linux-kernel - Re: [PATCH 6/11] ksm: remove old stable nodes more thoroughly

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Message-ID: <alpine.LNX.2.00.1301281747210.4947@eggly.anvils>
Date:	Mon, 28 Jan 2013 18:03:16 -0800 (PST)
From:	Hugh Dickins <hughd@...gle.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
cc:	Petr Holasek <pholasek@...hat.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Izik Eidus <izik.eidus@...ellosystems.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH 6/11] ksm: remove old stable nodes more thoroughly

On Mon, 28 Jan 2013, Andrew Morton wrote:
> On Fri, 25 Jan 2013 18:01:59 -0800 (PST)
> Hugh Dickins <hughd@...gle.com> wrote:
> 
> > +static int remove_all_stable_nodes(void)
> > +{
> > +	struct stable_node *stable_node;
> > +	int nid;
> > +	int err = 0;
> > +
> > +	for (nid = 0; nid < nr_node_ids; nid++) {
> > +		while (root_stable_tree[nid].rb_node) {
> > +			stable_node = rb_entry(root_stable_tree[nid].rb_node,
> > +						struct stable_node, node);
> > +			if (remove_stable_node(stable_node)) {
> > +				err = -EBUSY;
> 
> It's a bit rude to overwrite remove_stable_node()'s return value.

Well.... yes, but only the tiniest bit rude :)

> 
> > +				break;	/* proceed to next nid */
> > +			}
> > +			cond_resched();
> 
> Why is this here?

Because we don't have a limit on the length of this loop, and if
every node which remove_stable_node() finds is already stale, and
has no rmap_item still attached, then there would be no rescheduling
point in the unbounded loop without this one.  I was taught to worry
about bad latencies even in unpreemptible kernels.

Hugh
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