Re: [PATCH V2 1/7] m68k/mm: Change pmd_val()

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On 17.09.24 09:31, Anshuman Khandual wrote:
This changes platform's pmd_val() to access the pmd_t element directly like
other architectures rather than current pointer address based dereferencing
that prevents transition into pmdp_get().

Cc: Geert Uytterhoeven <geert@xxxxxxxxxxxxxx>
Cc: Guo Ren <guoren@xxxxxxxxxx>
Cc: Arnd Bergmann <arnd@xxxxxxxx>
Cc: linux-m68k@xxxxxxxxxxxxxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
Signed-off-by: Anshuman Khandual <anshuman.khandual@xxxxxxx>
---
  arch/m68k/include/asm/page.h | 2 +-
  1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/m68k/include/asm/page.h b/arch/m68k/include/asm/page.h
index 8cfb84b49975..be3f2c2a656c 100644
--- a/arch/m68k/include/asm/page.h
+++ b/arch/m68k/include/asm/page.h
@@ -19,7 +19,7 @@
   */
  #if !defined(CONFIG_MMU) || CONFIG_PGTABLE_LEVELS == 3
  typedef struct { unsigned long pmd; } pmd_t;
-#define pmd_val(x)	((&x)->pmd)
+#define pmd_val(x)	((x).pmd)
  #define __pmd(x)	((pmd_t) { (x) } )
  #endif

Trying to understand what's happening here, I stumbled over

commit ef22d8abd876e805b604e8f655127de2beee2869
Author: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Date:   Fri Jan 31 13:45:36 2020 +0100

    m68k: mm: Restructure Motorola MMU page-table layout
The Motorola 68xxx MMUs, 040 (and later) have a fixed 7,7,{5,6}
    page-table setup, where the last depends on the page-size selected (8k
    vs 4k resp.), and head.S selects 4K pages. For 030 (and earlier) we
    explicitly program 7,7,6 and 4K pages in %tc.
However, the current code implements this mightily weird. What it does
    is group 16 of those (6 bit) pte tables into one 4k page to not waste
    space. The down-side is that that forces pmd_t to be a 16-tuple
    pointing to consecutive pte tables.
This breaks the generic code which assumes READ_ONCE(*pmd) will be
    word sized.

Where we did

 #if !defined(CONFIG_MMU) || CONFIG_PGTABLE_LEVELS == 3
-typedef struct { unsigned long pmd[16]; } pmd_t;
-#define pmd_val(x)     ((&x)->pmd[0])
-#define __pmd(x)       ((pmd_t) { { (x) }, })
+typedef struct { unsigned long pmd; } pmd_t;
+#define pmd_val(x)     ((&x)->pmd)
+#define __pmd(x)       ((pmd_t) { (x) } )
 #endif

So I assume this should be fine

Acked-by: David Hildenbrand <david@xxxxxxxxxx>

--
Cheers,

David / dhildenb





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