[PATCH] powerpc/64: implement a slice mask cache

[PATCH] powerpc/64: implement a slice mask cache

[Nicholas Piggin]

Calculating the slice mask can become a signifcant overhead for
get_unmapped_area. The mask is relatively small and does not change
frequently, so we can cache it in the mm context.

This saves about 30% kernel time on a 4K user address allocation
in a microbenchmark.

Comments on the approach taken? I think there is the option for fixed
allocations to avoid some of the slice calculation entirely, but first
I think it will be good to have a general speedup that covers all
mmaps.

Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Anton Blanchard <anton@samba.org>
---
 arch/powerpc/include/asm/book3s/64/mmu.h |  8 +++++++
 arch/powerpc/mm/slice.c                  | 39 ++++++++++++++++++++++++++++++--
 2 files changed, 45 insertions(+), 2 deletions(-)

diff --git a/arch/powerpc/include/asm/book3s/64/mmu.h b/arch/powerpc/include/asm/book3s/64/mmu.h
index 5854263..0d15af4 100644
--- a/arch/powerpc/include/asm/book3s/64/mmu.h
+++ b/arch/powerpc/include/asm/book3s/64/mmu.h
@@ -71,6 +71,14 @@ typedef struct {
 #ifdef CONFIG_PPC_MM_SLICES
 	u64 low_slices_psize;	/* SLB page size encodings */
 	unsigned char high_slices_psize[SLICE_ARRAY_SIZE];
+	struct slice_mask mask_4k;
+# ifdef CONFIG_PPC_64K_PAGES
+	struct slice_mask mask_64k;
+# endif
+# ifdef CONFIG_HUGETLB_PAGE
+	struct slice_mask mask_16m;
+	struct slice_mask mask_16g;
+# endif
 #else
 	u16 sllp;		/* SLB page size encoding */
 #endif
diff --git a/arch/powerpc/mm/slice.c b/arch/powerpc/mm/slice.c
index 2b27458..559ea5f 100644
--- a/arch/powerpc/mm/slice.c
+++ b/arch/powerpc/mm/slice.c
@@ -147,7 +147,7 @@ static struct slice_mask slice_mask_for_free(struct mm_struct *mm)
 	return ret;
 }
 
-static struct slice_mask slice_mask_for_size(struct mm_struct *mm, int psize)
+static struct slice_mask calc_slice_mask_for_size(struct mm_struct *mm, int psize)
 {
 	unsigned char *hpsizes;
 	int index, mask_index;
@@ -171,6 +171,36 @@ static struct slice_mask slice_mask_for_size(struct mm_struct *mm, int psize)
 	return ret;
 }
 
+static void recalc_slice_mask_cache(struct mm_struct *mm)
+{
+	mm->context.mask_4k = calc_slice_mask_for_size(mm, MMU_PAGE_4K);
+#ifdef CONFIG_PPC_64K_PAGES
+	mm->context.mask_64k = calc_slice_mask_for_size(mm, MMU_PAGE_64K);
+#endif
+# ifdef CONFIG_HUGETLB_PAGE
+	/* Radix does not come here */
+	mm->context.mask_16m = calc_slice_mask_for_size(mm, MMU_PAGE_16M);
+	mm->context.mask_16g = calc_slice_mask_for_size(mm, MMU_PAGE_16G);
+# endif
+}
+
+static struct slice_mask slice_mask_for_size(struct mm_struct *mm, int psize)
+{
+	if (psize == MMU_PAGE_4K)
+		return mm->context.mask_4k;
+#ifdef CONFIG_PPC_64K_PAGES
+	if (psize == MMU_PAGE_64K)
+		return mm->context.mask_64k;
+#endif
+# ifdef CONFIG_HUGETLB_PAGE
+	if (psize == MMU_PAGE_16M)
+		return mm->context.mask_16m;
+	if (psize == MMU_PAGE_16G)
+		return mm->context.mask_16g;
+# endif
+	BUG();
+}
+
 static int slice_check_fit(struct slice_mask mask, struct slice_mask available)
 {
 	return (mask.low_slices & available.low_slices) == mask.low_slices &&
@@ -233,6 +263,8 @@ static void slice_convert(struct mm_struct *mm, struct slice_mask mask, int psiz
 
 	spin_unlock_irqrestore(&slice_convert_lock, flags);
 
