[release-branch.go1.22] runtime: update large object stats before freeSpan in sweep

Currently freeSpan is called before large object stats are updated when
sweeping large objects. This means heapStats.inHeap might get subtracted
before the large object is added to the largeFree field. The end result
is that the /memory/classes/heap/unused:bytes metric, which subtracts
live objects (alloc-free) from inHeap may overflow.

Fix this by always updating the large object stats before calling
freeSpan.

For #67019.
Fixes #67188.

Change-Id: Ib02bd8dcd1cf8cd1bc0110b6141e74f678c10445
Reviewed-on: https://go-review.googlesource.com/c/go/+/583380
Auto-Submit: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Felix Geisendörfer <felix.geisendoerfer@datadoghq.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
(cherry picked from commit 36d32f68f41561fb64677297e3733f5d5b866c2a)
Reviewed-on: https://go-review.googlesource.com/c/go/+/584339
Reviewed-by: Carlos Amedee <carlos@golang.org>

diff --git a/src/runtime/metrics_test.go b/src/runtime/metrics_test.go
index d7f41334cd6b625b0ac6a0b48bc15235bd4381c7..586610727563c2bb75b1fecda194f59081e88fa4 100644 (file)
--- a/src/runtime/metrics_test.go
+++ b/src/runtime/metrics_test.go
@@ -1290,3 +1290,38 @@ func (w *contentionWorker) run() {
        for w.fn() {
        }
 }
+
+func TestMetricHeapUnusedLargeObjectOverflow(t *testing.T) {
+       // This test makes sure /memory/classes/heap/unused:bytes
+       // doesn't overflow when allocating and deallocating large
+       // objects. It is a regression test for #67019.
+       done := make(chan struct{})
+       var wg sync.WaitGroup
+       wg.Add(1)
+       go func() {
+               defer wg.Done()
+               for {
+                       for i := 0; i < 10; i++ {
+                               runtime.Escape(make([]byte, 1<<20))
+                       }
+                       runtime.GC()
+                       select {
+                       case <-done:
+                               return
+                       default:
+                       }
+               }
+       }()
+       s := []metrics.Sample{
+               {Name: "/memory/classes/heap/unused:bytes"},
+       }
+       for i := 0; i < 1000; i++ {
+               metrics.Read(s)
+               if s[0].Value.Uint64() > 1<<40 {
+                       t.Errorf("overflow")
+                       break
+               }
+       }
+       done <- struct{}{}
+       wg.Wait()
+}

diff --git a/src/runtime/mgcsweep.go b/src/runtime/mgcsweep.go
index 3dbe9bcec72fb16b62e980cc5043f1b32426eb8d..35be7949472f8c45d0cd41f20a1e7ad6c9caa0fe 100644 (file)
--- a/src/runtime/mgcsweep.go
+++ b/src/runtime/mgcsweep.go
@@ -770,6 +770,19 @@ func (sl *sweepLocked) sweep(preserve bool) bool {
                if nfreed != 0 {
                        // Free large object span to heap.
 
+                       // Count the free in the consistent, external stats.
+                       //
+                       // Do this before freeSpan, which might update heapStats' inHeap
+                       // value. If it does so, then metrics that subtract object footprint
+                       // from inHeap might overflow. See #67019.
+                       stats := memstats.heapStats.acquire()
+                       atomic.Xadd64(&stats.largeFreeCount, 1)
+                       atomic.Xadd64(&stats.largeFree, int64(size))
+                       memstats.heapStats.release()
+
+                       // Count the free in the inconsistent, internal stats.
+                       gcController.totalFree.Add(int64(size))
+
                        // NOTE(rsc,dvyukov): The original implementation of efence
                        // in CL 22060046 used sysFree instead of sysFault, so that
                        // the operating system would eventually give the memory
@@ -802,16 +815,6 @@ func (sl *sweepLocked) sweep(preserve bool) bool {
                                // invalid pointer. See arena.go:(*mheap).allocUserArenaChunk.
                                *(*uintptr)(unsafe.Pointer(&s.largeType)) = 0
                        }
-
-                       // Count the free in the consistent, external stats.
-                       stats := memstats.heapStats.acquire()
-                       atomic.Xadd64(&stats.largeFreeCount, 1)
-                       atomic.Xadd64(&stats.largeFree, int64(size))
-                       memstats.heapStats.release()
-
-                       // Count the free in the inconsistent, internal stats.
-                       gcController.totalFree.Add(int64(size))
-
                        return true
                }