[PATCH] mm/debug_vm_pgtable: Fix build failure with powerpc 8xx

Since commit 9e343b467c70 ("READ_ONCE: Enforce atomicity for
{READ,WRITE}_ONCE() memory accesses"), READ_ONCE() cannot be used
anymore to read complex page table entries. This leads to:

  CC      mm/debug_vm_pgtable.o
In file included from ./include/asm-generic/bug.h:5,
                 from ./arch/powerpc/include/asm/bug.h:109,
                 from ./include/linux/bug.h:5,
                 from ./include/linux/mmdebug.h:5,
                 from ./include/linux/gfp.h:5,
                 from mm/debug_vm_pgtable.c:13:
In function 'pte_clear_tests',
    inlined from 'debug_vm_pgtable' at mm/debug_vm_pgtable.c:363:2:
./include/linux/compiler.h:392:38: error: call to '__compiletime_assert_210' declared with attribute error: Unsupported access size for {READ,WRITE}_ONCE().
  392 |  _compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__)
      |                                      ^
./include/linux/compiler.h:373:4: note: in definition of macro '__compiletime_assert'
  373 |    prefix ## suffix();    \
      |    ^~~~~~
./include/linux/compiler.h:392:2: note: in expansion of macro '_compiletime_assert'
  392 |  _compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__)
      |  ^~~~~~~~~~~~~~~~~~~
./include/linux/compiler.h:405:2: note: in expansion of macro 'compiletime_assert'
  405 |  compiletime_assert(__native_word(t) || sizeof(t) == sizeof(long long), \
      |  ^~~~~~~~~~~~~~~~~~
./include/linux/compiler.h:291:2: note: in expansion of macro 'compiletime_assert_rwonce_type'
  291 |  compiletime_assert_rwonce_type(x);    \
      |  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
mm/debug_vm_pgtable.c:249:14: note: in expansion of macro 'READ_ONCE'
  249 |  pte_t pte = READ_ONCE(*ptep);
      |              ^~~~~~~~~
make[2]: *** [mm/debug_vm_pgtable.o] Error 1

Fix it by using the recently added ptep_get() helper.

Fixes: 9e343b467c70 ("READ_ONCE: Enforce atomicity for {READ,WRITE}_ONCE() memory accesses")
Signed-off-by: Christophe Leroy <redacted>
---
 mm/debug_vm_pgtable.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/mm/debug_vm_pgtable.c b/mm/debug_vm_pgtable.c
index e45623016aea..61ab16fb2e36 100644
--- a/mm/debug_vm_pgtable.c
+++ b/mm/debug_vm_pgtable.c

@@ -246,13 +246,13 @@ static void __init pgd_populate_tests(struct mm_struct *mm, pgd_t *pgdp,
 static void __init pte_clear_tests(struct mm_struct *mm, pte_t *ptep,
 				   unsigned long vaddr)
 {
-	pte_t pte = READ_ONCE(*ptep);
+	pte_t pte = ptep_get(ptep);
 
 	pte = __pte(pte_val(pte) | RANDOM_ORVALUE);
 	set_pte_at(mm, vaddr, ptep, pte);
 	barrier();
 	pte_clear(mm, vaddr, ptep);
-	pte = READ_ONCE(*ptep);
+	pte = ptep_get(ptep);
 	WARN_ON(!pte_none(pte));
 }
 

-- 
2.25.0

On Thu, Jun 18, 2020 at 02:31:29PM +0000, Christophe Leroy wrote:

Acked-by: Will Deacon <will@kernel.org>

I wonder if there's a way to do this with coccinelle in one big go (but the
resulting diff would obviously need manual inspection)?

Will

On 06/18/2020 08:01 PM, Christophe Leroy wrote:

Tested this on arm64 and x86 platforms after applying the previous
series which adds ptep_get() and a follow up patch.

https://patchwork.kernel.org/project/linux-mm/list/?series=302949
https://patchwork.kernel.org/patch/11611929/

Build tested on s390 and arc platforms as well.

Reviewed-by: Anshuman Khandual <redacted>