+	recalc_slice_mask_cache(mm);
+
 	copro_flush_all_slbs(mm);
 }
 
@@ -625,7 +657,7 @@ void slice_set_user_psize(struct mm_struct *mm, unsigned int psize)
 		goto bail;
 
 	mm->context.user_psize = psize;
-	wmb();
+	wmb(); /* Why? */
 
 	lpsizes = mm->context.low_slices_psize;
 	for (i = 0; i < SLICE_NUM_LOW; i++)
@@ -652,6 +684,9 @@ void slice_set_user_psize(struct mm_struct *mm, unsigned int psize)
 		  mm->context.low_slices_psize,
 		  mm->context.high_slices_psize);
 
+	spin_unlock_irqrestore(&slice_convert_lock, flags);
+	recalc_slice_mask_cache(mm);
+	return;
  bail:
 	spin_unlock_irqrestore(&slice_convert_lock, flags);
 }
-- 
2.8.1

Re: [PATCH] powerpc/64: implement a slice mask cache

[Balbir Singh]

On Fri, Jul 22, 2016 at 10:57:28PM +1000, Nicholas Piggin wrote:
> Calculating the slice mask can become a signifcant overhead for
> get_unmapped_area. The mask is relatively small and does not change
> frequently, so we can cache it in the mm context.
> 
> This saves about 30% kernel time on a 4K user address allocation
> in a microbenchmark.
> 
> Comments on the approach taken? I think there is the option for fixed
> allocations to avoid some of the slice calculation entirely, but first
> I think it will be good to have a general speedup that covers all
> mmaps.
> 
> Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
> Cc: Anton Blanchard <anton@samba.org>
> ---
>  arch/powerpc/include/asm/book3s/64/mmu.h |  8 +++++++
>  arch/powerpc/mm/slice.c                  | 39 ++++++++++++++++++++++++++++++--
>  2 files changed, 45 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/powerpc/include/asm/book3s/64/mmu.h b/arch/powerpc/include/asm/book3s/64/mmu.h
> index 5854263..0d15af4 100644
> --- a/arch/powerpc/include/asm/book3s/64/mmu.h
> +++ b/arch/powerpc/include/asm/book3s/64/mmu.h
> @@ -71,6 +71,14 @@ typedef struct {
>  #ifdef CONFIG_PPC_MM_SLICES
>  	u64 low_slices_psize;	/* SLB page size encodings */
>  	unsigned char high_slices_psize[SLICE_ARRAY_SIZE];
> +	struct slice_mask mask_4k;
> +# ifdef CONFIG_PPC_64K_PAGES
> +	struct slice_mask mask_64k;
> +# endif
> +# ifdef CONFIG_HUGETLB_PAGE
> +	struct slice_mask mask_16m;
> +	struct slice_mask mask_16g;
> +# endif

Should we cache these in mmu_psize_defs? I am not 100% sure
if want to overload that structure, but it provides a convient
way of saying mmu_psize_defs[psize].mask instead of all
the if checks

