Re: [PATCH v2 5/8] s390/pgtable: cleanup description of swp pte layout

Gerald Schaefer
2022-03-30T16:50:13Z

On Tue, 29 Mar 2022 18:43:26 +0200
David Hildenbrand <david@redhat.com> wrote:

> Bit 52 and bit 55 don't have to be zero: they only trigger a
> translation-specifiation exception if the PTE is marked as valid, which
> is not the case for swap ptes.
>
> Document which bits are used for what, and which ones are unused.
>
> Signed-off-by: David Hildenbrand <david@redhat.com>
> ---
> arch/s390/include/asm/pgtable.h | 17 ++++++++---------
> 1 file changed, 8 insertions(+), 9 deletions(-)
>
> diff --git a/arch/s390/include/asm/pgtable.h b/arch/s390/include/asm/pgtable.h
> index 9df679152620..3982575bb586 100644
> --- a/arch/s390/include/asm/pgtable.h
> +++ b/arch/s390/include/asm/pgtable.h
> @@ -1712,18 +1712,17 @@ static inline int has_transparent_hugepage(void)
> /*
> * 64 bit swap entry format:
> * A page-table entry has some bits we have to treat in a special way.
> - * Bits 52 and bit 55 have to be zero, otherwise a specification
> - * exception will occur instead of a page translation exception. The
> - * specification exception has the bad habit not to store necessary
> - * information in the lowcore.
> - * Bits 54 and 63 are used to indicate the page type.
> + * Bits 54 and 63 are used to indicate the page type. Bit 53 marks the pte
> + * as invalid.
> * A swap pte is indicated by bit pattern (pte & 0x201) == 0x200
> - * This leaves the bits 0-51 and bits 56-62 to store type and offset.
> - * We use the 5 bits from 57-61 for the type and the 52 bits from 0-51
> - * for the offset.
> - * | offset |01100|type |00|
> + * | offset |X11XX|type |S0|
> * |0000000000111111111122222222223333333333444444444455|55555|55566|66|
> * |0123456789012345678901234567890123456789012345678901|23456|78901|23|
> + *
> + * Bits 0-51 store the offset.
> + * Bits 57-61 store the type.
> + * Bit 62 (S) is used for softdirty tracking.
> + * Bits 52, 55 and 56 (X) are unused.
> */
>
> #define __SWP_OFFSET_MASK ((1UL << 52) - 1)

Thanks David!

Reviewed-by: Gerald Schaefer <gerald.schaefer@linux.ibm.com>