GNU Hurd/ glibc/ mmap

Welcome to... ... the GNU Hurd!

The mmap call is generally supported on GNU Hurd, as indicated by _POSIX_MAPPED_FILES (sysconf (_SC_MAPPED_FILES)).

Flags

Flags contain mapping type, sharing type and options.

  • Mapping type (must choose one and only one of these).

    • MAP_FILE (Mapped from a file or device.)

    • MAP_ANON/MAP_ANONYMOUS (Allocated from anonymous virtual memory.)

    Even though it is not defined to zero (it is for the Linux kernel; why not for us?), MAP_FILE is the default and can be omitted.

  • Sharing types (must choose one and only one of these).

    • MAP_SHARED (Share changes.)

    • MAP_PRIVATE (Changes private; copy pages on write.)

    • MAP_COPY (Virtual copy of region at mapping time.)

    For us, MAP_PRIVATE is the default (is defined to zero), for the Linux kernel, one of MAP_SHARED or MAP_PRIVATE has to be specified explicitly.

    The Linux kernel does not support MAP_COPY, and as per the comment in elf/dl-load.c, MAP_PRIVATE | MAP_DENYWRITE is Linux' replacement for MAP_COPY. However, MAP_DENYWRITE is defunct (mmap manpage).

    In contrast to MAP_COPY, for MAP_PRIVATE it is unspecified whether changes made to the file after the mmap call are visible in the mapped region (mmap manpage).

    MAP_COPY:

    What exactly is that?  `elf/dl-load.c` has some explanation.
<http://lkml.indiana.edu/hypermail/linux/kernel/0110.1/1506.html>


It is only handled in `dl-sysdep.c`, when `flags &
(MAP_COPY|MAP_PRIVATE)` is used for
<a href="../microkernel/mach/interface/vm_map.html">`vm map`</a>'s `copy` parameter, and
`mmap.c` uses `! (flags & MAP_SHARED)` instead, which seems
inconsistent?


Usage in glibc:


  * `catgets/open_catalog.c:__open_catalog`,
    `locale/loadlocale.c:_nl_load_locale`: *Linux seems to lack read-only
    copy-on-write.*

  • MAP_TYPE (Mask for type field./Mask for type of mapping.)

    In bits/mman.h this is described and defined to be a mask for the mapping type, in the bits/mman.h files corresponding to Linux kernel it is described an defined to be a mask for the sharing type.

  • Other flags.

    • MAP_FIXED (Map address must be exactly as requested.)

      If the memory region is already in use, an unmap is attempted before (re-)mapping it.

      The following text should be improved:

      [glibc]/llio.texi says:

      @var{address} gives a preferred starting address for the mapping.
@code{NULL} expresses no preference. Any previous mapping at that
address is automatically removed. [...]

      The comments in misc/sys/mman.h, misc/mmap.c, misc/mmap64.c, ports/sysdeps/unix/sysv/linux/hppa/mmap.c, and sysdeps/mach/hurd/mmap.c have a better wording:

      A successful `mmap' call
deallocates any previous mapping for the affected region.

      This is correct insofar that for MAP_FIXED indeed it is first unmapped if already in use, and for the regular cases, an address will be chosen that has no previous mapping.

    • MAP_NOEXTEND (For MAP_FILE, don't change file size.)

      Referenced in [hurd]/TODO as unimplemented.

    • MAP_HASSEMPHORE (Region may contain semaphores.)

    • MAP_INHERIT (Region is retained after exec.)

  • Linux-specific flags

    • MAP_GROWSDOWN (Stack-like segment.), MAP_GROWSUP (Register stack-like segment.)

      See mmap manpage.

    • MAP_DENYWRITE (ETXTBSY)

      As per the comment in elf/dl-load.c, MAP_PRIVATE | MAP_DENYWRITE is Linux' replacement for MAP_COPY. However, MAP_DENYWRITE is defunct (mmap manpage).

    • MAP_EXECUTABLE (Mark it as an executable.)

    • MAP_LOCKED (Lock the mapping.)

      ... à la mlock. Not implemented for us, but probably could?open issue glibc.

    • MAP_NORESERVE (Don't check for reservations.)

      See mmap manpage.

      From guidelines: Not POSIX, but we could implement it.

    • MAP_POPULATE (Populate (prefault) pagetables.)

      From the mmap manpage:

      Populate (prefault) page tables for a mapping.  For a file mapping,
this causes read-ahead on the file.  Later accesses to the mapping
will not be blocked by page faults.  MAP_POPULATE is only supported
for private mappings since Linux 2.6.23.

