Date: Wed, 22 Jan 2014 12:16:43 -0800 (PST)
From: Hugh Dickins <hughd@google.com>
To: Oleg Nesterov <oleg@redhat.com>
cc: Alex Thorlton <athorlton@sgi.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        "Kirill A. Shutemov" <kirill@shutemov.name>,
        linux-kernel@vger.kernel.org, Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Rik van Riel <riel@redhat.com>,
        Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Andy Lutomirski <luto@amacapital.net>,
        Al Viro <viro@zeniv.linux.org.uk>, Kees Cook <keescook@chromium.org>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Martin Schwidefsky <schwidefsky@de.ibm.com>
Subject: Re: [PATCH 2/2] mm: thp: kill the bogus ->def_flags check in
 hugepage_madvise()
In-Reply-To: <20140122174646.GC29710@redhat.com>
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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@redhat.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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