linux-kernel - Re: [PATCH v3 11/16] zsmalloc: separate free_zspage from putback

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Date:	Mon, 18 Apr 2016 10:04:08 +0900
From:	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:	Minchan Kim <minchan@...nel.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	jlayton@...chiereds.net, bfields@...ldses.org,
	Vlastimil Babka <vbabka@...e.cz>,
	Joonsoo Kim <iamjoonsoo.kim@....com>, koct9i@...il.com,
	aquini@...hat.com, virtualization@...ts.linux-foundation.org,
	Mel Gorman <mgorman@...e.de>, Hugh Dickins <hughd@...gle.com>,
	Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
	Rik van Riel <riel@...hat.com>, rknize@...orola.com,
	Gioh Kim <gi-oh.kim@...fitbricks.com>,
	Sangseok Lee <sangseok.lee@....com>,
	Chan Gyun Jeong <chan.jeong@....com>,
	Al Viro <viro@...IV.linux.org.uk>,
	YiPing Xu <xuyiping@...ilicon.com>
Subject: Re: [PATCH v3 11/16] zsmalloc: separate free_zspage from
 putback_zspage

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?

>  		}
>  		/* 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?

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);
>  }