linux-kernel - Re: [PATCH 2/2] mm: thp: kill the bogus ->def_flags check in hugepage

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Date:	Wed, 22 Jan 2014 12:16:43 -0800 (PST)
From:	Hugh Dickins <hughd@...gle.com>
To:	Oleg Nesterov <oleg@...hat.com>
cc:	Alex Thorlton <athorlton@....com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Kirill A. Shutemov" <kirill@...temov.name>,
	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Rik van Riel <riel@...hat.com>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Andy Lutomirski <luto@...capital.net>,
	Al Viro <viro@...iv.linux.org.uk>,
	Kees Cook <keescook@...omium.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Martin Schwidefsky <schwidefsky@...ibm.com>
Subject: Re: [PATCH 2/2] mm: thp: kill the bogus ->def_flags check in
 hugepage_madvise()

On Wed, 22 Jan 2014, Oleg Nesterov wrote:

> hugepage_madvise() checks "mm->def_flags & VM_NOHUGEPAGE" but
> this can be never true, currently mm->def_flags can only have
> VM_LOCKED.

But line 1087 of arch/s390/mm/pgtable.c says
	mm->def_flags |= VM_NOHUGEPAGE;
from 3eabaee998c787e7e1565574821652548f7fc003
"KVM: s390: allow sie enablement for multi-threaded programs".

Hugh

> 
> And otoh we might want to add VM_NOHUGEPAGE into ->def_flags
> but override it in vma->vm_flags via madvise(MADV_HUGEPAGE).
> 
> Signed-off-by: Oleg Nesterov <oleg@...hat.com>
> ---
>  mm/huge_memory.c |    2 --
>  1 files changed, 0 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index bccd5a6..e8b656c 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -1941,8 +1941,6 @@ int hugepage_madvise(struct vm_area_struct *vma,
>  		 */
>  		if (*vm_flags & (VM_HUGEPAGE | VM_NO_THP))
>  			return -EINVAL;
> -		if (mm->def_flags & VM_NOHUGEPAGE)
> -			return -EINVAL;
>  		*vm_flags &= ~VM_NOHUGEPAGE;
>  		*vm_flags |= VM_HUGEPAGE;
>  		/*
> -- 
> 1.5.5.1
> 
> 
> --
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