linux-kernel - Re: [RFC v2 02/12] powerpc: Free up four 64K PTE bits in 64K backed hpte pages.

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Message-Id: <20170621201425.GM5845@ram.oc3035372033.ibm.com>
Date:   Wed, 21 Jun 2017 13:14:25 -0700
From:   Ram Pai <linuxram@...ibm.com>
To:     "Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
Cc:     linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
        benh@...nel.crashing.org, paulus@...ba.org, mpe@...erman.id.au,
        khandual@...ux.vnet.ibm.com, bsingharora@...il.com,
        dave.hansen@...el.com, hbabu@...ibm.com
Subject: Re: [RFC v2 02/12] powerpc: Free up four 64K PTE bits in 64K backed
 hpte pages.

On Wed, Jun 21, 2017 at 12:24:34PM +0530, Aneesh Kumar K.V wrote:
> Ram Pai <linuxram@...ibm.com> writes:
> 
> ....
> 
> > diff --git a/arch/powerpc/mm/hugetlbpage-hash64.c b/arch/powerpc/mm/hugetlbpage-hash64.c
> > index a84bb44..239ca86 100644
> > --- a/arch/powerpc/mm/hugetlbpage-hash64.c
> > +++ b/arch/powerpc/mm/hugetlbpage-hash64.c
> > @@ -22,6 +22,7 @@ int __hash_page_huge(unsigned long ea, unsigned long access, unsigned long vsid,
> >  		     pte_t *ptep, unsigned long trap, unsigned long flags,
> >  		     int ssize, unsigned int shift, unsigned int mmu_psize)
> >  {
> > +	real_pte_t rpte;
> >  	unsigned long vpn;
> >  	unsigned long old_pte, new_pte;
> >  	unsigned long rflags, pa, sz;
> > @@ -61,6 +62,7 @@ int __hash_page_huge(unsigned long ea, unsigned long access, unsigned long vsid,
> >  	} while(!pte_xchg(ptep, __pte(old_pte), __pte(new_pte)));
> >
> >  	rflags = htab_convert_pte_flags(new_pte);
> > +	rpte = __real_pte(__pte(old_pte), ptep);
> >
> >  	sz = ((1UL) << shift);
> >  	if (!cpu_has_feature(CPU_FTR_COHERENT_ICACHE))
> > @@ -71,15 +73,10 @@ int __hash_page_huge(unsigned long ea, unsigned long access, unsigned long vsid,
> >  	/* Check if pte already has an hpte (case 2) */
> >  	if (unlikely(old_pte & H_PAGE_HASHPTE)) {
> >  		/* There MIGHT be an HPTE for this pte */
> > -		unsigned long hash, slot;
> > +		unsigned long gslot;
> >
> > -		hash = hpt_hash(vpn, shift, ssize);
> > -		if (old_pte & H_PAGE_F_SECOND)
> > -			hash = ~hash;
> > -		slot = (hash & htab_hash_mask) * HPTES_PER_GROUP;
> > -		slot += (old_pte & H_PAGE_F_GIX) >> H_PAGE_F_GIX_SHIFT;
> > -
> > -		if (mmu_hash_ops.hpte_updatepp(slot, rflags, vpn, mmu_psize,
> > +		gslot = get_hidx_gslot(vpn, shift, ssize, rpte, 0);
> > +		if (mmu_hash_ops.hpte_updatepp(gslot, rflags, vpn, mmu_psize,
> >  					       mmu_psize, ssize, flags) == -1)
> >  			old_pte &= ~_PAGE_HPTEFLAGS;
> >  	}
> > @@ -106,8 +103,7 @@ int __hash_page_huge(unsigned long ea, unsigned long access, unsigned long vsid,
> >  			return -1;
> >  		}
> >
> > -		new_pte |= (slot << H_PAGE_F_GIX_SHIFT) &
> > -			(H_PAGE_F_SECOND | H_PAGE_F_GIX);
> > +		new_pte |= set_hidx_slot(ptep, rpte, 0, slot);
> 
> We don't really need rpte here. We are just need to track one entry
> here. May be it becomes simpler if use different helpers for 4k hpte and
> others ?

actually we need rpte here. the hidx for these 64K-hpte backed PTEs are
now stored in the second half of the pte. 
I have abstracted the helpers, so that the caller need not
know the location of the hidx. It comes in really handy.

RP