[PATCH v2 1/1] mm/hugetlb: fix memory offline with hugepage size > memory block size

[Gerald Schaefer <gerald.schaefer@de.ibm.com>]

dissolve_free_huge_pages() will either run into the VM_BUG_ON() or a
list corruption and addressing exception when trying to set a memory
block offline that is part (but not the first part) of a hugetlb page
with a size > memory block size.

When no other smaller hugetlb page sizes are present, the VM_BUG_ON()
will trigger directly. In the other case we will run into an addressing
exception later, because dissolve_free_huge_page() will not work on the
head page of the compound hugetlb page which will result in a NULL
hstate from page_hstate().

To fix this, first remove the VM_BUG_ON() because it is wrong, and then
use the compound head page in dissolve_free_huge_page().

Also change locking in dissolve_free_huge_page(), so that it only takes
the lock when actually removing a hugepage.

Signed-off-by: Gerald Schaefer <gerald.schaefer@de.ibm.com>
---
Changes in v2:
- Update comment in dissolve_free_huge_pages()
- Change locking in dissolve_free_huge_page()

 mm/hugetlb.c | 31 +++++++++++++++++++------------
 1 file changed, 19 insertions(+), 12 deletions(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 87e11d8..1522af8 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -1441,23 +1441,30 @@ static int free_pool_huge_page(struct hstate *h, nodemask_t *nodes_allowed,
  */
 static void dissolve_free_huge_page(struct page *page)
 {
+	struct page *head = compound_head(page);
+	struct hstate *h;
+	int nid;
+
+	if (page_count(head))
+		return;
+
+	h = page_hstate(head);
+	nid = page_to_nid(head);
+
 	spin_lock(&hugetlb_lock);
-	if (PageHuge(page) && !page_count(page)) {
-		struct hstate *h = page_hstate(page);
-		int nid = page_to_nid(page);
-		list_del(&page->lru);
-		h->free_huge_pages--;
-		h->free_huge_pages_node[nid]--;
-		h->max_huge_pages--;
-		update_and_free_page(h, page);
-	}
+	list_del(&head->lru);
+	h->free_huge_pages--;
+	h->free_huge_pages_node[nid]--;
+	h->max_huge_pages--;
+	update_and_free_page(h, head);
 	spin_unlock(&hugetlb_lock);
 }
 
 /*
  * Dissolve free hugepages in a given pfn range. Used by memory hotplug to
  * make specified memory blocks removable from the system.
- * Note that start_pfn should aligned with (minimum) hugepage size.
+ * Note that this will dissolve a free gigantic hugepage completely, if any
+ * part of it lies within the given range.
  */
 void dissolve_free_huge_pages(unsigned long start_pfn, unsigned long end_pfn)
 {
@@ -1466,9 +1473,9 @@ void dissolve_free_huge_pages(unsigned long start_pfn, unsigned long end_pfn)
 	if (!hugepages_supported())
 		return;
 
-	VM_BUG_ON(!IS_ALIGNED(start_pfn, 1 << minimum_order));
 	for (pfn = start_pfn; pfn < end_pfn; pfn += 1 << minimum_order)
-		dissolve_free_huge_page(pfn_to_page(pfn));
+		if (PageHuge(pfn_to_page(pfn)))
+			dissolve_free_huge_page(pfn_to_page(pfn));
 }
 
 /*

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