linux-kernel - Re: [RFC PATCH 1/7 v1]powerpc: Free up four PTE bits to accommodate memory keys

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Date:   Tue, 13 Jun 2017 10:22:43 +0530
From:   "Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
To:     Ram Pai <linuxram@...ibm.com>, linuxppc-dev@...ts.ozlabs.org,
        linux-kernel@...r.kernel.org
Cc:     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, linuxram@...ibm.com
Subject: Re: [RFC PATCH 1/7 v1]powerpc: Free up four PTE bits to accommodate memory keys

Ram Pai <linuxram@...ibm.com> writes:

> Rearrange  PTE   bits to  free  up  bits 3, 4, 5  and  6  for
> memory keys. Bit 3, 4, 5, 6 and 57  shall  be used for memory
> keys.
>
> The patch does the following change to the 64K PTE format
>
> H_PAGE_BUSY moves from bit 3 to bit 7
> H_PAGE_F_SECOND which occupied bit 4 moves to the second part
> 	of the pte.
> H_PAGE_F_GIX which  occupied bit 5, 6 and 7 also moves to the
> 	second part of the pte.
>
> The second part of the PTE will hold
>    a (H_PAGE_F_SECOND|H_PAGE_F_GIX)  for  64K page backed pte,
>    and sixteen (H_PAGE_F_SECOND|H_PAGE_F_GIX)  for 4k  backed
> 	pte.
>
> the four  bits((H_PAGE_F_SECOND|H_PAGE_F_GIX) that represent a slot
> is initialized to 0xF indicating a invalid slot. if a hashpage does
> get  allocated  to  the  0xF  slot, it is released and not used. In
> other words, even  though  0xF  is  a valid slot we discard it  and
> consider it as invalid slot(HPTE_SOFT_INVALID). This  gives  us  an
> opportunity to  not  depend on a bit in the primary PTE in order to
> determine the validity of a slot.
>
> When  we  release  a  0xF slot we also release a legitimate primary
> slot  and  unmap  that  entry. This  is  to  ensure  that we do get
> a legimate non-0xF slot the next time we retry for a slot.
>
> Though treating 0xF slot as invalid reduces the number of available
> slots and make have a effect on the performance, the probabilty
> of hitting a 0xF is extermely low.
>
> Compared  to the current scheme, the above described scheme reduces
> the number of false hash table updates  significantly  and  has the
> added  advantage  of  releasing  four  valuable  PTE bits for other
> purpose.
>
> This idea was jointly developed by Paul Mackerras, Aneesh, Michael
> Ellermen and myself.
>
> 4K PTE format remain unchanged currently.
>

Can you also split this patch into two. One which changes
__hash_page_4k() ie, linux pte format w.r.t 4k hash pte. Second patch
with changes w.r.t __hash_page_64k() ie, pte format w.r.t 64k hash pte.

-aneesh