linux-kernel - Re: [PATCH 0/2] mm->def_flags cleanups (Was: Change khugepaged to respect MMF_THP

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Message-ID: <20140122192514.GA1779@redhat.com>
Date:	Wed, 22 Jan 2014 20:25:14 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Alex Thorlton <athorlton@....com>
Cc:	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>
Subject: Re: [PATCH 0/2] mm->def_flags cleanups (Was: Change khugepaged to
	respect MMF_THP_DISABLE flag)

On 01/22, Alex Thorlton wrote:
>
> At a glance, without testing, it looks like a good idea to me.  By
> using def_flags, we leverage functionality that's already in place to
> achieve the same result.  We don't need to add any new checks into the
> fault path or into khugepaged, since we're just leveraging the
> VM_HUGEPAGE/NOHUGEPAGE flag, which we already check for.  We also get
> the behavior that you suggested (madvise is still respected, even with
> the new THP disable prctl set), for free with this method.

Yes, exactly.

> I like the idea, but I think that it should probably be a separate
> change from the other few cleanups that you proposed along with it,

Yes, sure, that is why I sent them separately,

> since
> they're somewhat unrelated to this particular issue.  Do you agree?

Not really. Note that without 1/2 VM_NOHUGEPAGE won't survive after
exec. And without 2/2 madvise(MADV_HUGEPAGE) won't work after
PR_SET_THP_DISABLE.

But again, I think that these 2 simple cleanups make sense even without
PR_SET_THP_DISABLE.

> > diff --git a/kernel/sys.c b/kernel/sys.c
> > index ac1842e..eb8b0fc 100644
> > --- a/kernel/sys.c
> > +++ b/kernel/sys.c
> > @@ -2029,6 +2029,19 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,
> >  		if (arg2 || arg3 || arg4 || arg5)
> >  			return -EINVAL;
> >  		return current->no_new_privs ? 1 : 0;
> > +	case PR_SET_THP_DISABLE:
> > +	case PR_GET_THP_DISABLE:
> > +		down_write(&me->mm->mmap_sem);
> > +		if (option == PR_SET_THP_DISABLE) {
> > +			if (arg2)
> > +				me->mm->def_flags |= VM_NOHUGEPAGE;
> > +			else
> > +				me->mm->def_flags &= ~VM_NOHUGEPAGE;
> > +		} else {
> > +			error = !!(me->mm->flags && VM_NOHUGEPAGE);
>
> Should be:
>
> error = !!(me->mm->def_flags && VM_NOHUGEPAGE);

No, we need to return 1 if this bit is set ;)

Oleg.

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