Subject: Re: [PATCH 1/2] mm: Update generic gup implementation to handle hugepage directory

From: Steve Capper <[email-masked]>
Date: Thu, 16 Oct 2014 16:42:29 +0100

On Thu, Oct 16, 2014 at 08:48:20PM +0530, Aneesh Kumar K.V wrote:
> Steve Capper <[email-masked]> writes:
> 
> > On Wed, Oct 15, 2014 at 10:04:47PM +0530, Aneesh Kumar K.V wrote:
> >> Update generic gup implementation with powerpc specific details.
> >> On powerpc at pmd level we can have hugepte, normal pmd pointer
> >> or a pointer to the hugepage directory.
> >> 
> >> Signed-off-by: Aneesh Kumar K.V <[email-masked]>
> >
> > Hi,
> > This patch causes compiler errors on arm and arm64 due to pgd_huge
> > being undefined. I've attached a fixup below, this fixup will require
> > that #define pgd_huge(pgd) 0 be added back into:
> > arch/powerpc/include/asm/page.h
> > For the second patch in this series.
> >
> > Another avenue would be to do something like:
> > #ifndef pgd_huge
> > #define pgd_huge(pgd)	(0)
> > #endif
> >
> > Then no changes would be required to arm and arm64 (or other
> > architectures).
> >
> > To help with bisectability, could we please have a suitable fix applied
> > to the two patches in the -mm tree:
> > http://ozlabs.org/~akpm/mmots/broken-out/mm-update-generic-gup-implementation-to-handle-hugepage-directory.patch
> > http://ozlabs.org/~akpm/mmots/broken-out/arch-powerpc-switch-to-generic-rcu-get_user_pages_fast.patch
> >
> > rather than applied afterwards?
> >
> > With pgd_huge(x) defined, this patch passes my futex test on arm
> > (Arndale platform) and arm64(Juno).
> >
> > Cheers,
> > -- 
> > Steve
> >
> >
> >
> > From 2fb7b0308f0aca94c50611257ba82d656abb0768 Mon Sep 17 00:00:00 2001
> > From: Steve Capper <[email-masked]>
> > Date: Thu, 16 Oct 2014 09:09:48 +0100
> > Subject: [PATCH] Fixup for Update generic gup implementation
> >
> > The patch:
> > mm: Update generic gup implementation to handle hugepage directory
> >
> > will not compile for arm or arm64 due to pgd_huge being undefined.
> >
> > Signed-off-by: Steve Capper <[email-masked]>
> > ---
> >  arch/arm/include/asm/pgtable.h   | 2 ++
> >  arch/arm64/include/asm/pgtable.h | 2 ++
> >  include/linux/hugetlb.h          | 1 -
> >  3 files changed, 4 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/arm/include/asm/pgtable.h b/arch/arm/include/asm/pgtable.h
> > index 90aa4583..46f81fb 100644
> > --- a/arch/arm/include/asm/pgtable.h
> > +++ b/arch/arm/include/asm/pgtable.h
> > @@ -181,6 +181,8 @@ extern pgd_t swapper_pg_dir[PTRS_PER_PGD];
> >  /* to find an entry in a kernel page-table-directory */
> >  #define pgd_offset_k(addr)	pgd_offset(&init_mm, addr)
> >
> > +#define pgd_huge(pgd)		(0)
> > +
> >  #define pmd_none(pmd)		(!pmd_val(pmd))
> >  #define pmd_present(pmd)	(pmd_val(pmd))
> >
> > diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
> > index 464c5ce..d4462ca 100644
> > --- a/arch/arm64/include/asm/pgtable.h
> > +++ b/arch/arm64/include/asm/pgtable.h
> > @@ -462,6 +462,8 @@ static inline pmd_t pmd_modify(pmd_t pmd, pgprot_t newprot)
> >  extern pgd_t swapper_pg_dir[PTRS_PER_PGD];
> >  extern pgd_t idmap_pg_dir[PTRS_PER_PGD];
> >
> > +#define pgd_huge(pgd)		(0)
> > +
> >  /*
> >   * Encode and decode a swap entry:
> >   *	bits 0-1:	present (must be zero)
> > diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
> > index 65e12a2..6e6d338 100644
> > --- a/include/linux/hugetlb.h
> > +++ b/include/linux/hugetlb.h
> > @@ -138,7 +138,6 @@ static inline void hugetlb_show_meminfo(void)
> >  #define prepare_hugepage_range(file, addr, len)	(-EINVAL)
> >  #define pmd_huge(x)	0
> >  #define pud_huge(x)	0
> > -#define pgd_huge(x)	0
> >  #define is_hugepage_only_range(mm, addr, len)	0
> >  #define hugetlb_free_pgd_range(tlb, addr, end, floor, ceiling) ({BUG(); 0; })
> >  #define hugetlb_fault(mm, vma, addr, flags)	({ BUG(); 0; })
> 
> don't do the last hunk, that will result in build failures on sub
> platforms on ppc64. can you do the arm patch without making the change
> to hugetlb.h ?
> 

Hi Aneesh,

The problem with leaving the empty pgd_huge in hugetlb.h is that we
would then have to resort to patterns like this for both arm and arm64:

#ifdef CONFIG_HUGETLB_PAGE
#define pgd_huge(pgd)		(0)
#endif

If possible, I'd much rather just have:
#define pgd_huge(pgd)		(0)

After the second patch in this series we already have the following
code pattern in arch/powerpc/include/asm/page.h:

 #define is_hugepd(hpd)               (hugepd_ok(hpd))
 int pgd_huge(pgd_t pgd);
 #else /* CONFIG_HUGETLB_PAGE */
 #define is_hugepd(pdep)                        0
 #endif /* CONFIG_HUGETLB_PAGE */
 #define __hugepd(x) ((hugepd_t) { (x) })

Can we not just add a:
#define pgd_huge(pgd)		(0)
above the "#endif /* CONFIG_HUGETLB_PAGE */" line in the second patch?
(or, more precisely, prevent the second patch from removing this line).

That way we get a clearer code overall?

Cheers,
-- 
Steve