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SubjectRe: [PATCH] Add nid sanity on alloc_pages_node
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);
-
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