      Unknown Linux kernel version, mm/mmap.c:

          if (vm_flags & VM_LOCKED) {
            if (!mlock_vma_pages_range(vma, addr, addr + len))
                    mm->locked_vm += (len >> PAGE_SHIFT);
    } else if ((flags & MAP_POPULATE) && !(flags & MAP_NONBLOCK))
            make_pages_present(addr, addr + len);
    return addr;

      Is only advisory, so can worked around with #define MAP_POPULATE 0, 8069478040336a7de3461be275432493cc7e4c91.

    • MAP_NONBLOCK (Do not block on IO.)

      From the mmap manpage:

      Only meaningful in conjunction with MAP_POPULATE.  Don't perform
read-ahead: only create page tables entries for pages that are
already present in RAM.  Since Linux 2.6.23, this flag causes
MAP_POPULATE to do nothing.  One day the combination of
MAP_POPULATE and MAP_NONBLOCK may be reimplemented.

    • MAP_STACK (Allocation is for a stack.)

      See mmap manpage.

    • MAP_HUGETLB (Create huge page mapping.)

      See mmap manpage.

    • MAP_32BIT (Only give out 32-bit addresses.)

      See mmap manpage.

Implementation

Essentially, mmap is implemented by means of io map (not for MAP_ANON) followed by vm map.

There are two implementations: sysdeps/mach/hurd/mmap.c (main implementation) and sysdeps/mach/hurd/dl-sysdep.c (Minimal mmap implementation sufficient for initial loading of shared libraries.).

mmap ("/dev/zero")

Do we implement that (equivalently to MAP_ANON)?

Mapping Size

From the mmap manpage:

A file is mapped in multiples of the page size.  For a file that is not a
multiple of the page size, the remaining memory is zeroed when mapped, and
writes to that region are not written out to the file.  The effect of
changing the size of the underlying file of a mapping on the pages that
correspond to added or removed regions of the file is unspecified.

Do we implement that?

Use of a Mapped Region

From the mmap manpage:

Use of a mapped region can result in these signals:

SIGSEGV Attempted write into a region mapped as read-only.

SIGBUS  Attempted access to a portion of the buffer that does not
        correspond to the file (for example, beyond the end of the file,
        including the case where another process has truncated the file).

Do we implement that?

Usage in glibc itself

Review of mmap usage in generic bits of glibc (omitted: nptl/, sysdeps/unix/sparc/, sysdepts/unix/sysv/linux/), based on a1bcbd4035ac2483dc10da150d4db46f3e1744f8 (2012-03-11). MAP_FILE is the interesting case; MAP_ANON is generally fine. Some of the mmap usages in glibc have fallback code for the MAP_FAILED case, some do not.

catgets/open_catalog.c:    (struct catalog_obj *) __mmap (NULL, st.st_size, PROT_READ,
catgets/open_catalog.c-                                  MAP_FILE|MAP_COPY, fd, 0);

Has fallback for MAP_FAILED.

elf/cache.c:    = mmap (NULL, st.st_size, PROT_READ, MAP_PRIVATE, fd, 0);
elf/cache.c:    = mmap (NULL, aux_cache_size, PROT_READ, MAP_PRIVATE, fd, 0);

No fallback for MAP_FAILED.

elf/dl-load.c:        l->l_map_start = (ElfW(Addr)) __mmap ((void *) mappref, maplength,
elf/dl-load.c-                                              c->prot,
elf/dl-load.c-                                              MAP_COPY|MAP_FILE,
elf/dl-load.c-                                              fd, c->mapoff);
elf/dl-load.c:            && (__mmap ((void *) (l->l_addr + c->mapstart),
elf/dl-load.c-                        c->mapend - c->mapstart, c->prot,
elf/dl-load.c-                        MAP_FIXED|MAP_COPY|MAP_FILE,
elf/dl-load.c-                        fd, c->mapoff)

No fallback for MAP_FAILED.

elf/dl-misc.c:            result = __mmap (NULL, *sizep, prot,
elf/dl-misc.c-#ifdef MAP_COPY
elf/dl-misc.c-                             MAP_COPY
elf/dl-misc.c-#else
elf/dl-misc.c-                             MAP_PRIVATE
elf/dl-misc.c-#endif
elf/dl-misc.c-#ifdef MAP_FILE
elf/dl-misc.c-                             | MAP_FILE
elf/dl-misc.c-#endif
elf/dl-misc.c-                             , fd, 0);

No fallback for MAP_FAILED.

elf/dl-profile.c:  addr = (struct gmon_hdr *) __mmap (NULL, expected_size, PROT_READ|PROT_WRITE,
elf/dl-profile.c-                                  MAP_SHARED|MAP_FILE, fd, 0);

No fallback for MAP_FAILED.

