Re: linux-next: build failure on powerpc 8xx with 16k pages

[Will Deacon]

Hi, [+Peter]

On Thu, Jun 04, 2020 at 10:48:03AM +0000, Christophe Leroy wrote:
> Using mpc885_ads_defconfig with CONFIG_PPC_16K_PAGES instead of
> CONFIG_PPC_4K_PAGES, getting the following build failure:
> 
>   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.78' at mm/gup.c:2465:8:
> ./include/linux/compiler.h:392:38: error: call to '__compiletime_assert_257'
> 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:2785:3:

At first glance, this looks like a real bug in the 16k page code -- you're
loading the pte non-atomically on the fast GUP path and so you're prone to
tearing, which probably isn't what you want. For a short-term hack, I'd
suggest having CONFIG_HAVE_FAST_GUP depend on !CONFIG_PPC_16K_PAGES, but if
you want to support this them you'll need to rework your pte_t so that it
can be loaded atomically.

Will