[HMM v15 01/16] mm/free_hot_cold_page: catch ZONE_DEVICE pages

["Jérôme Glisse" <jglisse@redhat.com>]

Catch page from ZONE_DEVICE in free_hot_cold_page(). This should never
happen as ZONE_DEVICE page must always have an elevated refcount.

This is safety-net to catch any refcounting issues in a sane way for any
ZONE_DEVICE pages.

Signed-off-by: JA(C)rA'me Glisse <jglisse@redhat.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Ross Zwisler <ross.zwisler@linux.intel.com>
---
 mm/page_alloc.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 1c24112..355beb4 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -2445,6 +2445,16 @@ void free_hot_cold_page(struct page *page, bool cold)
 	unsigned long pfn = page_to_pfn(page);
 	int migratetype;
 
+	/*
+	 * This should never happen ! Page from ZONE_DEVICE always must have an
+	 * active refcount. Complain about it and try to restore the refcount.
+	 */
+	if (is_zone_device_page(page)) {
+		VM_BUG_ON_PAGE(is_zone_device_page(page), page);
+		page_ref_inc(page);
+		return;
+	}
+
 	if (!free_pcp_prepare(page))
 		return;
 
-- 
2.4.3

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