linux-kernel - Re: [PATCH 01/13] mm: use add_page_to_lru

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Date:   Fri, 18 Sep 2020 04:05:19 -0600
From:   Yu Zhao <yuzhao@...gle.com>
To:     Michal Hocko <mhocko@...e.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Alex Shi <alex.shi@...ux.alibaba.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Roman Gushchin <guro@...com>,
        Shakeel Butt <shakeelb@...gle.com>,
        Chris Down <chris@...isdown.name>,
        Yafang Shao <laoar.shao@...il.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Huang Ying <ying.huang@...el.com>,
        Pankaj Gupta <pankaj.gupta.linux@...il.com>,
        Matthew Wilcox <willy@...radead.org>,
        Konstantin Khlebnikov <koct9i@...il.com>,
        Minchan Kim <minchan@...nel.org>,
        Jaewon Kim <jaewon31.kim@...sung.com>, cgroups@...r.kernel.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 01/13] mm: use add_page_to_lru_list()

On Fri, Sep 18, 2020 at 09:32:40AM +0200, Michal Hocko wrote:
> On Thu 17-09-20 21:00:39, Yu Zhao wrote:
> > This patch replaces the only open-coded lru list addition with
> > add_page_to_lru_list().
> > 
> > Before this patch, we have:
> > 	update_lru_size()
> > 	list_move()
> > 
> > After this patch, we have:
> > 	list_del()
> > 	add_page_to_lru_list()
> > 		update_lru_size()
> > 		list_add()
> > 
> > The only side effect is that page->lru is temporarily poisoned
> > after a page is deleted from its old list, which shouldn't be a
> > problem.
> 
> "because the lru lock is held" right?
> 
> Please always be explicit about your reasoning. "It shouldn't be a
> problem" without further justification is just pointless for anybody
> reading the code later on.

It's not my reasoning. We are simply assuming different contexts this
discussion is under. In the context I'm assuming, we all know we are
under lru lock, which is rule 101 of lru list operations. But I'd be
happy to remove such assumption and update the commit message.

Any concern with the code change?

>  
> > Signed-off-by: Yu Zhao <yuzhao@...gle.com>
> > ---
> >  mm/vmscan.c | 7 +++----
> >  1 file changed, 3 insertions(+), 4 deletions(-)
> > 
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index 9727dd8e2581..503fc5e1fe32 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -1850,8 +1850,8 @@ static unsigned noinline_for_stack move_pages_to_lru(struct lruvec *lruvec,
> >  	while (!list_empty(list)) {
> >  		page = lru_to_page(list);
> >  		VM_BUG_ON_PAGE(PageLRU(page), page);
> > +		list_del(&page->lru);
> >  		if (unlikely(!page_evictable(page))) {
> > -			list_del(&page->lru);
> >  			spin_unlock_irq(&pgdat->lru_lock);
> >  			putback_lru_page(page);
> >  			spin_lock_irq(&pgdat->lru_lock);
> > @@ -1862,9 +1862,7 @@ static unsigned noinline_for_stack move_pages_to_lru(struct lruvec *lruvec,
> >  		SetPageLRU(page);
> >  		lru = page_lru(page);
> >  
> > -		nr_pages = thp_nr_pages(page);
> > -		update_lru_size(lruvec, lru, page_zonenum(page), nr_pages);
> > -		list_move(&page->lru, &lruvec->lists[lru]);
> > +		add_page_to_lru_list(page, lruvec, lru);
> >  
> >  		if (put_page_testzero(page)) {
> >  			__ClearPageLRU(page);
> > @@ -1878,6 +1876,7 @@ static unsigned noinline_for_stack move_pages_to_lru(struct lruvec *lruvec,
> >  			} else
> >  				list_add(&page->lru, &pages_to_free);
> >  		} else {
> > +			nr_pages = thp_nr_pages(page);
> >  			nr_moved += nr_pages;
> >  			if (PageActive(page))
> >  				workingset_age_nonresident(lruvec, nr_pages);
> > -- 
> > 2.28.0.681.g6f77f65b4e-goog
> 
> -- 
> Michal Hocko
> SUSE Labs