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

[Christophe Leroy]

On 06/04/2020 12:00 PM, Peter Zijlstra wrote:
> On Thu, Jun 04, 2020 at 12:17:23PM +0100, 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.
>
> Looking at commit 55c8fc3f49302, they're all the exact same value, so
> what they could do is grow another special gup_get_pte() variant that
> just loads the first value.
>
> Also, per that very same commit, there's a distinct lack of WRITE_ONCE()
> in the pte_update() / __set_pte_at() paths for much of Power.

Thanks for the idea.

Now I get the same issue at

   CC      mm/mincore.o
In file included from ./include/asm-generic/bug.h:5:0,
                 from ./arch/powerpc/include/asm/bug.h:109,
                 from ./include/linux/bug.h:5,
                 from ./include/linux/mmdebug.h:5,
                 from ./include/linux/mm.h:9,
                 from ./include/linux/pagemap.h:8,
                 from mm/mincore.c:11:
In function 'huge_ptep_get',
    inlined from 'mincore_hugetlb' at mm/mincore.c:35:20:
./include/linux/compiler.h:392:38: error: call to 
'__compiletime_assert_218' 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);    \
  ^
./include/asm-generic/hugetlb.h:125:9: note: in expansion of macro 
'READ_ONCE'
  return READ_ONCE(*ptep);
         ^
make[2]: *** [mm/mincore.o] Error 1

I guess for this one I have to implement platform specific huge_ptep_get()

Christophe