On 07.12.22 14:55, Christophe Leroy wrote:

Le 06/12/2022 à 15:47, David Hildenbrand a écrit :

quoted

We already implemented support for 64bit book3s in commit bff9beaa2e80
("powerpc/pgtable: support __HAVE_ARCH_PTE_SWP_EXCLUSIVE for book3s")

Let's support __HAVE_ARCH_PTE_SWP_EXCLUSIVE also in 32bit by reusing yet
unused LSB 2 / MSB 29. There seems to be no real reason why that bit cannot
be used, and reusing it avoids having to steal one bit from the swap
offset.

While at it, mask the type in __swp_entry().

Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: Christophe Leroy <redacted>
Signed-off-by: David Hildenbrand <redacted>
---
   arch/powerpc/include/asm/book3s/32/pgtable.h | 38 +++++++++++++++++---
   1 file changed, 33 insertions(+), 5 deletions(-)

diff --git a/arch/powerpc/include/asm/book3s/32/pgtable.h b/arch/powerpc/include/asm/book3s/32/pgtable.h
index 75823f39e042..8107835b38c1 100644
--- a/arch/powerpc/include/asm/book3s/32/pgtable.h
+++ b/arch/powerpc/include/asm/book3s/32/pgtable.h

@@ -42,6 +42,9 @@
   #define _PMD_PRESENT_MASK (PAGE_MASK)
   #define _PMD_BAD	(~PAGE_MASK)
   
+/* We borrow the _PAGE_USER bit to store the exclusive marker in swap PTEs. */
+#define _PAGE_SWP_EXCLUSIVE	_PAGE_USER
+
   /* And here we include common definitions */
   
   #define _PAGE_KERNEL_RO		0

@@ -363,17 +366,42 @@ static inline void __ptep_set_access_flags(struct vm_area_struct *vma,
   #define pmd_page(pmd)		pfn_to_page(pmd_pfn(pmd))
   
   /*
- * Encode and decode a swap entry.
- * Note that the bits we use in a PTE for representing a swap entry
- * must not include the _PAGE_PRESENT bit or the _PAGE_HASHPTE bit (if used).
- *   -- paulus
+ * Encode/decode swap entries and swap PTEs. Swap PTEs are all PTEs that
+ * are !pte_none() && !pte_present().
+ *
+ * Format of swap PTEs (32bit PTEs):
+ *
+ *                         1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
+ *   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+ *   E H P <- type --> <----------------- offset ------------------>

That's in reversed order. _PAGE_HASHPTE is bit 30 and should be on the
right hand side. Etc ...

Ugh, messed it up while converting back and forth between LSB 0 and MSB 0.

/*
  * Format of swap PTEs (32bit PTEs):
  *
  *                         1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
  *   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  *   <----------------- offset ------------------> <- type --> E H P


Now the patch description ("unused LSB 2 / MSB 29") makes sense.

Thanks!

Any feedback if the bit could be problematic?

-- 
Thanks,

David / dhildenb