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Date:   Tue, 26 May 2020 11:55:49 -0400
From:   Johannes Weiner <hannes@cmpxchg.org>
To:     Qian Cai <cai@lca.pw>
Cc:     linux-mm@kvack.org, Rik van Riel <riel@surriel.com>,
        Minchan Kim <minchan.kim@gmail.com>,
        Michal Hocko <mhocko@suse.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        linux-kernel@vger.kernel.org, kernel-team@fb.com
Subject: Re: [PATCH 09/14] mm: deactivations shouldn't bias the LRU balance
Message-ID: <20200526155549.GB850116@cmpxchg.org>
References: <20200520232525.798933-1-hannes@cmpxchg.org>
 <20200520232525.798933-10-hannes@cmpxchg.org>
 <20200522133335.GA624@Qians-MacBook-Air.local>
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On Fri, May 22, 2020 at 09:33:35AM -0400, Qian Cai wrote:
> On Wed, May 20, 2020 at 07:25:20PM -0400, Johannes Weiner wrote:
> > Operations like MADV_FREE, FADV_DONTNEED etc. currently move any
> > affected active pages to the inactive list to accelerate their reclaim
> > (good) but also steer page reclaim toward that LRU type, or away from
> > the other (bad).
> > 
> > The reason why this is undesirable is that such operations are not
> > part of the regular page aging cycle, and rather a fluke that doesn't
> > say much about the remaining pages on that list; they might all be in
> > heavy use, and once the chunk of easy victims has been purged, the VM
> > continues to apply elevated pressure on those remaining hot pages. The
> > other LRU, meanwhile, might have easily reclaimable pages, and there
> > was never a need to steer away from it in the first place.
> > 
> > As the previous patch outlined, we should focus on recording actually
> > observed cost to steer the balance rather than speculating about the
> > potential value of one LRU list over the other. In that spirit, leave
> > explicitely deactivated pages to the LRU algorithm to pick up, and let
> > rotations decide which list is the easiest to reclaim.
> > 
> > Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
> > Acked-by: Minchan Kim <minchan@kernel.org>
> > Acked-by: Michal Hocko <mhocko@suse.com>
> > ---
> >  mm/swap.c | 4 ----
> >  1 file changed, 4 deletions(-)
> > 
> > diff --git a/mm/swap.c b/mm/swap.c
> > index 5d62c5a0c651..d7912bfb597f 100644
> > --- a/mm/swap.c
> > +++ b/mm/swap.c
> > @@ -515,14 +515,12 @@ static void lru_deactivate_file_fn(struct page *page, struct lruvec *lruvec,
> >  
> >  	if (active)
> >  		__count_vm_event(PGDEACTIVATE);
> > -	lru_note_cost(lruvec, !file, hpage_nr_pages(page));
> >  }
> >
> []
> 
> mm/swap.c: In function 'lru_deactivate_file_fn':
> mm/swap.c:504:11: warning: variable 'file' set but not used
> [-Wunused-but-set-variable]
>   int lru, file;
>            ^~~~ 

Oops, my gcc doesn't warn about that, but yes, it's clearly dead code.

$ make mm/swap.o
  GEN     Makefile
  CALL    /home/hannes/src/linux/linux/scripts/checksyscalls.sh
  CALL    /home/hannes/src/linux/linux/scripts/atomic/check-atomics.sh
  DESCEND  objtool
  CC      mm/swap.o
$
 
> This?
> 
> diff --git a/mm/swap.c b/mm/swap.c
> index fedf5847dfdb..9c38c1b545af 100644
> --- a/mm/swap.c
> +++ b/mm/swap.c
> @@ -501,7 +501,7 @@ void lru_cache_add_active_or_unevictable(struct page *page,
>  static void lru_deactivate_file_fn(struct page *page, struct lruvec *lruvec,
>  			      void *arg)
>  {
> -	int lru, file;
> +	int lru;
>  	bool active;
>  
>  	if (!PageLRU(page))
> @@ -515,7 +515,6 @@ static void lru_deactivate_file_fn(struct page *page, struct lruvec *lruvec,
>  		return;
>  
>  	active = PageActive(page);
> -	file = page_is_file_lru(page);
>  	lru = page_lru_base_type(page);
>  
>  	del_page_from_lru_list(page, lruvec, lru + active);

Looks good, and it appears Andrew has already queued it. Would you
mind providing the Signed-off-by: for it?