mm: fix page reference leak in soft_offline_page()

The conversion to move pfn_to_online_page() internal to
soft_offline_page() missed that the get_user_pages() reference taken by
the madvise() path needs to be dropped when pfn_to_online_page() fails.

Note the direct sysfs-path to soft_offline_page() does not perform a
get_user_pages() lookup.

When soft_offline_page() is handed a pfn_valid() && !pfn_to_online_page()
pfn the kernel hangs at dax-device shutdown due to a leaked reference.

Link: https://lkml.kernel.org/r/[e-mail]
Fixes: feec24a6139d ("mm, soft-offline: convert parameter to pfn")
Signed-off-by: Dan Williams <[e-mail]>
Reviewed-by: David Hildenbrand <[e-mail]>
Reviewed-by: Oscar Salvador <[e-mail]>
Reviewed-by: Naoya Horiguchi <[e-mail]>
Cc: Michal Hocko <[e-mail]>
Cc: Qian Cai <[e-mail]>
Cc: <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

mm/memory-failure.c

···
 	return rc;
 }
 
+static void put_ref_page(struct page *page)
+{
+	if (page)
+		put_page(page);
+}
+
 /**
  * soft_offline_page - Soft offline a page.
  * @pfn: pfn to soft-offline
···
 int soft_offline_page(unsigned long pfn, int flags)
 {
 	int ret;
-	struct page *page;
 	bool try_again = true;
+	struct page *page, *ref_page = NULL;
+
+	WARN_ON_ONCE(!pfn_valid(pfn) && (flags & MF_COUNT_INCREASED));
 
 	if (!pfn_valid(pfn))
 		return -ENXIO;
+	if (flags & MF_COUNT_INCREASED)
+		ref_page = pfn_to_page(pfn);
+
 	/* Only online pages can be soft-offlined (esp., not ZONE_DEVICE). */
 	page = pfn_to_online_page(pfn);
-	if (!page)
+	if (!page) {
+		put_ref_page(ref_page);
 		return -EIO;
+	}
 
 	if (PageHWPoison(page)) {
 		pr_info("%s: %#lx page already poisoned\n", __func__, pfn);
-		if (flags & MF_COUNT_INCREASED)
-			put_page(page);
+		put_ref_page(ref_page);
 		return 0;
 	}