>  #else
>  	u16 sllp;		/* SLB page size encoding */
>  #endif
> diff --git a/arch/powerpc/mm/slice.c b/arch/powerpc/mm/slice.c
> index 2b27458..559ea5f 100644
> --- a/arch/powerpc/mm/slice.c
> +++ b/arch/powerpc/mm/slice.c
> @@ -147,7 +147,7 @@ static struct slice_mask slice_mask_for_free(struct mm_struct *mm)
>  	return ret;
>  }
>  
> -static struct slice_mask slice_mask_for_size(struct mm_struct *mm, int psize)
> +static struct slice_mask calc_slice_mask_for_size(struct mm_struct *mm, int psize)
>  {
>  	unsigned char *hpsizes;
>  	int index, mask_index;
> @@ -171,6 +171,36 @@ static struct slice_mask slice_mask_for_size(struct mm_struct *mm, int psize)
>  	return ret;
>  }
>  
> +static void recalc_slice_mask_cache(struct mm_struct *mm)
> +{
> +	mm->context.mask_4k = calc_slice_mask_for_size(mm, MMU_PAGE_4K);
> +#ifdef CONFIG_PPC_64K_PAGES
> +	mm->context.mask_64k = calc_slice_mask_for_size(mm, MMU_PAGE_64K);
> +#endif
> +# ifdef CONFIG_HUGETLB_PAGE
> +	/* Radix does not come here */
> +	mm->context.mask_16m = calc_slice_mask_for_size(mm, MMU_PAGE_16M);
> +	mm->context.mask_16g = calc_slice_mask_for_size(mm, MMU_PAGE_16G);
> +# endif
> +}

Should the function above be called under slice_convert_lock?

> +
> +static struct slice_mask slice_mask_for_size(struct mm_struct *mm, int psize)
> +{
> +	if (psize == MMU_PAGE_4K)
> +		return mm->context.mask_4k;
> +#ifdef CONFIG_PPC_64K_PAGES
> +	if (psize == MMU_PAGE_64K)
> +		return mm->context.mask_64k;
> +#endif
> +# ifdef CONFIG_HUGETLB_PAGE
> +	if (psize == MMU_PAGE_16M)
> +		return mm->context.mask_16m;
> +	if (psize == MMU_PAGE_16G)
> +		return mm->context.mask_16g;
> +# endif
> +	BUG();
> +}
> +
>  static int slice_check_fit(struct slice_mask mask, struct slice_mask available)
>  {
>  	return (mask.low_slices & available.low_slices) == mask.low_slices &&
> @@ -233,6 +263,8 @@ static void slice_convert(struct mm_struct *mm, struct slice_mask mask, int psiz
>  
>  	spin_unlock_irqrestore(&slice_convert_lock, flags);
>  
> +	recalc_slice_mask_cache(mm);
> +
>  	copro_flush_all_slbs(mm);
>  }
>  
> @@ -625,7 +657,7 @@ void slice_set_user_psize(struct mm_struct *mm, unsigned int psize)
>  		goto bail;
>  
>  	mm->context.user_psize = psize;
> -	wmb();
> +	wmb(); /* Why? */
>  
>  	lpsizes = mm->context.low_slices_psize;
>  	for (i = 0; i < SLICE_NUM_LOW; i++)
> @@ -652,6 +684,9 @@ void slice_set_user_psize(struct mm_struct *mm, unsigned int psize)
>  		  mm->context.low_slices_psize,
>  		  mm->context.high_slices_psize);
>  
> +	spin_unlock_irqrestore(&slice_convert_lock, flags);
> +	recalc_slice_mask_cache(mm);
> +	return;
>   bail:
>  	spin_unlock_irqrestore(&slice_convert_lock, flags);
>  }
> -- 
> 2.8.1
> 
> _______________________________________________
> Linuxppc-dev mailing list
> Linuxppc-dev@lists.ozlabs.org
> https://lists.ozlabs.org/listinfo/linuxppc-dev

Re: [PATCH] powerpc/64: implement a slice mask cache

[Nicholas Piggin]

On Sat, 23 Jul 2016 12:19:37 +1000
Balbir Singh <bsingharora@gmail.com> wrote:

