linux-kernel - Re: [RFC PATCH 1/4] zbud: use page ref counter for zbud pages

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Date:	Wed, 07 Aug 2013 09:31:51 +0200
From:	Krzysztof Kozlowski <k.kozlowski@...sung.com>
To:	Seth Jennings <sjenning@...ux.vnet.ibm.com>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mgorman@...e.de>,
	Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Kyungmin Park <kyungmin.park@...sung.com>,
	Tomasz Stanislawski <t.stanislaws@...sung.com>,
	Bob Liu <bob.liu@...cle.com>
Subject: Re: [RFC PATCH 1/4] zbud: use page ref counter for zbud pages

Hi Seth,

On wto, 2013-08-06 at 13:51 -0500, Seth Jennings wrote:
> I like the idea.  I few things below.  Also agree with Bob the
> s/rebalance/adjust/ for rebalance_lists().
OK.

> s/else if/if/ since the if above returns if true.
Sure.

> > +		/* zbud_free() or zbud_alloc() */
> > +		int freechunks = num_free_chunks(zhdr);
> > +		list_add(&zhdr->buddy, &pool->unbuddied[freechunks]);
> > +	} else {
> > +		/* zbud_alloc() */
> > +		list_add(&zhdr->buddy, &pool->buddied);
> > +	}
> > +	/* Add/move zbud page to beginning of LRU */
> > +	if (!list_empty(&zhdr->lru))
> > +		list_del(&zhdr->lru);
> 
> We don't want to reinsert to the LRU list if we have called zbud_free()
> on a zbud page that previously had two buddies.  This code causes the
> zbud page to move to the front of the LRU list which is not what we want.

Right, I'll fix it.


> > @@ -326,10 +370,10 @@ found:
> >  void zbud_free(struct zbud_pool *pool, unsigned long handle)
> >  {
> >  	struct zbud_header *zhdr;
> > -	int freechunks;
> > 
> >  	spin_lock(&pool->lock);
> >  	zhdr = handle_to_zbud_header(handle);
> > +	BUG_ON(zhdr->last_chunks == 0 && zhdr->first_chunks == 0);
> 
> Not sure we need this.  Maybe, at most, VM_BUG_ON()?

Actually it is somehow a leftover after debugging so I don't mind
removing it completely.


> > @@ -411,11 +438,24 @@ int zbud_reclaim_page(struct zbud_pool *pool, unsigned int retries)
> >  		return -EINVAL;
> >  	}
> >  	for (i = 0; i < retries; i++) {
> > +		if (list_empty(&pool->lru)) {
> > +			/*
> > +			 * LRU was emptied during evict calls in previous
> > +			 * iteration but put_zbud_page() returned 0 meaning
> > +			 * that someone still holds the page. This may
> > +			 * happen when some other mm mechanism increased
> > +			 * the page count.
> > +			 * In such case we succedded with reclaim.
> > +			 */
> > +			return 0;
> > +		}
> >  		zhdr = list_tail_entry(&pool->lru, struct zbud_header, lru);
> > +		BUG_ON(zhdr->first_chunks == 0 && zhdr->last_chunks == 0);
> 
> Again here.
I agree.


Thanks for comments,
Krzysztof


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