linux-kernel - Re: [RFC v2 01/12] powerpc: Free up four 64K PTE bits in 4K backed hpte pages.

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Message-Id: <7348e178-b79e-421a-8dd5-4cfbcd56027a@linux.vnet.ibm.com>
Date:   Thu, 22 Jun 2017 14:37:27 +0530
From:   Anshuman Khandual <khandual@...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,
        aneesh.kumar@...ux.vnet.ibm.com, bsingharora@...il.com,
        dave.hansen@...el.com, hbabu@...ibm.com
Subject: Re: [RFC v2 01/12] powerpc: Free up four 64K PTE bits in 4K backed
 hpte pages.

On 06/17/2017 09:22 AM, Ram Pai wrote:
> Rearrange 64K PTE bits to  free  up  bits 3, 4, 5  and  6
> in the 4K backed hpte pages. These bits continue to be used
> for 64K backed hpte pages in this patch, but will be freed
> up in the next patch.
> 
> The patch does the following change to the 64K PTE format
> 
> H_PAGE_BUSY moves from bit 3 to bit 9
> 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 four  bits((H_PAGE_F_SECOND|H_PAGE_F_GIX) that represent a slot
> is  initialized  to  0xF  indicating  an invalid  slot.  If  a hpte
> gets cached in a 0xF  slot(i.e  7th  slot  of  secondary),  it   is
> released immediately. In  other  words, even  though   0xF   is   a
> valid slot we discard  and consider it as an invalid
> slot;i.e 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    hpte   in the 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  may  have an 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.

Scanned through the PTE format again for hash 64K and 4K. It seems
to me that there might be 5 free bits already present on the PTE
format. I might have seriously mistaken something here :) Please
correct me if that is not the case. _RPAGE_RPN* I think is applicable
only for hash page table format and will not be available for radix
later.

+#define _PAGE_FREE_1           0x0000000000000040UL /* Not used */
+#define _RPAGE_SW0             0x2000000000000000UL /* Not used */
+#define _RPAGE_SW1             0x0000000000000800UL /* Not used */
+#define _RPAGE_RPN42           0x0040000000000000UL /* Not used */
+#define _RPAGE_RPN41           0x0020000000000000UL /* Not used */