Index: sys/vm/vm_page.c
===================================================================
--- sys/vm/vm_page.c
+++ sys/vm/vm_page.c
@@ -2972,7 +2972,7 @@
 	struct vm_domain *vmd;
 	vm_paddr_t curr_low;
 	vm_page_t m_run, m_runs[NRUNS];
-	u_long count, reclaimed;
+	u_long count, minalign, reclaimed;
 	int error, i, options, req_class;
 
 	KASSERT(npages > 0, ("npages is 0"));
@@ -2997,6 +2997,18 @@
 	    (count < npages && req_class == VM_ALLOC_INTERRUPT))
 		return (false);
 
+	/*
+	 * The caller will attempt an allocation after some runs have been
+	 * reclaimed and added to the vm_phys buddy lists.  To ensure that runs
+	 * are not fragmented, round up the requested length to the next power
+	 * or two or maximum chunk size, and ensure that each run is suitably
+	 * aligned.
+	 */
+	minalign = 1ul << imin(flsl(npages - 1), VM_NFREEORDER - 1);
+	npages = roundup2(npages, minalign);
+	if (alignment < ptoa(minalign))
+		alignment = ptoa(minalign);
+
 	/*
 	 * Scan up to three times, relaxing the restrictions ("options") on
 	 * the reclamation of reservations and superpages each time.