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

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Date:	Thu, 23 Jan 2014 17:43:34 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Hugh Dickins <hughd@...gle.com>,
	Gerald Schaefer <gerald.schaefer@...ibm.com>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Heiko Carstens <heiko.carstens@...ibm.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 01/22, Hugh Dickins wrote:
>
> 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".

Argh. Thanks Hugh!

Another case when I forgot about /bin/grep. So the patch is wrong,
at least the changelog is certainly is. I am stupid.

But, perhaps, this all still can work? Looks like, s390 already
implements PR_SET_THP_DISABLE using the same idea, it would be
nice to avoid another hack.

Gerald, any chance we can revert 8e72033f2a489 "thp: make MADV_HUGEPAGE
check for mm->def_flags" ? The changelog says "In order to also prevent
MADV_HUGEPAGE on such an mm", is it really important? I mean, if the
application calls madvise(MADV_HUGEPAGE) it should probably know what
it does and, this can be useful after if PR_SET_THP_DISABLE or
KVM_S390_ENABLE_SIE. Of course I do not understand this code, perhaps
MADV_HUGEPAGE is simply impossible.

Another question, can't fork/exec preserve VM_NOHUGEPAGE in ->def_flags?
This is what PR_SET_THP_DISABLE wants.

I'll send you mbox with the previous discussion privately.

Thanks,

Oleg.

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