> On Fri, Jul 22, 2016 at 10:57:28PM +1000, Nicholas Piggin wrote:
> > Calculating the slice mask can become a signifcant overhead for
> > get_unmapped_area. The mask is relatively small and does not change
> > frequently, so we can cache it in the mm context.
> > 
> > This saves about 30% kernel time on a 4K user address allocation
> > in a microbenchmark.
> > 
> > Comments on the approach taken? I think there is the option for
> > fixed allocations to avoid some of the slice calculation entirely,
> > but first I think it will be good to have a general speedup that
> > covers all mmaps.
> > 
> > Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
> > Cc: Anton Blanchard <anton@samba.org>
> > ---
> >  arch/powerpc/include/asm/book3s/64/mmu.h |  8 +++++++
> >  arch/powerpc/mm/slice.c                  | 39
> > ++++++++++++++++++++++++++++++-- 2 files changed, 45 insertions(+),
> > 2 deletions(-)
> > 
> > diff --git a/arch/powerpc/include/asm/book3s/64/mmu.h
> > b/arch/powerpc/include/asm/book3s/64/mmu.h index 5854263..0d15af4
> > 100644 --- a/arch/powerpc/include/asm/book3s/64/mmu.h
> > +++ b/arch/powerpc/include/asm/book3s/64/mmu.h
> > @@ -71,6 +71,14 @@ typedef struct {
> >  #ifdef CONFIG_PPC_MM_SLICES
> >  	u64 low_slices_psize;	/* SLB page size encodings */
> >  	unsigned char high_slices_psize[SLICE_ARRAY_SIZE];
> > +	struct slice_mask mask_4k;
> > +# ifdef CONFIG_PPC_64K_PAGES
> > +	struct slice_mask mask_64k;
> > +# endif
> > +# ifdef CONFIG_HUGETLB_PAGE
> > +	struct slice_mask mask_16m;
> > +	struct slice_mask mask_16g;
> > +# endif  
> 
> Should we cache these in mmu_psize_defs? I am not 100% sure
> if want to overload that structure, but it provides a convient
> way of saying mmu_psize_defs[psize].mask instead of all
> the if checks

I'm not sure if we can, can we? mmu_psize_defs is global
whereas we need per-process structure.

The branches are a bit annoying, but we can't directly use an array
because it's too big. But see the comment at MMU_PAGE_* defines.
Perhaps we could change this structure to be sized at compile time to
only include possible page sizes, and would enable building a
structure like the above with simply

struct type blah[MMU_POSSIBLE_PAGE_COUNT];

Perhaps we can consider that as a follow on patch? It's probably a bit
more work to implement.


> >  #else
> >  	u16 sllp;		/* SLB page size encoding */
> >  #endif
> > diff --git a/arch/powerpc/mm/slice.c b/arch/powerpc/mm/slice.c
> > index 2b27458..559ea5f 100644
> > --- a/arch/powerpc/mm/slice.c
> > +++ b/arch/powerpc/mm/slice.c
> > @@ -147,7 +147,7 @@ static struct slice_mask
> > slice_mask_for_free(struct mm_struct *mm) return ret;
> >  }
> >  
> > -static struct slice_mask slice_mask_for_size(struct mm_struct *mm,
> > int psize) +static struct slice_mask
> > calc_slice_mask_for_size(struct mm_struct *mm, int psize) {
> >  	unsigned char *hpsizes;
> >  	int index, mask_index;
> > @@ -171,6 +171,36 @@ static struct slice_mask
> > slice_mask_for_size(struct mm_struct *mm, int psize) return ret;
> >  }
> >  
> > +static void recalc_slice_mask_cache(struct mm_struct *mm)
> > +{
> > +	mm->context.mask_4k = calc_slice_mask_for_size(mm,
> > MMU_PAGE_4K); +#ifdef CONFIG_PPC_64K_PAGES
> > +	mm->context.mask_64k = calc_slice_mask_for_size(mm,
> > MMU_PAGE_64K); +#endif
> > +# ifdef CONFIG_HUGETLB_PAGE
> > +	/* Radix does not come here */
> > +	mm->context.mask_16m = calc_slice_mask_for_size(mm,
> > MMU_PAGE_16M);
> > +	mm->context.mask_16g = calc_slice_mask_for_size(mm,
> > MMU_PAGE_16G); +# endif
> > +}  
> 
> Should the function above be called under slice_convert_lock?

