X Demo that it is possible to change mmapping while under pagefault to it

--- FAIL: TestCacheControl (0.05s)
    cachecontrol_test.go:154: original: file mmap: got "Debian GNU/"  ; want "hello world"

If there was no mmapFixed(fmmap, "/etc/issue") under DataNode.Read the
test would not fail (i.e. it will see the old - not mmapped page)

fuse/test/cachecontrol_test.go

@@ -8,7 +8,7 @@ package test
 
 import (
 	"os"
-	"io/ioutil"
+	//"io/ioutil"
 	"testing"
 
 	"golang.org/x/sys/unix"
@@ -38,7 +38,21 @@ func (d *DataNode) GetAttr(out *fuse.Attr, _ nodefs.File, _ *fuse.Context) fuse.
 	return fuse.OK
 }
 
+var fetc *os.File
+
+func init() {
+	var err error
+	fetc, err = os.Open("/etc/issue")
+	if err != nil {
+		panic(err)
+	}
+}
+
 func (d *DataNode) Read(_ nodefs.File, dest []byte, off int64, _ *fuse.Context) (fuse.ReadResult, fuse.Status) {
+	err := mmapFixed(fmmap, int(fetc.Fd()), unix.PROT_READ, unix.MAP_SHARED)
+	if err != nil {
+		panic(err)
+	}
 	l := int64(len(d.data))
 	end := off + l
 	if end > l {
@@ -48,6 +62,8 @@ func (d *DataNode) Read(_ nodefs.File, dest []byte, off int64, _ *fuse.Context)
 	return fuse.ReadResultData(d.data[off:end]), fuse.OK
 }
 
+var fmmap []byte
+
 // TestCacheControl verifies that FUSE server process can store/retrieve kernel data cache.
 func TestCacheControl(t *testing.T) {
 	dir := testutil.TempDir()
@@ -63,6 +79,7 @@ func TestCacheControl(t *testing.T) {
 	opts := nodefs.NewOptions()
 	opts.Debug = testutil.VerboseTest()
 	srv, fsconn, err := nodefs.MountRoot(dir, root, opts)
+	_ = fsconn
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -82,6 +99,7 @@ func TestCacheControl(t *testing.T) {
 		}
 	}()
 
+	/*
 	// assertFileRead asserts that the file content reads as dataOK.
 	assertFileRead := func(subj, dataOK string) {
 		t.Helper()
@@ -97,13 +115,14 @@ func TestCacheControl(t *testing.T) {
 
 	// make sure the file reads correctly
 	assertFileRead("original", data0)
+	*/
 
 	// pin file content into OS cache
 	f, err := os.Open(dir + "/hello.txt")
 	if err != nil {
 		t.Fatal(err)
 	}
-	fmmap, err := unix.Mmap(int(f.Fd()), 0, len(data0), unix.PROT_READ, unix.MAP_SHARED)
+	fmmap, err = unix.Mmap(int(f.Fd()), 0, len(data0), unix.PROT_READ, unix.MAP_SHARED)
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -117,10 +136,10 @@ func TestCacheControl(t *testing.T) {
 			t.Fatal(err)
 		}
 	}()
-	err = unix.Mlock(fmmap)
-	if err != nil {
-		t.Fatal(err)
-	}
+	//err = unix.Mlock(fmmap)
+	//if err != nil {
+	//	t.Fatal(err)
+	//}
 
 	// assertMmapRead asserts that file's mmaped memory reads as dataOK.
 	assertMmapRead := func(subj, dataOK string) {
@@ -133,6 +152,7 @@ func TestCacheControl(t *testing.T) {
 	// make sure the cache has original data
 	assertMmapRead("original", data0)
 
+	/*
 	// store changed data into OS cache
 	st := fsconn.FileNotifyStoreCache(file.Inode(), 7, []byte("123"))
 	if st != fuse.OK {
@@ -155,4 +175,5 @@ func TestCacheControl(t *testing.T) {
 	// make sure mmapped data and file read as original data
 	assertMmapRead("after invalcache", data0)
 	assertFileRead("after invalcache", data0)
+	*/
 }

fuse/test/test.go

@@ -5,3 +5,39 @@
 // Package test holds the tests for Go-FUSE and is not for end-user
 // consumption.
 package test
+
+import (
+	"reflect"
+	"unsafe"
+	"syscall"
+)
+
+/*
+#include <sys/mman.h>
+#include <errno.h>
+#include <stdio.h>
+#include <stdlib.h>
+
+int mmapFixed(int fd, void *addr, size_t length, int prot, int flags) {
+	void *ok;
+	ok = mmap(addr, length, prot, flags | MAP_FIXED, fd, 0);
+	if (ok == MAP_FAILED) {
+		return errno;
+	}
+	if (ok != addr) {
+		fprintf(stderr, "mmap(fixed): ret != addr\n");
+		abort();
+	}
+	return 0;
+}
+*/
+import "C"
+
+func mmapFixed(where []byte, fd int, prot int, flags int) error {
+	bp := (*reflect.SliceHeader)(unsafe.Pointer(&where))
+	errno := C.mmapFixed(C.int(fd), unsafe.Pointer(bp.Data), C.size_t(bp.Len), C.int(prot), C.int(flags))
+	if errno != 0 {
+		return syscall.Errno(errno)
+	}
+	return nil
+}