pkg/buffer: Reorganize internal structure to allow dynamic sizes.

This change changes `buffer.data` into a `[]byte`, from `[bufferSize]byte`.
In exchange, each `buffer` is now grouped together to reduce the number of
allocation. Plus, `View` now holds an embeded list of `buffer` (via `pool`) to
support the happy path which the number of buffer is small. Expect no extra
allocation for the happy path.

It is to enable the use case for PacketBuffer, which
* each `View` is small (way less than `defaultBufferSize`), and
* needs to dynamically transfer ownership of `[]byte` to `View`.
  (to allow gradual migration)

PiperOrigin-RevId: 333197252

1 files changed, 83 insertions, 0 deletions

diff --git a/pkg/buffer/pool.go b/pkg/buffer/pool.go
new file mode 100644
index 000000000..7ad6132ab
--- /dev/null
+++ b/pkg/buffer/pool.go
@@ -0,0 +1,83 @@
+// Copyright 2020 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package buffer
+
+const (
+	// embeddedCount is the number of buffer structures embedded in the pool. It
+	// is also the number for overflow allocations.
+	embeddedCount = 8
+
+	// defaultBufferSize is the default size for each underlying storage buffer.
+	//
+	// It is slightly less than two pages. This is done intentionally to ensure
+	// that the buffer object aligns with runtime internals. This two page size
+	// will effectively minimize internal fragmentation, but still have a large
+	// enough chunk to limit excessive segmentation.
+	defaultBufferSize = 8144
+)
+
+// pool allocates buffer.
+//
+// It contains an embedded buffer storage for fast path when the number of
+// buffers needed is small.
+//
+// +stateify savable
+type pool struct {
+	bufferSize      int
+	avail           []buffer              `state:"nosave"`
+	embeddedStorage [embeddedCount]buffer `state:"wait"`
+}
+
+// get gets a new buffer from p.
+func (p *pool) get() *buffer {
+	if p.avail == nil {
+		p.avail = p.embeddedStorage[:]
+	}
+	if len(p.avail) == 0 {
+		p.avail = make([]buffer, embeddedCount)
+	}
+	if p.bufferSize <= 0 {
+		p.bufferSize = defaultBufferSize
+	}
+	buf := &p.avail[0]
+	buf.init(p.bufferSize)
+	p.avail = p.avail[1:]
+	return buf
+}
+
+// put releases buf.
+func (p *pool) put(buf *buffer) {
+	// Remove reference to the underlying storage, allowing it to be garbage
+	// collected.
+	buf.data = nil
+}
+
+// setBufferSize sets the size of underlying storage buffer for future
+// allocations. It can be called at any time.
+func (p *pool) setBufferSize(size int) {
+	p.bufferSize = size
+}
+
+// afterLoad is invoked by stateify.
+func (p *pool) afterLoad() {
+	// S/R does not save subslice into embeddedStorage correctly. Restore
+	// available portion of embeddedStorage manually. Restore as nil if none used.
+	for i := len(p.embeddedStorage); i > 0; i-- {
+		if p.embeddedStorage[i-1].data != nil {
+			p.avail = p.embeddedStorage[i:]
+			break
+		}
+	}
+}