Re: [PATCH v3 11/16] zsmalloc: separate free_zspage from putback_zspage

From: Minchan Kim <minchan@kernel.org>
To: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	<linux-kernel@vger.kernel.org>, <linux-mm@kvack.org>,
	<jlayton@poochiereds.net>, <bfields@fieldses.org>,
	Vlastimil Babka <vbabka@suse.cz>,
	Joonsoo Kim <iamjoonsoo.kim@lge.com>, <koct9i@gmail.com>,
	<aquini@redhat.com>, <virtualization@lists.linux-foundation.org>,
	Mel Gorman <mgorman@suse.de>, Hugh Dickins <hughd@google.com>,
	Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
	Rik van Riel <riel@redhat.com>, <rknize@motorola.com>,
	Gioh Kim <gi-oh.kim@profitbricks.com>,
	Sangseok Lee <sangseok.lee@lge.com>,
	Chan Gyun Jeong <chan.jeong@lge.com>,
	Al Viro <viro@ZenIV.linux.org.uk>,
	YiPing Xu <xuyiping@hisilicon.com>
Subject: Re: [PATCH v3 11/16] zsmalloc: separate free_zspage from putback_zspage
Date: Tue, 19 Apr 2016 16:51:18 +0900	[thread overview]
Message-ID: <20160419075118.GD18448@bbox> (raw)
In-Reply-To: <20160418010408.GB5882@swordfish>

Hi Sergey,

On Mon, Apr 18, 2016 at 10:04:08AM +0900, Sergey Senozhatsky wrote:
> Hello Minchan,
> 
> On (03/30/16 16:12), Minchan Kim wrote:
> [..]
> > @@ -1835,23 +1827,31 @@ static void __zs_compact(struct zs_pool *pool, struct size_class *class)
> >  			if (!migrate_zspage(pool, class, &cc))
> >  				break;
> >  
> > -			putback_zspage(pool, class, dst_page);
> > +			VM_BUG_ON_PAGE(putback_zspage(pool, class,
> > +				dst_page) == ZS_EMPTY, dst_page);
> 
> can this VM_BUG_ON_PAGE() condition ever be true?

I guess it is remained thing after I rebased to catch any mistake.
But I'm heavily chainging this part.
Please review next version instead of this after a few days. :)

> 
> >  		}
> >  		/* Stop if we couldn't find slot */
> >  		if (dst_page == NULL)
> >  			break;
> > -		putback_zspage(pool, class, dst_page);
> > -		if (putback_zspage(pool, class, src_page) == ZS_EMPTY)
> > +		VM_BUG_ON_PAGE(putback_zspage(pool, class,
> > +				dst_page) == ZS_EMPTY, dst_page);
> 
> hm... this VM_BUG_ON_PAGE(dst_page) is sort of confusing. under what
> circumstances it can be true?
> 
> a minor nit, it took me some time (need some coffee I guess) to
> correctly parse this macro wrapper
> 
> 		VM_BUG_ON_PAGE(putback_zspage(pool, class,
> 			dst_page) == ZS_EMPTY, dst_page);
> 
> may be do it like:
> 		fullness = putback_zspage(pool, class, dst_page);
> 		VM_BUG_ON_PAGE(fullness == ZS_EMPTY, dst_page);
> 
> 
> well, if we want to VM_BUG_ON_PAGE() at all. there haven't been any
> problems with compaction, is there any specific reason these macros
> were added?
> 
> 
> 
> > +		if (putback_zspage(pool, class, src_page) == ZS_EMPTY) {
> >  			pool->stats.pages_compacted += class->pages_per_zspage;
> > -		spin_unlock(&class->lock);
> > +			spin_unlock(&class->lock);
> > +			free_zspage(pool, class, src_page);
> 
> do we really need to free_zspage() out of class->lock?
> wouldn't something like this
> 
> 		if (putback_zspage(pool, class, src_page) == ZS_EMPTY) {
> 			pool->stats.pages_compacted += class->pages_per_zspage;
> 			free_zspage(pool, class, src_page);
> 		}
> 		spin_unlock(&class->lock);
> 
> be simpler?

The reason I did out of class->lock is deadlock between page_lock
and class->lock with upcoming page migration.
However, as I said, I'm now heavily changing the part. :)

> 
> besides, free_zspage() now updates class stats out of class lock,
> not critical but still.
> 
> 	-ss
> 
> > +		} else {
> > +			spin_unlock(&class->lock);
> > +		}
> > +
> >  		cond_resched();
> >  		spin_lock(&class->lock);
> >  	}
> >  
> >  	if (src_page)
> > -		putback_zspage(pool, class, src_page);
> > +		VM_BUG_ON_PAGE(putback_zspage(pool, class,
> > +				src_page) == ZS_EMPTY, src_page);
> >  
> >  	spin_unlock(&class->lock);
> >  }