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@xxxxxxxxxx> --- 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 008a6c856fa4..64fbe5fd3853 100644 --- a/arch/s390/include/asm/pgtable.h +++ b/arch/s390/include/asm/pgtable.h @@ -1669,18 +1669,17 @@ 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) -- 2.35.1 > > If bit 56 could be used for _PAGE_SWP_EXCLUSIVE, that would be better > than stealing a bit from the offset, or using potentially dangerous > bit 52. It is defined as _PAGE_UNUSED and only used for kvm, not sure > if this is also relevant for swap ptes, similar to bit 62. I don't think it is, and I also don't think there is anything wrong with reusing bit 52. > > Adding Christian on cc, maybe he has some insight on _PAGE_UNUSED > bit 56 and swap ptes. -- Thanks, David / dhildenb
