[PATCH] mm, soft offline: split thp at the beginning of soft_offline_page()

[Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>]

When we try to soft-offline a thp tail page, put_page() is called on the
tail page unthinkingly and VM_BUG_ON is triggered in put_compound_page().
This patch splits thp before going into the main body of soft-offlining.

The interface of soft-offlining is open for userspace, so this bug can
lead to DoS attack and should be fixed immedately.

Signed-off-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Cc: stable@vger.kernel.org
---
 mm/memory-failure.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git v3.7-rc7.orig/mm/memory-failure.c v3.7-rc7/mm/memory-failure.c
index 8fe3640..e48e235 100644
--- v3.7-rc7.orig/mm/memory-failure.c
+++ v3.7-rc7/mm/memory-failure.c
@@ -1548,9 +1548,17 @@ int soft_offline_page(struct page *page, int flags)
 {
 	int ret;
 	unsigned long pfn = page_to_pfn(page);
+	struct page *hpage = compound_trans_head(page);
 
 	if (PageHuge(page))
 		return soft_offline_huge_page(page, flags);
+	if (PageTransHuge(hpage)) {
+		if (PageAnon(hpage) && unlikely(split_huge_page(hpage))) {
+			pr_info("soft offline: %#lx: failed to split THP\n",
+				pfn);
+			return -EBUSY;
+		}
+	}
 
 	ret = get_any_page(page, pfn, flags);
 	if (ret < 0)
-- 
1.7.11.7

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