[PATCH] s390/pgtable: cleanup description of swp pte layout

[David Hildenbrand]

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