Good question. The slice_convert_lock is... interesting. It only
protects the update-side of the slice page size arrays. I thought
this was okay last time I looked, but now you make me think again
maybe it is not. I need to check again what's providing exclusion
on the read side too.

I wanted to avoid doing more work under slice_convert_lock, but
we should just make that a per-mm lock anyway shouldn't we?

Thanks,
Nick

Re: [PATCH] powerpc/64: implement a slice mask cache

[Benjamin Herrenschmidt]

On Sat, 2016-07-23 at 17:10 +1000, Nicholas Piggin wrote:
> I wanted to avoid doing more work under slice_convert_lock, but
> we should just make that a per-mm lock anyway shouldn't we?

Aren't the readers under the mm sem taken for writing or has this
changed ?

Cheers,
Ben.

Re: [PATCH] powerpc/64: implement a slice mask cache

[Balbir Singh]

On Sat, Jul 23, 2016 at 05:10:36PM +1000, Nicholas Piggin wrote:
> On Sat, 23 Jul 2016 12:19:37 +1000
> Balbir Singh <bsingharora@gmail.com> wrote:
> 
> > On Fri, Jul 22, 2016 at 10:57:28PM +1000, Nicholas Piggin wrote:
> > > Calculating the slice mask can become a signifcant overhead for
> > > get_unmapped_area. The mask is relatively small and does not change
> > > frequently, so we can cache it in the mm context.
> > > 
> > > This saves about 30% kernel time on a 4K user address allocation
> > > in a microbenchmark.
> > > 
> > > Comments on the approach taken? I think there is the option for
> > > fixed allocations to avoid some of the slice calculation entirely,
> > > but first I think it will be good to have a general speedup that
> > > covers all mmaps.
> > > 
> > > Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
> > > Cc: Anton Blanchard <anton@samba.org>
> > > ---
> > >  arch/powerpc/include/asm/book3s/64/mmu.h |  8 +++++++
> > >  arch/powerpc/mm/slice.c                  | 39
> > > ++++++++++++++++++++++++++++++-- 2 files changed, 45 insertions(+),
> > > 2 deletions(-)
> > > 
> > > diff --git a/arch/powerpc/include/asm/book3s/64/mmu.h
> > > b/arch/powerpc/include/asm/book3s/64/mmu.h index 5854263..0d15af4
> > > 100644 --- a/arch/powerpc/include/asm/book3s/64/mmu.h
> > > +++ b/arch/powerpc/include/asm/book3s/64/mmu.h
> > > @@ -71,6 +71,14 @@ typedef struct {
> > >  #ifdef CONFIG_PPC_MM_SLICES
> > >  	u64 low_slices_psize;	/* SLB page size encodings */
> > >  	unsigned char high_slices_psize[SLICE_ARRAY_SIZE];
> > > +	struct slice_mask mask_4k;
> > > +# ifdef CONFIG_PPC_64K_PAGES
> > > +	struct slice_mask mask_64k;
> > > +# endif
> > > +# ifdef CONFIG_HUGETLB_PAGE
> > > +	struct slice_mask mask_16m;
> > > +	struct slice_mask mask_16g;
> > > +# endif  
> > 
> > Should we cache these in mmu_psize_defs? I am not 100% sure
> > if want to overload that structure, but it provides a convient
> > way of saying mmu_psize_defs[psize].mask instead of all
> > the if checks
> 
> I'm not sure if we can, can we? mmu_psize_defs is global
> whereas we need per-process structure.
>

Oh! sorry, I meant a structure like mmu_psize_defs.
 
