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

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

On Tue, Nov 27, 2012 at 01:08:38PM -0800, Andi Kleen wrote:
> Naoya Horiguchi <n-horiguchi@ah.jp.nec.com> writes:
> 
> > 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.
> 
> Looks good.
> 
> >
> > The interface of soft-offlining is open for userspace, so this bug can
> > lead to DoS attack and should be fixed immedately.
> 
> The interface is root only and root can do everything anyways, so it's
> not really a security issue.

OK, this description had better not be here. I replace with attached one.

(.. and I forgot to disable suppress-cc for stable@vger.kernel.org in
the initial post so did it now, sorry.)

---
From: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Date: Tue, 27 Nov 2012 15:38:23 -0500
Subject: [PATCH] mm, soft offline: split thp at the beginning of
 soft_offline_page()

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.

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 a/mm/memory-failure.c b/mm/memory-failure.c
index 8fe3640..e48e235 100644
--- a/mm/memory-failure.c
+++ b/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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