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

Geert Uytterhoeven
2023-02-26T20:19:07Z

Hi David,

On Fri, Jan 13, 2023 at 6:16 PM David Hildenbrand <david@redhat.com> 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@monstr.eu>
> Signed-off-by: David Hildenbrand <david@redhat.com>

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

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

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

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?

>
> /*
> * 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?

> * - 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?

Gr{oetje,eeting}s,

Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
-- Linus Torvalds