  include/asm-generic/nommu_context.h       | 19 ++++++++
>>  4 files changed, 64 insertions(+), 16 deletions(-)
>>  create mode 100644 include/asm-generic/nommu_context.h
>> 
>> diff --git a/arch/microblaze/include/asm/mmu_context.h b/arch/microblaze/include/asm/mmu_context.h
>> index f74f9da07fdc..34004efb3def 100644
>> --- a/arch/microblaze/include/asm/mmu_context.h
>> +++ b/arch/microblaze/include/asm/mmu_context.h
>> @@ -2,5 +2,5 @@
>>  #ifdef CONFIG_MMU
>>  # include <asm/mmu_context_mm.h>
>>  #else
>> -# include <asm-generic/mmu_context.h>
>> +# include <asm-generic/nommu_context.h>
>>  #endif
>> diff --git a/arch/sh/include/asm/mmu_context.h b/arch/sh/include/asm/mmu_context.h
>> index f664e51e8a15..461b1304580b 100644
>> --- a/arch/sh/include/asm/mmu_context.h
>> +++ b/arch/sh/include/asm/mmu_context.h
>> @@ -133,7 +133,7 @@ static inline void switch_mm(struct mm_struct *prev,
>>  #define set_TTB(pgd)			do { } while (0)
>>  #define get_TTB()			(0)
>>  
>> -#include <asm-generic/mmu_context.h>
>> +#include <asm-generic/nommu_context.h>
>>  
>>  #endif /* CONFIG_MMU */
>>  
>> diff --git a/include/asm-generic/mmu_context.h b/include/asm-generic/mmu_context.h
>> index 6be9106fb6fb..86cea80a50df 100644
>> --- a/include/asm-generic/mmu_context.h
>> +++ b/include/asm-generic/mmu_context.h
>> @@ -3,44 +3,73 @@
>>  #define __ASM_GENERIC_MMU_CONTEXT_H
>>  
>>  /*
>> - * Generic hooks for NOMMU architectures, which do not need to do
>> - * anything special here.
>> + * Generic hooks to implement no-op functionality.
>>   */
>>  
>> -#include <asm-generic/mm_hooks.h>
>> -
>>  struct task_struct;
>>  struct mm_struct;
>>  
>> +/*
>> + * enter_lazy_tlb - Called when "tsk" is about to enter lazy TLB mode.
>> + *
>> + * @mm:  the currently active mm context which is becoming lazy
>> + * @tsk: task which is entering lazy tlb
>> + *
>> + * tsk->mm will be NULL
>> + */
>> +#ifndef enter_lazy_tlb
>>  static inline void enter_lazy_tlb(struct mm_struct *mm,
>>  			struct task_struct *tsk)
>>  {
>>  }
>> +#endif
>>  
>> +/**
>> + * init_new_context - Initialize context of a new mm_struct.
>> + * @tsk: task struct for the mm
>> + * @mm:  the new mm struct
> 
> 'make *docs' will complain here about missing Return: description

Thanks, I added something. Looks like we can't get rid of it, sparc
and um can return error. Oh well.

>> + */
>> +#ifndef init_new_context
>>  static inline int init_new_context(struct task_struct *tsk,
>>  			struct mm_struct *mm)
>>  {
>>  	return 0;
>>  }
>> +#endif
> 
> Most architectures have non-trivial init_new_context, maybe this
> should go into nommu_context.h?

Hmm, I guess you could go either way. A few do have no-op functions so I 
think that qualifies them if the rule is less ambiguous and we provide 
no-op for functions unless it does not make any sense (e.g., switch_mm 
for MMU). More importantly I already have the code written and am lazy  
:)

Thanks,
Nick