Subject: Re: [RFC REBASED 4/5] powerpc/mm/slice: Use const pointers to cached slice masks where possible

From: "Aneesh Kumar K.V" <[email-masked]>
Date: Tue, 27 Feb 2018 12:59:53 +0530

Christophe Leroy <[email-masked]> writes:

> The slice_mask cache was a basic conversion which copied the slice
> mask into caller's structures, because that's how the original code
> worked. In most cases the pointer can be used directly instead, saving
> a copy and an on-stack structure.
>
> This also converts the slice_mask bit operation helpers to be the usual
> 3-operand kind, which is clearer to work with. And we remove some
> unnecessary intermediate bitmaps, reducing stack and copy overhead
> further.

Can we move the reduce unncessary intermediate bitmaps as another patch?

>
> Signed-off-by: Nicholas Piggin <[email-masked]>
> Signed-off-by: Christophe Leroy <[email-masked]>
> ---
>  arch/powerpc/include/asm/book3s/64/slice.h |  7 +++
>  arch/powerpc/include/asm/nohash/32/slice.h |  6 +++
>  arch/powerpc/mm/slice.c                    | 77 ++++++++++++++++++------------
>  3 files changed, 59 insertions(+), 31 deletions(-)
>
> diff --git a/arch/powerpc/include/asm/book3s/64/slice.h b/arch/powerpc/include/asm/book3s/64/slice.h
> index f9a2c8bd7a77..be1ce8e91ad1 100644
> --- a/arch/powerpc/include/asm/book3s/64/slice.h
> +++ b/arch/powerpc/include/asm/book3s/64/slice.h
> @@ -63,6 +63,13 @@ static inline void slice_bitmap_set(unsigned long *map, unsigned int start,
>  {
>  	bitmap_set(map, start, nbits);
>  }
> +
> +static inline void slice_bitmap_copy(unsigned long *dst,
> +				     const unsigned long *src,
> +				     unsigned int nbits)
> +{
> +	bitmap_copy(dst, src, nbits);
> +}
>  #endif /* __ASSEMBLY__ */
>  
>  #else /* CONFIG_PPC_MM_SLICES */
> diff --git a/arch/powerpc/include/asm/nohash/32/slice.h b/arch/powerpc/include/asm/nohash/32/slice.h
> index bcb4924f7d22..38f041e01a0a 100644
> --- a/arch/powerpc/include/asm/nohash/32/slice.h
> +++ b/arch/powerpc/include/asm/nohash/32/slice.h
> @@ -58,6 +58,12 @@ static inline void slice_bitmap_set(unsigned long *map, unsigned int start,
>  				    unsigned int nbits)
>  {
>  }
> +
> +static inline void slice_bitmap_copy(unsigned long *dst,
> +				     const unsigned long *src,
> +				     unsigned int nbits)
> +{
> +}
>  #endif /* __ASSEMBLY__ */
>  
>  #endif /* CONFIG_PPC_MM_SLICES */
> diff --git a/arch/powerpc/mm/slice.c b/arch/powerpc/mm/slice.c
> index 311168ca3939..b8b691369c29 100644
> --- a/arch/powerpc/mm/slice.c
> +++ b/arch/powerpc/mm/slice.c
> @@ -468,21 +468,30 @@ static unsigned long slice_find_area(struct mm_struct *mm, unsigned long len,
>  		return slice_find_area_bottomup(mm, len, mask, psize, high_limit);
>  }
>  
> -static inline void slice_or_mask(struct slice_mask *dst,
> +static inline void slice_copy_mask(struct slice_mask *dst,
>  					const struct slice_mask *src)
>  {
> -	dst->low_slices |= src->low_slices;
> -	slice_bitmap_or(dst->high_slices, dst->high_slices, src->high_slices,
> +	dst->low_slices = src->low_slices;
> +	slice_bitmap_copy(dst->high_slices, src->high_slices, SLICE_NUM_HIGH);
> +}
> +
> +static inline void slice_or_mask(struct slice_mask *dst,
> +				 const struct slice_mask *src1,
> +				 const struct slice_mask *src2)
> +{
> +	dst->low_slices = src1->low_slices | src2->low_slices;
> +	slice_bitmap_or(dst->high_slices, src1->high_slices, src2->high_slices,
>  			SLICE_NUM_HIGH);
>  }
>  
>  static inline void slice_andnot_mask(struct slice_mask *dst,
> -					const struct slice_mask *src)
