Re: [RFC PATCH] powerpc/mm: Reduce memory usage for mm_context_t for radix

[Nicholas Piggin <npiggin@gmail.com>]

Christophe Leroy's on April 3, 2019 4:31 am:
> 
> 
> Le 02/04/2019 à 16:34, Aneesh Kumar K.V a écrit :
>> Currently, our mm_context_t on book3s64 include all hash specific
>> context details like slice mask, subpage protection details. We
>> can skip allocating those on radix. This will help us to save
>> 8K per mm_context with radix translation.
>> 
>> With the patch applied we have
>> 
>> sizeof(mm_context_t)  = 136
>> sizeof(struct hash_mm_context)  = 8288
>> 
>> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
>> ---
>> NOTE:
>> 
>> If we want to do this, I am still trying to figure out how best we can do this
>> without all the #ifdef and other overhead for 8xx book3e
>> 
>> 
>>   arch/powerpc/include/asm/book3s/64/mmu-hash.h |  2 +-
>>   arch/powerpc/include/asm/book3s/64/mmu.h      | 48 +++++++++++--------
>>   arch/powerpc/include/asm/book3s/64/slice.h    |  6 +--
>>   arch/powerpc/kernel/paca.c                    |  9 ++--
>>   arch/powerpc/kernel/setup-common.c            |  7 ++-
>>   arch/powerpc/mm/hash_utils_64.c               | 10 ++--
>>   arch/powerpc/mm/mmu_context_book3s64.c        | 16 ++++++-
>>   arch/powerpc/mm/slb.c                         |  2 +-
>>   arch/powerpc/mm/slice.c                       | 48 +++++++++----------
>>   arch/powerpc/mm/subpage-prot.c                |  8 ++--
>>   10 files changed, 91 insertions(+), 65 deletions(-)
>> 
>> diff --git a/arch/powerpc/include/asm/book3s/64/mmu-hash.h b/arch/powerpc/include/asm/book3s/64/mmu-hash.h
>> index a28a28079edb..d801be977623 100644
>> --- a/arch/powerpc/include/asm/book3s/64/mmu-hash.h
>> +++ b/arch/powerpc/include/asm/book3s/64/mmu-hash.h
>> @@ -657,7 +657,7 @@ extern void slb_set_size(u16 size);
>>   
>>   /* 4 bits per slice and we have one slice per 1TB */
>>   #define SLICE_ARRAY_SIZE	(H_PGTABLE_RANGE >> 41)
>> -#define TASK_SLICE_ARRAY_SZ(x)	((x)->context.slb_addr_limit >> 41)
>> +#define TASK_SLICE_ARRAY_SZ(x)	((x)->context.hash_context->slb_addr_limit >> 41)
>>   
>>   #ifndef __ASSEMBLY__
>>   
>> diff --git a/arch/powerpc/include/asm/book3s/64/mmu.h b/arch/powerpc/include/asm/book3s/64/mmu.h
>> index a809bdd77322..07e76e304a3b 100644
>> --- a/arch/powerpc/include/asm/book3s/64/mmu.h
>> +++ b/arch/powerpc/include/asm/book3s/64/mmu.h
>> @@ -114,6 +114,33 @@ struct slice_mask {
>>   	DECLARE_BITMAP(high_slices, SLICE_NUM_HIGH);
>>   };
>>   
>> +struct hash_mm_context {
>> +
>> +	u16 user_psize; /* page size index */
> 
> Could we keep that in mm_context_t ?

Why do you want it there?


>> @@ -155,15 +155,15 @@ static struct slice_mask *slice_mask_for_size(struct mm_struct *mm, int psize)
>>   {
>>   #ifdef CONFIG_PPC_64K_PAGES
>>   	if (psize == MMU_PAGE_64K)
>> -		return &mm->context.mask_64k;
>> +		return &mm->context.hash_context->mask_64k;
> 
> You should take the two patches below, that would help:
> https://patchwork.ozlabs.org/patch/1059056/
> https://patchwork.ozlabs.org/patch/1059058/

Above patches seem good I think. What I think Aneesh should have is
is a macro or inline that gives a pointer to the hash_mm_context from
an mm_context pointer.

Architectures which always want it should just put the hash struct
in their mm_context struct and that avoids the pointer overhead
completely.

Thanks,
Nick