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SubjectRe: [PATCH v5 01/11] mm: x86: move _PAGE_SWP_SOFT_DIRTY from bit 7 to bit 1
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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 <n-horiguchi@ah.jp.nec.com>
> Signed-off-by: Zi Yan <zi.yan@cs.rutgers.edu>
> ---
> 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
>

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