> +				     const struct slice_mask *src1,
> +				     const struct slice_mask *src2)
>  {
> -	dst->low_slices &= ~src->low_slices;
> +	dst->low_slices = src1->low_slices & ~src2->low_slices;
>  
> -	slice_bitmap_andnot(dst->high_slices, dst->high_slices,
> -			    src->high_slices, SLICE_NUM_HIGH);
> +	slice_bitmap_andnot(dst->high_slices, src1->high_slices,
> +			    src2->high_slices, SLICE_NUM_HIGH);
>  }
>  
>  #ifdef CONFIG_PPC_64K_PAGES
> @@ -495,10 +504,10 @@ unsigned long slice_get_unmapped_area(unsigned long addr, unsigned long len,
>  				      unsigned long flags, unsigned int psize,
>  				      int topdown)
>  {
> -	struct slice_mask mask;
>  	struct slice_mask good_mask;
>  	struct slice_mask potential_mask;
> -	struct slice_mask compat_mask;
> +	const struct slice_mask *maskp;
> +	const struct slice_mask *compat_maskp = NULL;
>  	int fixed = (flags & MAP_FIXED);
>  	int pshift = max_t(int, mmu_psize_defs[psize].shift, PAGE_SHIFT);
>  	unsigned long page_size = 1UL << pshift;
> @@ -537,9 +546,6 @@ unsigned long slice_get_unmapped_area(unsigned long addr, unsigned long len,
>  	potential_mask.low_slices = 0;
>  	slice_bitmap_zero(potential_mask.high_slices, SLICE_NUM_HIGH);
>  
> -	compat_mask.low_slices = 0;
> -	slice_bitmap_zero(compat_mask.high_slices, SLICE_NUM_HIGH);
> -
>  	/* Sanity checks */
>  	BUG_ON(mm->task_size == 0);
>  	BUG_ON(mm->context.slb_addr_limit == 0);
> @@ -562,7 +568,7 @@ unsigned long slice_get_unmapped_area(unsigned long addr, unsigned long len,
>  	/* First make up a "good" mask of slices that have the right size
>  	 * already
>  	 */
> -	good_mask = *slice_mask_for_size(mm, psize);
> +	maskp = slice_mask_for_size(mm, psize);
>  	slice_print_mask(" good_mask", &good_mask);
>  
>  	/*
> @@ -587,11 +593,16 @@ unsigned long slice_get_unmapped_area(unsigned long addr, unsigned long len,
>  #ifdef CONFIG_PPC_64K_PAGES
>  	/* If we support combo pages, we can allow 64k pages in 4k slices */
>  	if (psize == MMU_PAGE_64K) {
> -		compat_mask = *slice_mask_for_size(mm, MMU_PAGE_4K);
> +		compat_maskp = slice_mask_for_size(mm, MMU_PAGE_4K);
>  		if (fixed)
> -			slice_or_mask(&good_mask, &compat_mask);
> -	}
> +			slice_or_mask(&good_mask, maskp, compat_maskp);
> +		else
> +			slice_copy_mask(&good_mask, maskp);
> +	} else
>  #endif
> +	{
> +		slice_copy_mask(&good_mask, maskp);
> +	}
>  
>  	/* First check hint if it's valid or if we have MAP_FIXED */
>  	if (addr || fixed) {
> @@ -621,7 +632,7 @@ unsigned long slice_get_unmapped_area(unsigned long addr, unsigned long len,
>  	 * empty and thus can be converted
>  	 */
>  	slice_mask_for_free(mm, &potential_mask, high_limit);
> -	slice_or_mask(&potential_mask, &good_mask);
> +	slice_or_mask(&potential_mask, &potential_mask, &good_mask);
>  	slice_print_mask(" potential", &potential_mask);
>  
>  	if (addr || fixed) {
> @@ -658,7 +669,7 @@ unsigned long slice_get_unmapped_area(unsigned long addr, unsigned long len,
>  #ifdef CONFIG_PPC_64K_PAGES
>  	if (addr == -ENOMEM && psize == MMU_PAGE_64K) {
>  		/* retry the search with 4k-page slices included */
> -		slice_or_mask(&potential_mask, &compat_mask);
> +		slice_or_mask(&potential_mask, &potential_mask, compat_maskp);
>  		addr = slice_find_area(mm, len, &potential_mask,
>  				       psize, topdown, high_limit);
>  	}
> @@ -667,16 +678,17 @@ unsigned long slice_get_unmapped_area(unsigned long addr, unsigned long len,
>  	if (addr == -ENOMEM)
>  		return -ENOMEM;
>  
> -	slice_range_to_mask(addr, len, &mask);
> +	slice_range_to_mask(addr, len, &potential_mask);

