On 9 Feb 2017, at 3:14, Naoya Horiguchi wrote: > On Sun, Feb 05, 2017 at 11:12:42AM -0500, Zi Yan wrote: >> From: Naoya Horiguchi >> >> 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. >> >> Signed-off-by: Naoya Horiguchi >> >> ChangeLog v3: >> - Move _PAGE_SWP_SOFT_DIRTY to bit 1, it was placed at bit 6. Because >> some CPUs might accidentally set bit 5 or 6. >> >> Signed-off-by: Zi Yan >> --- > > More documenting will be helpful, could you do like follows? Sure. Thanks for helping. > > Thanks, > Naoya Horiguchi > --- > From: Naoya Horiguchi > Date: Sun, 5 Feb 2017 11:12:42 -0500 > Subject: [PATCH] mm: x86: move _PAGE_SWP_SOFT_DIRTY from bit 7 to bit 1 > > 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. > > 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..07c98c85cc96 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 > * > * 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 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 8b4de22d6429..3695abd58ef6 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 > -- > 2.7.4 -- Best Regards Yan Zi