powerpc/8xx: Provide ptep_get() with 16k pages

READ_ONCE() now enforces atomic read, which leads to:

  CC      mm/gup.o
In file included from ./include/linux/kernel.h:11:0,
                 from mm/gup.c:2:
In function 'gup_hugepte.constprop',
    inlined from 'gup_huge_pd.isra.79' at mm/gup.c:2465:8:
./include/linux/compiler.h:392:38: error: call to '__compiletime_assert_222' declared with attribute error: Unsupported access size for {READ,WRITE}_ONCE().
  _compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__)
                                      ^
./include/linux/compiler.h:373:4: note: in definition of macro '__compiletime_assert'
    prefix ## suffix();    \
    ^
./include/linux/compiler.h:392:2: note: in expansion of macro '_compiletime_assert'
  _compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__)
  ^
./include/linux/compiler.h:405:2: note: in expansion of macro 'compiletime_assert'
  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'
  compiletime_assert_rwonce_type(x);    \
  ^
mm/gup.c:2428:8: note: in expansion of macro 'READ_ONCE'
  pte = READ_ONCE(*ptep);
        ^
In function 'gup_get_pte',
    inlined from 'gup_pte_range' at mm/gup.c:2228:9,
    inlined from 'gup_pmd_range' at mm/gup.c:2613:15,
    inlined from 'gup_pud_range' at mm/gup.c:2641:15,
    inlined from 'gup_p4d_range' at mm/gup.c:2666:15,
    inlined from 'gup_pgd_range' at mm/gup.c:2694:15,
    inlined from 'internal_get_user_pages_fast' at mm/gup.c:2795:3:
./include/linux/compiler.h:392:38: error: call to '__compiletime_assert_219' declared with attribute error: Unsupported access size for {READ,WRITE}_ONCE().
  _compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__)
                                      ^
./include/linux/compiler.h:373:4: note: in definition of macro '__compiletime_assert'
    prefix ## suffix();    \
    ^
./include/linux/compiler.h:392:2: note: in expansion of macro '_compiletime_assert'
  _compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__)
  ^
./include/linux/compiler.h:405:2: note: in expansion of macro 'compiletime_assert'
  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'
  compiletime_assert_rwonce_type(x);    \
  ^
mm/gup.c:2199:9: note: in expansion of macro 'READ_ONCE'
  return READ_ONCE(*ptep);
         ^
make[2]: *** [mm/gup.o] Error 1

Define ptep_get() on 8xx when using 16k pages.

Fixes: 9e343b46 ("READ_ONCE: Enforce atomicity for {READ,WRITE}_ONCE() memory accesses")
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Acked-by: Will Deacon <will@kernel.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/341688399c1b102756046d19ea6ce39db1ae4742.1592225558.git.christophe.leroy@csgroup.eu

arch/powerpc/include/asm/nohash/32/pgtable.h

@@ -284,6 +284,16 @@ static inline pte_t ptep_get_and_clear(struct mm_struct *mm, unsigned long addr,
 	return __pte(pte_update(mm, addr, ptep, ~0, 0, 0));
 }
 
+#if defined(CONFIG_PPC_8xx) && defined(CONFIG_PPC_16K_PAGES)
+#define __HAVE_ARCH_PTEP_GET
+static inline pte_t ptep_get(pte_t *ptep)
+{
+	pte_t pte = {READ_ONCE(ptep->pte), 0, 0, 0};
+
+	return pte;
+}
+#endif
+
 #define __HAVE_ARCH_PTEP_SET_WRPROTECT
 static inline void ptep_set_wrprotect(struct mm_struct *mm, unsigned long addr,
 				      pte_t *ptep)