> The branches are a bit annoying, but we can't directly use an array
> because it's too big. But see the comment at MMU_PAGE_* defines.
> Perhaps we could change this structure to be sized at compile time to
> only include possible page sizes, and would enable building a
> structure like the above with simply
> 
> struct type blah[MMU_POSSIBLE_PAGE_COUNT];
> 
> Perhaps we can consider that as a follow on patch? It's probably a bit
> more work to implement.
> 


Yeah.. good idea
MMU_PAGE_COUNT is 15, the size is going to be 15*8 bytes?


> 
> > >  #else
> > >  	u16 sllp;		/* SLB page size encoding */
> > >  #endif
> > > diff --git a/arch/powerpc/mm/slice.c b/arch/powerpc/mm/slice.c
> > > index 2b27458..559ea5f 100644
> > > --- a/arch/powerpc/mm/slice.c
> > > +++ b/arch/powerpc/mm/slice.c
> > > @@ -147,7 +147,7 @@ static struct slice_mask
> > > slice_mask_for_free(struct mm_struct *mm) return ret;
> > >  }
> > >  
> > > -static struct slice_mask slice_mask_for_size(struct mm_struct *mm,
> > > int psize) +static struct slice_mask
> > > calc_slice_mask_for_size(struct mm_struct *mm, int psize) {
> > >  	unsigned char *hpsizes;
> > >  	int index, mask_index;
> > > @@ -171,6 +171,36 @@ static struct slice_mask
> > > slice_mask_for_size(struct mm_struct *mm, int psize) return ret;
> > >  }
> > >  
> > > +static void recalc_slice_mask_cache(struct mm_struct *mm)
> > > +{
> > > +	mm->context.mask_4k = calc_slice_mask_for_size(mm,
> > > MMU_PAGE_4K); +#ifdef CONFIG_PPC_64K_PAGES
> > > +	mm->context.mask_64k = calc_slice_mask_for_size(mm,
> > > MMU_PAGE_64K); +#endif
> > > +# ifdef CONFIG_HUGETLB_PAGE
> > > +	/* Radix does not come here */
> > > +	mm->context.mask_16m = calc_slice_mask_for_size(mm,
> > > MMU_PAGE_16M);
> > > +	mm->context.mask_16g = calc_slice_mask_for_size(mm,
> > > MMU_PAGE_16G); +# endif
> > > +}  
> > 
> > Should the function above be called under slice_convert_lock?
> 
> Good question. The slice_convert_lock is... interesting. It only
> protects the update-side of the slice page size arrays. I thought
> this was okay last time I looked, but now you make me think again
> maybe it is not. I need to check again what's providing exclusion
> on the read side too.
> 
> I wanted to avoid doing more work under slice_convert_lock, but
> we should just make that a per-mm lock anyway shouldn't we?
>

Yeah and Ben's comment in the reply suggest we already hold a
per mm lock on the read side.

Balbir Singh

 

Re: [PATCH] powerpc/64: implement a slice mask cache

[Nicholas Piggin]

On Sat, 23 Jul 2016 18:49:06 +1000
Benjamin Herrenschmidt <benh@kernel.crashing.org> wrote:

> On Sat, 2016-07-23 at 17:10 +1000, Nicholas Piggin wrote:
> > I wanted to avoid doing more work under slice_convert_lock, but
> > we should just make that a per-mm lock anyway shouldn't we?  
> 
> Aren't the readers under the mm sem taken for writing or has this
> changed ?


I don't think this has changed, but I look at the writers now,
which aren't synchronized by mm sem. But neither are readers
under the slice_convert_lock, so I'm looking at what the locking
actually is. Is it just using atomicity of dword stores vs loads?

