Re: [PATCH] Add nid sanity on alloc_pages_node

[Hugh Dickins <hugh@veritas.com>]

On Tue, 17 Jul 2007, Andrew Morton wrote:
> 
> Given that we've now gone and added deliberate-but-we-hope-benign 
> races into this code, an elaborate comment which explains and justifies
> it all is pretty much obligatory, IMO.

[PATCH] Remove nid_lock from alloc_fresh_huge_page

The fix to that race in alloc_fresh_huge_page() which could give an illegal
node ID did not need nid_lock at all: the fix was to replace static int nid
by static int prev_nid and do the work on local int nid.  nid_lock did make
sure that racers strictly roundrobin the nodes, but that's not something we
need to enforce strictly.  Kill nid_lock.

Signed-off-by: Hugh Dickins <hugh@veritas.com>
---
 mm/hugetlb.c |   10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

--- 2.6.22-git9/mm/hugetlb.c	2007-07-17 20:29:33.000000000 +0100
+++ linux/mm/hugetlb.c	2007-07-17 21:29:58.000000000 +0100
@@ -107,15 +107,19 @@ static int alloc_fresh_huge_page(void)
 {
 	static int prev_nid;
 	struct page *page;
-	static DEFINE_SPINLOCK(nid_lock);
 	int nid;
 
-	spin_lock(&nid_lock);
+	/*
+	 * Copy static prev_nid to local nid, work on that, then copy it
+	 * back to prev_nid afterwards: otherwise there's a window in which
+	 * a racer might pass invalid nid MAX_NUMNODES to alloc_pages_node.
+	 * But we don't need to use a spin_lock here: it really doesn't
+	 * matter if occasionally a racer chooses the same nid as we do.
+	 */
 	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);