Re: [PATCH mmotm] thp: transparent hugepage core fixlet

[Hugh Dickins <hughd@google.com>]

On Tue, 11 Jan 2011, Andrea Arcangeli wrote:
> On Tue, Jan 11, 2011 at 03:04:21PM +0100, Andrea Arcangeli wrote:
> > architectural bug to me. Why can't pud_huge simply return 0 for
> > x86_32? Any other place dealing with hugepages and calling pud_huge on
> > x86 noPAE would be at risk, otherwise, no?
> 
> Isn't this better solution?

[Better solution than my patch to follow_page() in mmotm, to fix crash
with Transparent Huge Pages by duplicating Andrea's pmd_huge VM_HUGETLB
check to the pud_huge line too.]

The truth is, I'm sure one of the solutions is better than the other,
but I'm too confused by p?d folding to know which is which ;)

Certainly I don't oppose your patch as a replacement for mine,
if you're sure yours is better.

There are only two places which are using pud_huge() anyway:
follow_page() and apply_to_pmd_range().  Is the latter's
BUG_ON(pud_huge) safe?  Safe in the THP world?

And I never quite understood why we have both pmd_huge and pmd_large,
pud_huge and pud_large.

There are answers to these questions, but it would take me hours and
hours of easily-confused research (across several arches) to decide.

I'm hoping someone else has a surer grasp: Andi introduced pud_huge(),
and Jeremy is the most active in the pagetable layers nowadays -
perhaps they can tell us more quickly.

Hugh

> 
> ======
> Subject: avoid confusing hugetlbfs code when pmd_trans_huge is set
> 
> From: Andrea Arcangeli <aarcange@redhat.com>
> 
> If pmd is set huge by THP, pud_huge shouldn't return 1 when pud doesn't exist
> and it's just a 1:1 bypass over the pmd (like it happens on 32bit x86 because
> there are at most 2 or 3 level of pagetables). Only pmd_huge can return 1.
> 
> Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
> ---
> 
> diff --git a/arch/x86/mm/hugetlbpage.c b/arch/x86/mm/hugetlbpage.c
> --- a/arch/x86/mm/hugetlbpage.c
> +++ b/arch/x86/mm/hugetlbpage.c
> @@ -227,7 +227,15 @@ int pmd_huge(pmd_t pmd)
>  
>  int pud_huge(pud_t pud)
>  {
> +#ifdef CONFIG_X86_64
>  	return !!(pud_val(pud) & _PAGE_PSE);
> +#else
> +	/*
> +	 * pud is a bypass with 2 or 3 level pagetables, only pmd_huge
> +	 * can return 1.
> +	 */
> +	return 0;
> +#endif
>  }
>  
>  struct page *

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