From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pa0-x231.google.com (mail-pa0-x231.google.com [IPv6:2607:f8b0:400e:c03::231]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by lists.ozlabs.org (Postfix) with ESMTPS id 9E63E1A0686 for ; Fri, 19 Feb 2016 15:08:27 +1100 (AEDT) Received: by mail-pa0-x231.google.com with SMTP id fl4so43036337pad.0 for ; Thu, 18 Feb 2016 20:08:27 -0800 (PST) Subject: Re: [PATCH] powerpc/Kconfig: Update config option based on page size. To: Rashmica Gupta , linuxppc-dev@lists.ozlabs.org References: <1455846943-32147-1-git-send-email-rashmicy@gmail.com> From: Balbir Singh Message-ID: <56C69535.90907@gmail.com> Date: Fri, 19 Feb 2016 15:08:21 +1100 MIME-Version: 1.0 In-Reply-To: <1455846943-32147-1-git-send-email-rashmicy@gmail.com> Content-Type: text/plain; charset=utf-8 List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , On 19/02/16 12:55, Rashmica Gupta wrote: > Currently on PPC64 changing kernel pagesize from 4K to 64K leaves > FORCE_MAX_ZONEORDER set to 13 - which produces a compile error. > > The error occurs because of the following constraint (from > include/linux/mmzone.h) being violated: > > MAX_ORDER -1 + PAGESHIFT <= SECTION_SIZE_BITS. IA64 has this cool hack 12 #ifdef CONFIG_FORCE_MAX_ZONEORDER 13 #if ((CONFIG_FORCE_MAX_ZONEORDER - 1 + PAGE_SHIFT) > SECTION_SIZE_BITS) 14 #undef SECTION_SIZE_BITS 15 #define SECTION_SIZE_BITS (CONFIG_FORCE_MAX_ZONEORDER - 1 + PAGE_SHIFT) 16 #endif But coming back (we can revisit the SECTION_SIZE_BITS definition later) MAX_ORDER -1 + 16 <= 24 for 64 K and MAX_ORDER -1 + 12 < = 24 for 4K Your calculations are correct > > Expanding this out, we get: > > FORCE_MAX_ZONEBITS <= 25 - PAGESHIFT, > > which requires, for a 64K page, FORCE_MAX_ZONEBITS <= 9. Thus > set max value of FORCE_MAX_ZONEORDER for 64K pages to 9. > > Also, check the minimum value: > In include/linux/huge_mm.h, we have the constraint HPAGE_PMD_ORDER < > MAX_ORDER which expands out to: > > PTE_INDEX_SIZE < FORCE_MAX_ZONEORDER. > PTE_INDEX_SIZE is: > 9 (4k hash or no hash 4K pgtable) or > 8 (64K hash or no hash 64K pgtable). > Thus a min value of 9 for 64K pages is reasonable. For 4K pages we end up with 9 < FORCE_MAX_ZONE_ORDER FORCE_MAX_ZONE_ORDER -1 + 12 < = 24 The range is 9 to 13 For 64K we end up with 8 < FORCE_MAX_ZONE_ORDER FORCE_MAX_ZONE_ORDER -1 + 16 <= 24 or FORCE_MAX_ZONE_ORDER <= 9 The range is really between 8 and 9 unless we tweak the SECTION_SIZE_BITS > So, update the range of FORCE_MAX_ZONEORDER from 9-64 to 9-9. > > Signed-off-by: Rashmica Gupta > --- > arch/powerpc/Kconfig | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig > index e4824fd04bb7..3bd3465b93ba 100644 > --- a/arch/powerpc/Kconfig > +++ b/arch/powerpc/Kconfig > @@ -585,7 +585,7 @@ endchoice > > config FORCE_MAX_ZONEORDER > int "Maximum zone order" > - range 9 64 if PPC64 && PPC_64K_PAGES > + range 9 9 if PPC64 && PPC_64K_PAGES range 8 9? > default "9" if PPC64 && PPC_64K_PAGES > range 13 64 if PPC64 && !PPC_64K_PAGES Should this be fixed as well? range 9 13? > default "13" if PPC64 && !PPC_64K_PAGES Please check my calculations Reviewed-by: Balbir Singh Balbir Singh