From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pg0-f71.google.com (mail-pg0-f71.google.com [74.125.83.71]) by kanga.kvack.org (Postfix) with ESMTP id 84BFD2806EA for ; Fri, 19 May 2017 11:55:13 -0400 (EDT) Received: by mail-pg0-f71.google.com with SMTP id q125so61945325pgq.8 for ; Fri, 19 May 2017 08:55:13 -0700 (PDT) Received: from mga14.intel.com (mga14.intel.com. [192.55.52.115]) by mx.google.com with ESMTPS id f1si8420487pgc.109.2017.05.19.08.55.12 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Fri, 19 May 2017 08:55:12 -0700 (PDT) Subject: Re: [PATCH v5 01/11] mm: x86: move _PAGE_SWP_SOFT_DIRTY from bit 7 to bit 1 References: <20170420204752.79703-1-zi.yan@sent.com> <20170420204752.79703-2-zi.yan@sent.com> From: Dave Hansen Message-ID: <76a36bee-0f1c-a2f4-6f5c-78394ac46ee4@intel.com> Date: Fri, 19 May 2017 08:55:11 -0700 MIME-Version: 1.0 In-Reply-To: <20170420204752.79703-2-zi.yan@sent.com> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Zi Yan , n-horiguchi@ah.jp.nec.com, kirill.shutemov@linux.intel.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org Cc: akpm@linux-foundation.org, minchan@kernel.org, vbabka@suse.cz, mgorman@techsingularity.net, mhocko@kernel.org, khandual@linux.vnet.ibm.com, zi.yan@cs.rutgers.edu, dnellans@nvidia.com On 04/20/2017 01:47 PM, Zi Yan wrote: > pmd_present() checks _PAGE_PSE along with _PAGE_PRESENT to avoid > false negative return when it races with thp spilt > (during which _PAGE_PRESENT is temporary cleared.) I don't think that > dropping _PAGE_PSE check in pmd_present() works well because it can > hurt optimization of tlb handling in thp split. > In the current kernel, bits 1-4 are not used in non-present format > since commit 00839ee3b299 ("x86/mm: Move swap offset/type up in PTE to > work around erratum"). So let's move _PAGE_SWP_SOFT_DIRTY to bit 1. > Bit 7 is used as reserved (always clear), so please don't use it for > other purpose. This description lacks a problem statement. What's the problem? _PAGE_PSE is used to distinguish between a truly non-present (_PAGE_PRESENT=0) PMD, and a PMD which is undergoing a THP split and should be treated as present. But _PAGE_SWP_SOFT_DIRTY currently uses the _PAGE_PSE bit, which would cause confusion between one of those PMDs undergoing a THP split, and a soft-dirty PMD. Thus, we need to move the bit. Does that capture it? > Signed-off-by: Naoya Horiguchi > Signed-off-by: Zi Yan > --- > arch/x86/include/asm/pgtable_64.h | 12 +++++++++--- > arch/x86/include/asm/pgtable_types.h | 10 +++++----- > 2 files changed, 14 insertions(+), 8 deletions(-) > > diff --git a/arch/x86/include/asm/pgtable_64.h b/arch/x86/include/asm/pgtable_64.h > index 73c7ccc38912..770b5ae271ed 100644 > --- a/arch/x86/include/asm/pgtable_64.h > +++ b/arch/x86/include/asm/pgtable_64.h > @@ -157,15 +157,21 @@ static inline int pgd_large(pgd_t pgd) { return 0; } > /* > * Encode and de-code a swap entry > * > - * | ... | 11| 10| 9|8|7|6|5| 4| 3|2|1|0| <- bit number > - * | ... |SW3|SW2|SW1|G|L|D|A|CD|WT|U|W|P| <- bit names > - * | OFFSET (14->63) | TYPE (9-13) |0|X|X|X| X| X|X|X|0| <- swp entry > + * | ... | 11| 10| 9|8|7|6|5| 4| 3|2| 1|0| <- bit number > + * | ... |SW3|SW2|SW1|G|L|D|A|CD|WT|U| W|P| <- bit names > + * | OFFSET (14->63) | TYPE (9-13) |0|0|X|X| X| X|X|SD|0| <- swp entry So, this diagram was incomplete before? It should have had "SD" under bit 7 for swap entries? > * G (8) is aliased and used as a PROT_NONE indicator for > * !present ptes. We need to start storing swap entries above > * there. We also need to avoid using A and D because of an > * erratum where they can be incorrectly set by hardware on > * non-present PTEs. > + * > + * SD (1) in swp entry is used to store soft dirty bit, which helps us > + * remember soft dirty over page migration > + * > + * Bit 7 in swp entry should be 0 because pmd_present checks not only P, > + * but also L and G. > */ > #define SWP_TYPE_FIRST_BIT (_PAGE_BIT_PROTNONE + 1) > #define SWP_TYPE_BITS 5 > diff --git a/arch/x86/include/asm/pgtable_types.h b/arch/x86/include/asm/pgtable_types.h > index df08535f774a..9a4ac934659e 100644 > --- a/arch/x86/include/asm/pgtable_types.h > +++ b/arch/x86/include/asm/pgtable_types.h > @@ -97,15 +97,15 @@ > /* > * Tracking soft dirty bit when a page goes to a swap is tricky. > * We need a bit which can be stored in pte _and_ not conflict > - * with swap entry format. On x86 bits 6 and 7 are *not* involved > - * into swap entry computation, but bit 6 is used for nonlinear > - * file mapping, so we borrow bit 7 for soft dirty tracking. > + * with swap entry format. On x86 bits 1-4 are *not* involved > + * into swap entry computation, but bit 7 is used for thp migration, > + * so we borrow bit 1 for soft dirty tracking. > * > * Please note that this bit must be treated as swap dirty page > - * mark if and only if the PTE has present bit clear! > + * mark if and only if the PTE/PMD has present bit clear! > */ > #ifdef CONFIG_MEM_SOFT_DIRTY > -#define _PAGE_SWP_SOFT_DIRTY _PAGE_PSE > +#define _PAGE_SWP_SOFT_DIRTY _PAGE_RW > #else > #define _PAGE_SWP_SOFT_DIRTY (_AT(pteval_t, 0)) > #endif > -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org