linux-kernel - Re: [RFC] mm: change find

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Message-ID: <20151214211132.GA7390@node.shutemov.name>
Date:	Mon, 14 Dec 2015 23:11:32 +0200
From:	"Kirill A. Shutemov" <kirill@...temov.name>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	yalin wang <yalin.wang2010@...il.com>, akpm@...ux-foundation.org,
	kirill.shutemov@...ux.intel.com, gang.chen.5i5j@...il.com,
	mhocko@...e.com, kwapulinski.piotr@...il.com, aarcange@...hat.com,
	dcashman@...gle.com, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC] mm: change find_vma() function

On Mon, Dec 14, 2015 at 06:55:09PM +0100, Oleg Nesterov wrote:
> On 12/14, Kirill A. Shutemov wrote:
> >
> > On Mon, Dec 14, 2015 at 07:02:25PM +0800, yalin wang wrote:
> > > change find_vma() to break ealier when found the adderss
> > > is not in any vma, don't need loop to search all vma.
> > >
> > > Signed-off-by: yalin wang <yalin.wang2010@...il.com>
> > > ---
> > >  mm/mmap.c | 3 +++
> > >  1 file changed, 3 insertions(+)
> > >
> > > diff --git a/mm/mmap.c b/mm/mmap.c
> > > index b513f20..8294c9b 100644
> > > --- a/mm/mmap.c
> > > +++ b/mm/mmap.c
> > > @@ -2064,6 +2064,9 @@ struct vm_area_struct *find_vma(struct mm_struct *mm, unsigned long addr)
> > >  			vma = tmp;
> > >  			if (tmp->vm_start <= addr)
> > >  				break;
> > > +			if (!tmp->vm_prev || tmp->vm_prev->vm_end <= addr)
> > > +				break;
> > > +
> >
> > This 'break' would return 'tmp' as found vma.
> 
> But this would be right?

Hm. Right. Sorry for my tone.

I think the right condition is 'tmp->vm_prev->vm_end < addr', not '<=' as
vm_end is the first byte after the vma. But it's equivalent in practice
here.

Anyway, I don't think it's possible to gain anything measurable from this
optimization.

> 
> Not that I think this optimization makes sense, I simply do not know,
> but to me this change looks technically correct at first glance...
> 
> But the changelog is wrong or I missed something. This change can stop
> the main loop earlier; if "tmp" is the first vma,

For the first vma, we don't get anything comparing to what we have now:
check for !rb_node on the next iteration would have the same trade off and
effect as the proposed check.

> or if the previous one is below the address.

Yes, but would it compensate additional check on each 'tmp->vm_end > addr'
iteration to the point? That's not obvious.

> Or perhaps I just misread that "not in any vma" note in the changelog.
> 
> No?
> 
> Oleg.
> 

-- 
 Kirill A. Shutemov
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