linux-kernel - Re: [PATCH] zcache: don't limit number of pools per client

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Date:	Tue, 05 Jun 2012 22:41:46 +0200
From:	Sasha Levin <levinsasha928@...il.com>
To:	Seth Jennings <sjenning@...ux.vnet.ibm.com>
Cc:	gregkh@...uxfoundation.org, dan.magenheimer@...cle.com,
	konrad.wilk@...cle.com, devel@...verdev.osuosl.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] zcache: don't limit number of pools per client

On Tue, 2012-06-05 at 15:19 -0500, Seth Jennings wrote:
> > +	pool = idr_find(&cli->tmem_pools, poolid);
> > +	if (pool != NULL)
> > +		atomic_inc(&pool->refcount);
> 
> 
> This is called on the main path, so it needs to be fast.  There is so much
> contention elsewhere in the stack I don't think it'll be an issue. It looks
> like idr_find() is fast, even though it contains a loop.  Just needs to be
> considered.

Agreed. idr actually uses something which looks like a hash table, so it
should be fast enough for this case.

> > +retry:
> > +	r = idr_pre_get(&cli->tmem_pools, GFP_ATOMIC);
> 
> > +	if (r != 1) {
> > +		kfree(pool);
> > +		pr_info("zcache: pool creation failed: out of memory\n");
> > +		goto out;
> > +	}
> > +	r = idr_get_new(&cli->tmem_pools, pool, &poolid);
> > +	switch (r) {
> > +	case 0:
> > +		break;
> > +	case -EAGAIN:
> > +		goto retry;
> > +	default:
> > +		pr_info("zcache: pool creation failed: error %d\n", r);
> >  		kfree(pool);
> > -		poolid = -1;
> >  		goto out;
> >  	}
> > +
> 
> 
> how about:
> =====
> do {
> 	r = idr_pre_get(&cli->tmem_pools, GFP_ATOMIC);
> 	if (r != 1) {
> 		kfree(pool);
> 		pr_info("zcache: pool creation failed: out of memory\n");
> 		goto out;
> 	}
> 	r = idr_get_new(&cli->tmem_pools, pool, &poolid);
> }
> while (r == -EAGAIN)
> 
> if (r) {
> 	pr_info("zcache: pool creation failed: error %d\n", r);
> 	kfree(pool);
> 	goto out;
> }
> =====
> so we can lose the label/goto.
> 
> Also, do we want GFP_ATOMIC?  Why not GFP_KERNEL?

I wasn't sure about the context this code runs at, and assumed it's
atomic one due to the kmalloc allocating with GFP_ATOMIC just couple of
lines above the idr_pre_get. If that's wrong, we can switch that kmalloc
as well.

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