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

[Christophe Leroy]

Le 04/04/2019 à 18:13, Nicholas Piggin a écrit :
> 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 at 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?

It was just to avoid so many changes and also the pointer complexity for 
just saving 2 bytes. But your suggestion below seems good.

> 
> 
>>> @@ -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.

Good idea. But please call it something else, as it has nothing to do 
with hash outside of book3s64 (ie the 8xx is a nohash32).

Christophe

> 
> 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
>