Re: [PATCH mm-unstable v1 11/26] microblaze/mm: support __HAVE_ARCH_PTE_SWP_EXCLUSIVE

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On 26.02.23 21:13, Geert Uytterhoeven wrote:
Hi David,

Hi Geert,


On Fri, Jan 13, 2023 at 6:16 PM David Hildenbrand <david@xxxxxxxxxx> wrote:
Let's support __HAVE_ARCH_PTE_SWP_EXCLUSIVE by stealing one bit
from the type. Generic MM currently only uses 5 bits for the type
(MAX_SWAPFILES_SHIFT), so the stolen bit is effectively unused.

The shift by 2 when converting between PTE and arch-specific swap entry
makes the swap PTE layout a little bit harder to decipher.

While at it, drop the comment from paulus---copy-and-paste leftover
from powerpc where we actually have _PAGE_HASHPTE---and mask the type in
__swp_entry_to_pte() as well.

Cc: Michal Simek <monstr@xxxxxxxxx>
Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>

Thanks for your patch, which is now commit b5c88f21531c3457
("microblaze/mm: support __HAVE_ARCH_PTE_SWP_EXCLUSIVE") in


Right, it went upstream, so we can only fixup.

  arch/m68k/include/asm/mcf_pgtable.h   |  4 +--

What is this m68k change doing here?
Sorry for not noticing this earlier.

Thanks for the late review, still valuable :)

That hunk should have gone into the previous patch, looks like I messed that up when reworking.


Furthermore, several things below look strange to me...

  arch/microblaze/include/asm/pgtable.h | 45 +++++++++++++++++++++------
  2 files changed, 37 insertions(+), 12 deletions(-)

diff --git a/arch/m68k/include/asm/mcf_pgtable.h b/arch/m68k/include/asm/mcf_pgtable.h
index 3f8f4d0e66dd..e573d7b649f7 100644
--- a/arch/m68k/include/asm/mcf_pgtable.h
+++ b/arch/m68k/include/asm/mcf_pgtable.h
@@ -46,8 +46,8 @@
  #define _CACHEMASK040          (~0x060)
  #define _PAGE_GLOBAL040                0x400   /* 68040 global bit, used for kva descs */

-/* We borrow bit 7 to store the exclusive marker in swap PTEs. */
-#define _PAGE_SWP_EXCLUSIVE    0x080
+/* We borrow bit 24 to store the exclusive marker in swap PTEs. */
+#define _PAGE_SWP_EXCLUSIVE    CF_PAGE_NOCACHE

CF_PAGE_NOCACHE is 0x80, so this is still bit 7, thus the new comment
is wrong?

You're right, it's still bit 7 (and we use LSB-0 bit numbering in that file). I'll send a fixup.



  /*
   * Externally used page protection values.
diff --git a/arch/microblaze/include/asm/pgtable.h b/arch/microblaze/include/asm/pgtable.h
index 42f5988e998b..7e3de54bf426 100644
--- a/arch/microblaze/include/asm/pgtable.h
+++ b/arch/microblaze/include/asm/pgtable.h
@@ -131,10 +131,10 @@ extern pte_t *va_to_pte(unsigned long address);
   * of the 16 available.  Bit 24-26 of the TLB are cleared in the TLB
   * miss handler.  Bit 27 is PAGE_USER, thus selecting the correct
   * zone.
- * - PRESENT *must* be in the bottom two bits because swap cache
- * entries use the top 30 bits.  Because 4xx doesn't support SMP
- * anyway, M is irrelevant so we borrow it for PAGE_PRESENT.  Bit 30
- * is cleared in the TLB miss handler before the TLB entry is loaded.
+ * - PRESENT *must* be in the bottom two bits because swap PTEs use the top
+ * 30 bits.  Because 4xx doesn't support SMP anyway, M is irrelevant so we
+ * borrow it for PAGE_PRESENT.  Bit 30 is cleared in the TLB miss handler
+ * before the TLB entry is loaded.

So the PowerPC 4xx comment is still here?

I only dropped the comment above __swp_type(). I guess you mean that we could also drop the "Because 4xx doesn't support SMP anyway, M is irrelevant so we borrow it for PAGE_PRESENT." sentence, correct? Not sure about the "Bit 30 is cleared in the TLB miss handler" comment, if that can similarly be dropped.


   * - All other bits of the PTE are loaded into TLBLO without
   *  * modification, leaving us only the bits 20, 21, 24, 25, 26, 30 for
   * software PTE bits.  We actually use bits 21, 24, 25, and
@@ -155,6 +155,9 @@ extern pte_t *va_to_pte(unsigned long address);
  #define _PAGE_ACCESSED 0x400   /* software: R: page referenced */
  #define _PMD_PRESENT   PAGE_MASK

+/* We borrow bit 24 to store the exclusive marker in swap PTEs. */
+#define _PAGE_SWP_EXCLUSIVE    _PAGE_DIRTY

_PAGE_DIRTY is 0x80, so this is also bit 7, thus the new comment is
wrong?

In the example, I use MSB-0 bit numbering (which I determined to be correct in microblaze context eventually, but I got confused a couple a times because it's very inconsistent). That should be MSB-0 bit 24.

Thanks!

--
Thanks,

David / dhildenb




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