[PATCH] mm: prep_zero_page() in irq is a bug

prep_zero_page() uses KM_USER0 and hence may not be used from IRQ context, at
least for highmem pages.

Cc: Nick Piggin <nickpiggin@yahoo.com.au>
Cc: Christoph Lameter <christoph@lameter.com>
Signed-off-by: NAndrew Morton <akpm@osdl.org>
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>

mm/page_alloc.c

@@ -217,6 +217,11 @@ static inline void prep_zero_page(struct page *page, int order, gfp_t gfp_flags)
 	int i;
 
 	BUG_ON((gfp_flags & (__GFP_WAIT | __GFP_HIGHMEM)) == __GFP_HIGHMEM);
+	/*
+	 * clear_highpage() will use KM_USER0, so it's a bug to use __GFP_ZERO
+	 * and __GFP_HIGHMEM from hard or soft interrupt context.
+	 */
+	BUG_ON((gfp_flags & __GFP_HIGHMEM) && in_interrupt());
 	for (i = 0; i < (1 << order); i++)
 		clear_highpage(page + i);
 }