linux-kernel - [patch v2] mm, page_alloc: make first

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Date:	Tue, 4 Feb 2014 16:22:53 -0800 (PST)
From:	David Rientjes <rientjes@...gle.com>
To:	Andrew Morton <akpm@...ux-foundation.org>,
	Holger Kiehl <Holger.Kiehl@....de>
cc:	Christoph Lameter <cl@...ux.com>,
	Rafael Aquini <aquini@...hat.com>,
	Vlastimil Babka <vbabka@...e.cz>,
	Michal Hocko <mhocko@...e.cz>, Mel Gorman <mgorman@...e.de>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: [patch v2] mm, page_alloc: make first_page visible before PageTail

Commit bf6bddf1924e ("mm: introduce compaction and migration for ballooned
pages") introduces page_count(page) into memory compaction which
dereferences page->first_page if PageTail(page).

Introduce a store memory barrier to ensure page->first_page is properly
initialized so that code that does page_count(page) on pages off the lru
always have a valid p->first_page.

Reported-by: Holger Kiehl <Holger.Kiehl@....de>
Signed-off-by: David Rientjes <rientjes@...gle.com>
---
 v2: with commentary, per checkpatch

 mm/page_alloc.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -369,9 +369,11 @@ void prep_compound_page(struct page *page, unsigned long order)
 	__SetPageHead(page);
 	for (i = 1; i < nr_pages; i++) {
 		struct page *p = page + i;
-		__SetPageTail(p);
 		set_page_count(p, 0);
 		p->first_page = page;
+		/* Make sure p->first_page is always valid for PageTail() */
+		smp_wmb();
+		__SetPageTail(p);
 	}
 }
 
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