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// Copyright 2018 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 pgalloc
import (
"bytes"
"fmt"
"io"
"runtime"
"sync/atomic"
"syscall"
"gvisor.googlesource.com/gvisor/pkg/log"
"gvisor.googlesource.com/gvisor/pkg/sentry/usage"
"gvisor.googlesource.com/gvisor/pkg/sentry/usermem"
"gvisor.googlesource.com/gvisor/pkg/state"
)
// SaveTo writes f's state to the given stream.
func (f *MemoryFile) SaveTo(w io.Writer) error {
// Wait for reclaim.
f.mu.Lock()
defer f.mu.Unlock()
for f.reclaimable {
f.reclaimCond.Signal()
f.mu.Unlock()
runtime.Gosched()
f.mu.Lock()
}
// Ensure that there are no pending evictions.
if len(f.evictable) != 0 {
panic(fmt.Sprintf("evictions still pending for %d users; call StartEvictions and WaitForEvictions before SaveTo", len(f.evictable)))
}
// Ensure that all pages that contain data have knownCommitted set, since
// we only store knownCommitted pages below.
zeroPage := make([]byte, usermem.PageSize)
err := f.updateUsageLocked(0, func(bs []byte, committed []byte) error {
for pgoff := 0; pgoff < len(bs); pgoff += usermem.PageSize {
i := pgoff / usermem.PageSize
pg := bs[pgoff : pgoff+usermem.PageSize]
if !bytes.Equal(pg, zeroPage) {
committed[i] = 1
continue
}
committed[i] = 0
// Reading the page caused it to be committed; decommit it to
// reduce memory usage.
//
// "MADV_REMOVE [...] Free up a given range of pages and its
// associated backing store. This is equivalent to punching a hole
// in the corresponding byte range of the backing store (see
// fallocate(2))." - madvise(2)
if err := syscall.Madvise(pg, syscall.MADV_REMOVE); err != nil {
// This doesn't impact the correctness of saved memory, it
// just means that we're incrementally more likely to OOM.
// Complain, but don't abort saving.
log.Warningf("Decommitting page %p while saving failed: %v", pg, err)
}
}
return nil
})
if err != nil {
return err
}
// Save metadata.
if err := state.Save(w, &f.fileSize, nil); err != nil {
return err
}
if err := state.Save(w, &f.usage, nil); err != nil {
return err
}
// Dump out committed pages.
for seg := f.usage.FirstSegment(); seg.Ok(); seg = seg.NextSegment() {
if !seg.Value().knownCommitted {
continue
}
// Write a header to distinguish from objects.
if err := state.WriteHeader(w, uint64(seg.Range().Length()), false); err != nil {
return err
}
// Write out data.
var ioErr error
err := f.forEachMappingSlice(seg.Range(), func(s []byte) {
if ioErr != nil {
return
}
_, ioErr = w.Write(s)
})
if ioErr != nil {
return ioErr
}
if err != nil {
return err
}
}
return nil
}
// LoadFrom loads MemoryFile state from the given stream.
func (f *MemoryFile) LoadFrom(r io.Reader) error {
// Load metadata.
if err := state.Load(r, &f.fileSize, nil); err != nil {
return err
}
if err := f.file.Truncate(f.fileSize); err != nil {
return err
}
newMappings := make([]uintptr, f.fileSize>>chunkShift)
f.mappings.Store(newMappings)
if err := state.Load(r, &f.usage, nil); err != nil {
return err
}
// Try to map committed chunks concurrently: For any given chunk, either
// this loop or the following one will mmap the chunk first and cache it in
// f.mappings for the other, but this loop is likely to run ahead of the
// other since it doesn't do any work between mmaps. The rest of this
// function doesn't mutate f.usage, so it's safe to iterate concurrently.
mapperDone := make(chan struct{})
mapperCanceled := int32(0)
go func() { // S/R-SAFE: see comment
defer func() { close(mapperDone) }()
for seg := f.usage.FirstSegment(); seg.Ok(); seg = seg.NextSegment() {
if atomic.LoadInt32(&mapperCanceled) != 0 {
return
}
if seg.Value().knownCommitted {
f.forEachMappingSlice(seg.Range(), func(s []byte) {})
}
}
}()
defer func() {
atomic.StoreInt32(&mapperCanceled, 1)
<-mapperDone
}()
// Load committed pages.
for seg := f.usage.FirstSegment(); seg.Ok(); seg = seg.NextSegment() {
if !seg.Value().knownCommitted {
continue
}
// Verify header.
length, object, err := state.ReadHeader(r)
if err != nil {
return err
}
if object {
// Not expected.
return fmt.Errorf("unexpected object")
}
if expected := uint64(seg.Range().Length()); length != expected {
// Size mismatch.
return fmt.Errorf("mismatched segment: expected %d, got %d", expected, length)
}
// Read data.
var ioErr error
err = f.forEachMappingSlice(seg.Range(), func(s []byte) {
if ioErr != nil {
return
}
_, ioErr = io.ReadFull(r, s)
})
if ioErr != nil {
return ioErr
}
if err != nil {
return err
}
// Update accounting for restored pages. We need to do this here since
// these segments are marked as "known committed", and will be skipped
// over on accounting scans.
usage.MemoryAccounting.Inc(seg.End()-seg.Start(), seg.Value().kind)
}
return nil
}
// MemoryFileProvider provides the MemoryFile method.
//
// This type exists to work around a save/restore defect. The only object in a
// saved object graph that S/R allows to be replaced at time of restore is the
// starting point of the restore, kernel.Kernel. However, the MemoryFile
// changes between save and restore as well, so objects that need persistent
// access to the MemoryFile must instead store a pointer to the Kernel and call
// Kernel.MemoryFile() as required. In most cases, depending on the kernel
// package directly would create a package dependency loop, so the stored
// pointer must instead be a MemoryProvider interface object. Correspondingly,
// kernel.Kernel is the only implementation of this interface.
type MemoryFileProvider interface {
// MemoryFile returns the Kernel MemoryFile.
MemoryFile() *MemoryFile
}
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