linux-kernel - Re: [PATCH mm-new v6 2/5] mm: khugepaged: refine scan progress number

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Message-ID: <6zltgzs24wpypzu36ldwgtzilhv2z3ofuu45azp5u45huiwqvj@6jhhp5r24po6>
Date: Fri, 6 Feb 2026 19:12:46 +0800
From: Vernon Yang <vernon2gm@...il.com>
To: "David Hildenbrand (Arm)" <david@...nel.org>
Cc: Dev Jain <dev.jain@....com>, akpm@...ux-foundation.org, 
	lorenzo.stoakes@...cle.com, ziy@...dia.com, baohua@...nel.org, lance.yang@...ux.dev, 
	linux-mm@...ck.org, linux-kernel@...r.kernel.org, 
	Vernon Yang <yanglincheng@...inos.cn>
Subject: Re: [PATCH mm-new v6 2/5] mm: khugepaged: refine scan progress number

On Fri, Feb 06, 2026 at 10:02:48AM +0100, David Hildenbrand (Arm) wrote:
> On 2/5/26 15:25, Dev Jain wrote:
> >
> > On 05/02/26 5:41 pm, David Hildenbrand (arm) wrote:
> > > On 2/5/26 07:08, Vernon Yang wrote:
> > > > On Thu, Feb 5, 2026 at 5:35 AM David Hildenbrand (arm)
> > > > <david@...nel.org> wrote:
> > > >
> > > > I guess, your meaning is "min(_pte - pte + 1, HPAGE_PMD_NR)", not max().
> > >
> > > Yes!
> > >
> > > >
> > > >
> > > > I'm also worried that the compiler can't optimize this since the body of
> > > > the loop is complex, as with Dev's opinion [1].
> > >
> > > Why do we even have to optimize this? :)
> > >
> > > Premature ... ? :)
> >
> >
> > I mean .... we don't, but the alternate is a one liner using max().
>
> I'm fine with the max(), but it still seems like adding complexity to
> optimize something that is nowhere prove to really be a problem.

Hi David, Dev,

I use "*cur_progress += 1" at the beginning of the loop, the compiler
optimize that. Assembly as follows:

60c1:	4d 29 ca        sub    %r9,%r10		// r10 is _pte, r9 is pte, r10 = _pte - pte
60c4:	b8 00 02 00 00  mov    $0x200,%eax	// eax = HPAGE_PMD_NR
60c9:	44 89 5c 24 10  mov    %r11d,0x10(%rsp)	//
60ce:	49 c1 fa 03     sar    $0x3,%r10	//
60d2:	49 83 c2 01     add    $0x1,%r10	// r10 += 1
60d6:	49 39 c2        cmp    %rax,%r10	// r10 = min(r10, eax)
60d9:	4c 0f 4f d0     cmovg  %rax,%r10	//
60dd:	44 89 55 00     mov    %r10d,0x0(%rbp)	// *cur_progress = r10

To make the code simpler, Let us use "*cur_progress += 1".

--
Thanks,
Vernon