linux-kernel - Re: [PATCH 3/3] zsmalloc: do not take class lock in zs_pages_to

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Date:	Thu, 16 Jul 2015 08:38:34 +0900
From:	Minchan Kim <minchan@...nel.org>
To:	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Subject: Re: [PATCH 3/3] zsmalloc: do not take class lock in
 zs_pages_to_compact()

Hi Sergey,

On Wed, Jul 15, 2015 at 01:07:03PM +0900, Sergey Senozhatsky wrote:
> On (07/11/15 18:45), Sergey Senozhatsky wrote:
> [..]
> > We re-do this calculations during compaction on a per class basis
> > anyway.
> > 
> > zs_unregister_shrinker() will not return until we have an active
> > shrinker, so classes won't unexpectedly disappear while
> > zs_pages_to_compact(), invoked by zs_shrinker_count(), iterates
> > them.
> > 
> > When called from zram, we are protected by zram's ->init_lock,
> > so, again, classes will be there until zs_pages_to_compact()
> > iterates them.
> > 
> > Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@...il.com>
> > ---
> >  mm/zsmalloc.c | 2 --
> >  1 file changed, 2 deletions(-)
> > 
> > diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
> > index b10a228..824c182 100644
> > --- a/mm/zsmalloc.c
> > +++ b/mm/zsmalloc.c
> > @@ -1811,9 +1811,7 @@ unsigned long zs_pages_to_compact(struct zs_pool *pool)
> >  		if (class->index != i)
> >  			continue;
> >  
> > -		spin_lock(&class->lock);
> >  		pages_to_free += zs_can_compact(class);
> > -		spin_unlock(&class->lock);
> >  	}
> >  
> >  	return pages_to_free;
> 
> This patch still makes sense. Agree?

There is already race window between shrink_count and shrink_slab so
it would be okay if we return stale stat with removing the lock if
the difference is not huge.

Even, now we don't obey nr_to_scan of shrinker in zs_shrinker_scan
so the such accuracy would be pointless.

Please resend the patch and correct zs_can_compact's comment.

Thanks.
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