1 /*
2 * Copyright Amazon.com Inc. or its affiliates. All Rights Reserved.
3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
4 *
5 * This code is free software; you can redistribute it and/or modify it
6 * under the terms of the GNU General Public License version 2 only, as
7 * published by the Free Software Foundation.
8 *
9 * This code is distributed in the hope that it will be useful, but WITHOUT
10 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
11 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
12 * version 2 for more details (a copy is included in the LICENSE file that
13 * accompanied this code).
14 *
15 * You should have received a copy of the GNU General Public License version
16 * 2 along with this work; if not, write to the Free Software Foundation,
17 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
18 *
19 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
20 * or visit www.oracle.com if you need additional information or have any
21 * questions.
22 */
23
24 package gc.stress.ihash;
25
26 /*
27 * @test id=Shenandoah
28 * @bug 8387285
29 * @summary Full GC must not underflow the humongous compaction window when an
30 * identity-hashed, exact-region-multiple humongous object needs one more
31 * region after hash expansion.
32 * @requires vm.gc.Shenandoah
33 * @run main/othervm
34 * -XX:+UseCompactObjectHeaders -XX:+UseShenandoahGC
35 * -XX:+UnlockDiagnosticVMOptions -XX:+VerifyDuringGC -XX:+ShenandoahVerify
36 * -XX:-ExplicitGCInvokesConcurrent
37 * -Xms16m -Xmx16m
38 * -XX:+UnlockExperimentalVMOptions -XX:ShenandoahRegionSize=256K
39 * gc.stress.ihash.TestHumongousHashFullGC
40 */
41
42 /*
43 * @test id=Shenandoah-aggressive
44 * @bug 8387285
45 * @summary Full GC must not underflow the humongous compaction window when an
46 * identity-hashed, exact-region-multiple humongous object needs one more
47 * region after hash expansion.
48 * @requires vm.gc.Shenandoah
49 * @run main/othervm
50 * -XX:+UseCompactObjectHeaders -XX:+UseShenandoahGC
51 * -XX:+UnlockDiagnosticVMOptions -XX:+VerifyDuringGC -XX:+ShenandoahVerify
52 * -XX:-ExplicitGCInvokesConcurrent
53 * -XX:ShenandoahGCHeuristics=aggressive
54 * -Xms16m -Xmx16m
55 * -XX:+UnlockExperimentalVMOptions -XX:ShenandoahRegionSize=256K
56 * gc.stress.ihash.TestHumongousHashFullGC
57 */
58
59 /**
60 * Regression test for the Full-GC half of JDK-8387285.
61 *
62 * ShenandoahFullGC::calculate_target_humongous_objects() slides movable humongous
63 * objects toward the end of the heap within a window [to_begin, to_end). With
64 * compact object headers an identity-hashed humongous object grows by one word when
65 * relocated, so an object whose size is an exact multiple of the region size needs
66 * one extra region after expansion (k regions -> k+1).
67 *
68 * The bug: the fit test computed "start = to_end - num_regions" before checking it
69 * fits. When the expanded num_regions exceeds the remaining window, the unsigned
70 * subtraction underflows to ~SIZE_MAX, the guards pass, and the code dereferences a
71 * bogus region -> SIGSEGV during STW Full GC.
72 *
73 * Reproduction (verified to SIGSEGV on the unfixed VM, exit cleanly on the fix):
74 * a small heap with the minimum (256K) region size keeps the humongous objects at
75 * the bottom of the heap, so the backward compaction scan reaches a movable
76 * exact-2-region humongous start while the remaining window is smaller than its
77 * expanded (3-region) size. The arrays are identity-hashed so relocation must
78 * expand them, and -XX:-ExplicitGCInvokesConcurrent makes System.gc() a STW Full GC.
79 */
80 public class TestHumongousHashFullGC {
81
82 static final int REGION_SIZE = 256 * 1024;
83
84 // byte[] allocation size (with compact headers) = align_up(8 + length, 8).
85 // length = k * REGION_SIZE - 8 yields an allocation of exactly k regions.
86 static byte[] exactRegions(int k) {
87 return new byte[k * REGION_SIZE - 8];
88 }
89
90 static final int COUNT = 6;
91
92 static Object[] keep;
93
94 public static void main(String[] args) {
95 // Allocate exact-2-region humongous arrays at the bottom of the small heap
96 // and identity-hash each so a relocating GC must expand it by one word.
97 keep = new Object[COUNT];
98 int[] hashes = new int[COUNT];
99 for (int i = 0; i < COUNT; i++) {
100 byte[] o = exactRegions(2);
101 hashes[i] = System.identityHashCode(o);
102 keep[i] = o;
103 }
104
105 // Fragment: drop a couple of arrays low in the heap so the backward Full GC
106 // compaction scan reaches a movable exact-2-region humongous start while the
107 // remaining window is smaller than its expanded (3-region) size.
108 keep[1] = null;
109 keep[3] = null;
110
111 // STW Full GC compaction. With the bug this SIGSEGVs in
112 // calculate_target_humongous_objects(); with the fix it slides cleanly.
113 System.gc();
114
115 // Validate surviving objects are intact across expansion/relocation.
116 for (int i = 0; i < COUNT; i++) {
117 byte[] o = (byte[]) keep[i];
118 if (o == null) {
119 continue;
120 }
121 if (System.identityHashCode(o) != hashes[i]) {
122 throw new RuntimeException("hash mismatch at " + i);
123 }
124 }
125 System.out.println("OK");
126 }
127 }