.

wcfs/internal/wcfs_misc.cpp

@@ -21,9 +21,13 @@
 
 #include "wcfs_misc.h"
 
+#include <golang/libgolang.h>
+using namespace golang;
+
 #include <stdarg.h>
 #include <string.h>
 #include <unistd.h>
+#include <sys/mman.h>
 
 #include <memory>
 
@@ -86,10 +90,10 @@ namespace mm {
     // flags is MAP_*  from mmap(2); MAP_FIXED is added to flags.
     //
     // XXX split from File?
-    error map_into(void *addr, size_t size, int prot, int flags, const File &f, off_t offset) {
+    error map_into(void *addr, size_t size, int prot, int flags, const os::File &f, off_t offset) {
         void *addr2;
 
-        addr2 = mmap(addr, size, PROT_READ, MAP_FIXED | MAP_SHARED, f->fd(), offset);
+        addr2 = mmap(addr, size, prot, MAP_FIXED | flags, f.fd(), offset);
         if (addr2 == MAP_FAILED)
             return _pathError("mmap", f.name(), errno);
         if (addr2 != addr)

wcfs/internal/wcfs_misc.h

@@ -63,8 +63,8 @@ class File {
 public:
     friend tuple<File, error> open(const string &path, int flags, mode_t mode);
     // XXX empty ctor -> fd=-1, path=?
-    int fd();
-    string name();
+    int     fd()    const;
+    string  name()  const;
     error   close();
     error   stat(struct stat *st);
 

wcfs/internal/wcfs_virtmem.cpp

@@ -58,8 +58,10 @@ typedef uint64_t Oid;   // XXX ok?
 // TidHead is invalid Tid which is larged Tid value and means @head
 const Tid TidHead = -1ULL;
 
-string h(uint64_t v); // v -> 016x hex representation
+static string h(uint64_t v); // v -> 016x hex representation
 #define h_(v)   (h(v).c_str())
+static error mmap_zero_into_ro(void *addr, size_t size);
+
 
 
 // XXX ok?
@@ -275,11 +277,12 @@ error _Mapping::_remmapblk(int64_t blk, Tid at) {
     // block is beyond file size - mmap with zeros - else access to memory
     // after file.size will raise SIGBUS. (assumes head/f size ↑=)
     if ((blk+1)*f->blksize > st.st_size) {
-        mm.map_zero_into_ro(blkmem);    // XXX err
+        err = mmap_zero_into_ro(blkmem, 1*f->blksize);
+        // XXX err handle
     }
     // block is inside file - mmap file data
     else {
-        err = mm::map_into(blkmem, f->blksize, PROT_READ, MAP_SHARED, fsfile, blk*f->blksize);
+        err = mm::map_into(blkmem, 1*f->blksize, PROT_READ, MAP_SHARED, fsfile, blk*f->blksize);
         // XXX err handle
     }
 
@@ -306,6 +309,32 @@ tuple<os::File, error> WCFS::_open(const string &path, int flags) {
 
 // ---- misc ----
 
-string h(uint64_t v) {
+static string h(uint64_t v) {
     return fmt::sprintf("%016x", v);
 }
+
+// map_zero_ro mmaps read-only zeros into [addr +size) so that region as all zeros.
+// created mapping, even after it is accessed, does not consume memory.
+static error _mmap_zero_into_ro(void *addr, size_t size);
+static error mmap_zero_into_ro(void *addr, size_t size) {
+    error err = _mmap_zero_into_ro(addr, size);
+    if (err != nil)
+        err = fmt::errorf("mmap zero"); // XXX -> xerr::contextf?
+    return err;
+}
+static error _mmap_zero_into_ro(void *addr, size_t size) {
+    // mmap /dev/zero with MAP_NORESERVE and MAP_SHARED
+    // this way the mapping will be able to be read, but no memory will be allocated to keep it.
+    os::File z;
+    error err;
+    tie(z, err) = os::open("/dev/zero");
+    if (err != nil)
+        return err;
+    defer([&]() {
+        z.close();
+    });
+    err = mm::map_into(addr, size, PROT_READ, MAP_SHARED | MAP_NORESERVE, z, 0);
+    if (err != nil)
+        return err;
+    return nil;
+}