mm: drop VM_BUG_ON from __get_free_pages

There is no real reason to blow up just because the caller doesn't know
that __get_free_pages cannot return highmem pages.  Simply fix that up
silently.  Even if we have some confused users such a fixup will not be
harmful.

[akpm@linux-foundation.org: mask off __GFP_HIGHMEM]
Link: http://lkml.kernel.org/r/20180622162841.25114-1-mhocko@kernel.org
Signed-off-by: Michal Hocko <mhocko@suse.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Jiankang Chen <chenjiankang1@huawei.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Yisheng Xie <xieyisheng1@huawei.com>
Cc: Hanjun Guo <guohanjun@huawei.com>
Cc: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/page_alloc.c

@@ -4404,19 +4404,15 @@ out:
 EXPORT_SYMBOL(__alloc_pages_nodemask);
 
 /*
- * Common helper functions.
+ * Common helper functions. Never use with __GFP_HIGHMEM because the returned
+ * address cannot represent highmem pages. Use alloc_pages and then kmap if
+ * you need to access high mem.
  */
 unsigned long __get_free_pages(gfp_t gfp_mask, unsigned int order)
 {
 	struct page *page;
 
-	/*
-	 * __get_free_pages() returns a virtual address, which cannot represent
-	 * a highmem page
-	 */
-	VM_BUG_ON((gfp_mask & __GFP_HIGHMEM) != 0);
-
-	page = alloc_pages(gfp_mask, order);
+	page = alloc_pages(gfp_mask & ~__GFP_HIGHMEM, order);
 	if (!page)
 		return 0;
 	return (unsigned long) page_address(page);