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

[Christophe Leroy]

On 06/04/2020 11:17 AM, Will Deacon wrote:
> 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.

What do you mean by *rework* pte_t ?
pte are 32 bits words in size and are spread every 4 words in memory. 
Therefore pte_t has to be 128 bits because unlike huge_pte handling 
which always use huge_pte_offset() in loops, many many places in the 
kernel do pte++, so we need the pte type to be the size of the interval 
from one pte to the next one.

Christophe