linux-kernel - Re: [PATCH v3 03/10] Add additional isolation mode

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Date:	Fri, 10 Jun 2011 00:00:45 +0900
From:	Minchan Kim <minchan.kim@...il.com>
To:	Mel Gorman <mgorman@...e.de>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-mm <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Rik van Riel <riel@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Subject: Re: [PATCH v3 03/10] Add additional isolation mode

On Thu, Jun 09, 2011 at 02:59:02PM +0100, Mel Gorman wrote:
> On Tue, Jun 07, 2011 at 11:38:16PM +0900, Minchan Kim wrote:
> > There are some places to isolate lru page and I believe
> > users of isolate_lru_page will be growing.
> > The purpose of them is each different so part of isolated pages
> > should put back to LRU, again.
> > 
> > The problem is when we put back the page into LRU,
> > we lose LRU ordering and the page is inserted at head of LRU list.
> > It makes unnecessary LRU churning so that vm can evict working set pages
> > rather than idle pages.
> > 
> > This patch adds new modes when we isolate page in LRU so we don't isolate pages
> > if we can't handle it. It could reduce LRU churning.
> > 
> > This patch doesn't change old behavior. It's just used by next patches.
> > 
> > Cc: KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
> > Cc: Mel Gorman <mgorman@...e.de>
> > Cc: Andrea Arcangeli <aarcange@...hat.com>
> > Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
> > Acked-by: Rik van Riel <riel@...hat.com>
> > Acked-by: Johannes Weiner <hannes@...xchg.org>
> > Signed-off-by: Minchan Kim <minchan.kim@...il.com>
> > ---
> >  include/linux/swap.h |    2 ++
> >  mm/vmscan.c          |    6 ++++++
> >  2 files changed, 8 insertions(+), 0 deletions(-)
> > 
> > diff --git a/include/linux/swap.h b/include/linux/swap.h
> > index 48d50e6..731f5dd 100644
> > --- a/include/linux/swap.h
> > +++ b/include/linux/swap.h
> > @@ -248,6 +248,8 @@ enum ISOLATE_MODE {
> >  	ISOLATE_NONE,
> >  	ISOLATE_INACTIVE = 1,	/* Isolate inactive pages */
> >  	ISOLATE_ACTIVE = 2,	/* Isolate active pages */
> > +	ISOLATE_CLEAN = 8,      /* Isolate clean file */
> > +	ISOLATE_UNMAPPED = 16,  /* Isolate unmapped file */
> >  };
> 
> This really should be a bitwise type like gfp_t.

Agree. As I said, I will change it.

> 
> >  
> >  /* linux/mm/vmscan.c */
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index 4cbe114..26aa627 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -990,6 +990,12 @@ int __isolate_lru_page(struct page *page, enum ISOLATE_MODE mode, int file)
> >  
> >  	ret = -EBUSY;
> >  
> > +	if (mode & ISOLATE_CLEAN && (PageDirty(page) || PageWriteback(page)))
> > +		return ret;
> > +
> > +	if (mode & ISOLATE_UNMAPPED && page_mapped(page))
> > +		return ret;
> > +
> >  	if (likely(get_page_unless_zero(page))) {
> >  		/*
> >  		 * Be careful not to clear PageLRU until after we're
> 
> This patch does notuse ISOLATE_CLEAN or ISOLATE_UMAPPED anywhere. While
> I can guess how they will be used, it would be easier to review if one
> patch introduced ISOLATE_CLEAN and updated the call sites where it was
> relevant. Same with ISOLATE_UNMAPPED.

Totally agree.
I also always wanted it to others. :(

> 
> Also when using & like this, I thought the compiler warned if it wasn't
> in parenthesis but maybe that's wrong. The problem is the operator

My compiler(gcc version 4.4.3 (Ubuntu 4.4.3-4ubuntu5) was smart.

> precedence for bitwise AND and logical AND is easy to forget as it's
> so rarely an issue.

I will update the part for readability as well as compiler warning unexpected

> 
> i.e. it's easy to forget if
> 
> mode & ISOLATE_UNMAPPED && page_mapped(page)
> 
> means
> 
> mode & (ISOLATE_UNMAPPED && page_mapped(page))
> 
> or
> 
> (mode & ISOLATE_UNMAPPED) && page_mapped(page)
> 
> Be nice and specific for this one.
> 
> -- 
> Mel Gorman
> SUSE Labs

-- 
Kind regards
Minchan Kim
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