Thanks,
Nick

Re: [PATCH] powerpc/64: implement a slice mask cache

[Nicholas Piggin]

On Sat, 23 Jul 2016 20:36:42 +1000
Balbir Singh <bsingharora@gmail.com> wrote:

> On Sat, Jul 23, 2016 at 05:10:36PM +1000, Nicholas Piggin wrote:
> > On Sat, 23 Jul 2016 12:19:37 +1000
> > Balbir Singh <bsingharora@gmail.com> wrote:
> >   
> > > On Fri, Jul 22, 2016 at 10:57:28PM +1000, Nicholas Piggin wrote:  
> > > > Calculating the slice mask can become a signifcant overhead for
> > > > get_unmapped_area. The mask is relatively small and does not
> > > > change frequently, so we can cache it in the mm context.
> > > > 
> > > > This saves about 30% kernel time on a 4K user address allocation
> > > > in a microbenchmark.
> > > > 
> > > > Comments on the approach taken? I think there is the option for
> > > > fixed allocations to avoid some of the slice calculation
> > > > entirely, but first I think it will be good to have a general
> > > > speedup that covers all mmaps.
> > > > 
> > > > Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
> > > > Cc: Anton Blanchard <anton@samba.org>
> > > > ---
> > > >  arch/powerpc/include/asm/book3s/64/mmu.h |  8 +++++++
> > > >  arch/powerpc/mm/slice.c                  | 39
> > > > ++++++++++++++++++++++++++++++-- 2 files changed, 45
> > > > insertions(+), 2 deletions(-)
> > > > 
> > > > diff --git a/arch/powerpc/include/asm/book3s/64/mmu.h
> > > > b/arch/powerpc/include/asm/book3s/64/mmu.h index
> > > > 5854263..0d15af4 100644 ---
> > > > a/arch/powerpc/include/asm/book3s/64/mmu.h +++
> > > > b/arch/powerpc/include/asm/book3s/64/mmu.h @@ -71,6 +71,14 @@
> > > > typedef struct { #ifdef CONFIG_PPC_MM_SLICES
> > > >  	u64 low_slices_psize;	/* SLB page size
> > > > encodings */ unsigned char high_slices_psize[SLICE_ARRAY_SIZE];
> > > > +	struct slice_mask mask_4k;
> > > > +# ifdef CONFIG_PPC_64K_PAGES
> > > > +	struct slice_mask mask_64k;
> > > > +# endif
> > > > +# ifdef CONFIG_HUGETLB_PAGE
> > > > +	struct slice_mask mask_16m;
> > > > +	struct slice_mask mask_16g;
> > > > +# endif    
> > > 
> > > Should we cache these in mmu_psize_defs? I am not 100% sure
> > > if want to overload that structure, but it provides a convient
> > > way of saying mmu_psize_defs[psize].mask instead of all
> > > the if checks  
> > 
> > I'm not sure if we can, can we? mmu_psize_defs is global
> > whereas we need per-process structure.
> >  
> 
> Oh! sorry, I meant a structure like mmu_psize_defs.

In that case, sure. Avoiding the branches might be worthwhile.


> > The branches are a bit annoying, but we can't directly use an array
> > because it's too big. But see the comment at MMU_PAGE_* defines.
> > Perhaps we could change this structure to be sized at compile time
> > to only include possible page sizes, and would enable building a
> > structure like the above with simply
> > 
> > struct type blah[MMU_POSSIBLE_PAGE_COUNT];
> > 
> > Perhaps we can consider that as a follow on patch? It's probably a
> > bit more work to implement.
> >   
> 
> 
> Yeah.. good idea
> MMU_PAGE_COUNT is 15, the size is going to be 15*8 bytes?

Unfortunately, slice_mask is 16 bytes. Only 10 are used, but it
seemed too ugly to try squashing things together.


> > Good question. The slice_convert_lock is... interesting. It only
> > protects the update-side of the slice page size arrays. I thought
> > this was okay last time I looked, but now you make me think again
> > maybe it is not. I need to check again what's providing exclusion
> > on the read side too.
> > 
> > I wanted to avoid doing more work under slice_convert_lock, but
> > we should just make that a per-mm lock anyway shouldn't we?
> >  
> 
> Yeah and Ben's comment in the reply suggest we already hold a
> per mm lock on the read side.

Let's discuss this further in my reply to Ben.

Thanks,
Nick