Re: [PATCH] Add nid sanity on alloc_pages_node

[Andrew Morton <akpm@linux-foundation.org>]

On Tue, 17 Jul 2007 16:04:54 +0100 (BST) Hugh Dickins <hugh@veritas.com> wrote:

> On Thu, 12 Jul 2007, Andrew Morton wrote:
> > 
> > It'd be much better to fix the race within alloc_fresh_huge_page().  That
> > function is pretty pathetic.
> > 
> > Something like this?
> > 
> > --- a/mm/hugetlb.c~a
> > +++ a/mm/hugetlb.c
> > @@ -105,13 +105,20 @@ static void free_huge_page(struct page *
> >  
> >  static int alloc_fresh_huge_page(void)
> >  {
> > -	static int nid = 0;
> > +	static int prev_nid;
> > +	static DEFINE_SPINLOCK(nid_lock);
> >  	struct page *page;
> > -	page = alloc_pages_node(nid, htlb_alloc_mask|__GFP_COMP|__GFP_NOWARN,
> > -					HUGETLB_PAGE_ORDER);
> > -	nid = next_node(nid, node_online_map);
> > +	int nid;
> > +
> > +	spin_lock(&nid_lock);
> > +	nid = next_node(prev_nid, node_online_map);
> >  	if (nid == MAX_NUMNODES)
> >  		nid = first_node(node_online_map);
> > +	prev_nid = nid;
> > +	spin_unlock(&nid_lock);
> > +
> > +	page = alloc_pages_node(nid, htlb_alloc_mask|__GFP_COMP|__GFP_NOWARN,
> > +					HUGETLB_PAGE_ORDER);
> >  	if (page) {
> >  		set_compound_page_dtor(page, free_huge_page);
> >  		spin_lock(&hugetlb_lock);
> 
> Now that it's gone into the tree, I look at it and wonder, does your
> nid_lock really serve any purpose?  We're just doing a simple assignment
> to prev_nid, and it doesn't matter if occasionally two racers choose the
> same node, and there's no protection here against a node being offlined
> before the alloc_pages_node anyway (unsupported? I'm ignorant).

umm, actually, yes, the code as it happens to be structured does mean that
ther is no longer a way in which a race can cause us to pass MAX_NUMNODES
into alloc_pages_node().

Or not.  We can call next_node(MAX_NUMNODES, node_online_map) in that race
window, with perhaps bad results.

I think I like the lock ;)