Can we avoid reusing variables like that. I had to look at the change
below to ensure that this is intended change. The print below is wrong
then. In my patch series I did try to remove  potential_mask and compat
mask and used tmp_mask in its place. IMHO that resulted in much simpiler
code. I do recompute tmp_mask for 4K size because of lack of variable.
But then i guess with your change to cache mask for different page size,
it should not have an impact?

>  	slice_dbg(" found potential area at 0x%lx\n", addr);
> -	slice_print_mask(" mask", &mask);
> +	slice_print_mask(" mask", maskp);
>  
>   convert:
> -	slice_andnot_mask(&mask, &good_mask);
> -	slice_andnot_mask(&mask, &compat_mask);
> -	if (mask.low_slices ||
> -	    !slice_bitmap_empty(mask.high_slices, SLICE_NUM_HIGH)) {
> -		slice_convert(mm, &mask, psize);
> +	slice_andnot_mask(&potential_mask, &potential_mask, &good_mask);
> +	if (compat_maskp && !fixed)
> +		slice_andnot_mask(&potential_mask, &potential_mask, compat_maskp);
> +	if (potential_mask.low_slices ||
> +	    !slice_bitmap_empty(potential_mask.high_slices, SLICE_NUM_HIGH)) {
> +		slice_convert(mm, &potential_mask, psize);
>  		if (psize > MMU_PAGE_BASE)
>  			on_each_cpu(slice_flush_segments, mm, 1);
>  	}
> @@ -834,19 +846,22 @@ void slice_set_range_psize(struct mm_struct *mm, unsigned long start,
>  int is_hugepage_only_range(struct mm_struct *mm, unsigned long addr,
>  			   unsigned long len)
>  {
> -	struct slice_mask available;
> +	const struct slice_mask *maskp;
>  	unsigned int psize = mm->context.user_psize;
>  
>  	if (radix_enabled())
>  		return 0;
>  
> -	available = *slice_mask_for_size(mm, psize);
> +	maskp = slice_mask_for_size(mm, psize);
>  #ifdef CONFIG_PPC_64K_PAGES
>  	/* We need to account for 4k slices too */
>  	if (psize == MMU_PAGE_64K) {
> -		struct slice_mask compat_mask;
> -		compat_mask = *slice_mask_for_size(mm, MMU_PAGE_4K);
> -		slice_or_mask(&available, &compat_mask);
> +		const struct slice_mask *compat_maskp;
> +		struct slice_mask available;
> +
> +		compat_maskp = slice_mask_for_size(mm, MMU_PAGE_4K);
> +		slice_or_mask(&available, maskp, compat_maskp);
> +		return !slice_check_range_fits(mm, &available, addr, len);
>  	}
>  #endif
>  
> @@ -856,6 +871,6 @@ int is_hugepage_only_range(struct mm_struct *mm, unsigned long addr,
>  	slice_print_mask(" mask", &mask);
>  	slice_print_mask(" available", &available);
>  #endif
> -	return !slice_check_range_fits(mm, &available, addr, len);
> +	return !slice_check_range_fits(mm, maskp, addr, len);
>  }
>  #endif
> -- 
> 2.13.3