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

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Message-ID: <20140122200214.GV18196@sgi.com>
Date:	Wed, 22 Jan 2014 14:02:14 -0600
From:	Alex Thorlton <athorlton@....com>
To:	Oleg Nesterov <oleg@...hat.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 Wed, Jan 22, 2014 at 08:43:27PM +0100, Oleg Nesterov wrote:
> On 01/22, Oleg Nesterov wrote:
> >
> > On 01/22, Alex Thorlton wrote:
> > > > +	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 ;)
> 
> Damn, you are right of course, we need "&". I didn't notice "&&"
> in the patch I sent and misunderstood your "&&" above ;) Sorry.

Actually, I didn't catch that either!  Looking at it, though, we
definitely do want bitwise AND here, not logical.

However, what I was originally referring to is:  Shouldn't we be
checking mm->***def_flags*** for the VM_NOHUGEPAGE bit, as opposed
to mm->flags?  i.e. I think we want this:

error = !!(me->mm->def_flags & VM_NOHUGEPAGE);

As opposed to:

error = !!(me->mm->flags && VM_NOHUGEPAGE);

The way I understand it, the VM_NOHUGEPAGE bit is defined for
mm->vma->flags, but not for mm->flags.  Am I wrong here?

- Alex
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