elf/readlib.c:  file_contents = mmap (0, statbuf.st_size, PROT_READ, MAP_SHARED,
elf/readlib.c-                        fileno (file), 0);

No fallback for MAP_FAILED.

elf/sprof.c:      result->symbol_map = mmap (NULL, max_offset - min_offset,
elf/sprof.c-                           PROT_READ, MAP_SHARED|MAP_FILE, symfd,
elf/sprof.c-                           min_offset);
elf/sprof.c:  addr = mmap (NULL, st.st_size, PROT_READ, MAP_SHARED|MAP_FILE, fd, 0);

No fallback for MAP_FAILED.

iconv/gconv_cache.c:  gconv_cache = __mmap (NULL, cache_size, PROT_READ, MAP_SHARED, fd, 0);
iconv/iconv_charmap.c:            && ((addr = mmap (NULL, st.st_size, PROT_READ, MAP_PRIVATE,
iconv/iconv_charmap.c-                              fd, 0)) != MAP_FAILED))
iconv/iconv_prog.c:           && ((addr = mmap (NULL, st.st_size, PROT_READ, MAP_PRIVATE,
iconv/iconv_prog.c-                             fd, 0)) != MAP_FAILED))

Have fallback for MAP_FAILED.

intl/loadmsgcat.c:  data = (struct mo_file_header *) mmap (NULL, size, PROT_READ,
intl/loadmsgcat.c-                                     MAP_PRIVATE, fd, 0);

Has fallback for MAP_FAILED.

libio/fileops.c:        p = __mmap (NULL, st.st_size, PROT_READ, MAP_SHARED,
libio/fileops.c-                    fp->_fileno, 0);
libio/fileops.c:      p = __mmap (NULL, st.st_size, PROT_READ, MAP_SHARED, fp->_fileno, 0);

Has fallback for MAP_FAILED.

locale/loadarchive.c:      result = __mmap64 (NULL, mapsize, PROT_READ, MAP_FILE|MAP_COPY, fd, 0);
locale/loadarchive.c:   result = __mmap64 (NULL, mapsize, PROT_READ, MAP_FILE|MAP_COPY,
locale/loadarchive.c-                      fd, 0);
locale/loadarchive.c:   addr = __mmap64 (NULL, to - from, PROT_READ, MAP_FILE|MAP_COPY,
locale/loadarchive.c-                    fd, from);

Some have fallback for MAP_FAILED.

locale/programs/locale.c:               void *mapped = mmap64 (NULL, st.st_size, PROT_READ,
locale/programs/locale.c-                                      MAP_SHARED, fd, 0);
locale/programs/locale.c:                   && ((mapped = mmap64 (NULL, st.st_size, PROT_READ,
locale/programs/locale.c-                                         MAP_SHARED, fd, 0))
locale/programs/locale.c:  addr = mmap64 (NULL, len, PROT_READ, MAP_SHARED, fd, 0);
locale/programs/locarchive.c:      void *p = mmap64 (NULL, RESERVE_MMAP_SIZE, PROT_NONE, MAP_SHARED, fd, 0);
locale/programs/locarchive.c:  p = mmap64 (p, total, PROT_READ | PROT_WRITE, MAP_SHARED | xflags, fd, 0);
locale/programs/locarchive.c:  void *p = mmap64 (ah->addr + start, st.st_size - start,
locale/programs/locarchive.c-             PROT_READ | PROT_WRITE, MAP_SHARED | MAP_FIXED,
locale/programs/locarchive.c-             ah->fd, start);
locale/programs/locarchive.c:    ah->addr = mmap64 (ah->addr, st.st_size, PROT_READ | PROT_WRITE,
locale/programs/locarchive.c-                MAP_SHARED | MAP_FIXED, ah->fd, 0);
locale/programs/locarchive.c:      ah->addr = mmap64 (NULL, st.st_size, PROT_READ | PROT_WRITE,
locale/programs/locarchive.c-                  MAP_SHARED, ah->fd, 0);
locale/programs/locarchive.c:  p = mmap64 (p, total, PROT_READ | PROT_WRITE, MAP_SHARED | xflags, fd, 0);
locale/programs/locarchive.c:  ah->addr = mmap64 (p, st.st_size, PROT_READ | (readonly ? 0 : PROT_WRITE),
locale/programs/locarchive.c-              MAP_SHARED | xflags, fd, 0);
locale/programs/locarchive.c:     data[cnt].addr = mmap64 (NULL, st.st_size, PROT_READ, MAP_SHARED,
locale/programs/locarchive.c-                              fd, 0);

No fallback for MAP_FAILED.

nscd/connections.c:           else if ((mem = mmap (NULL, dbs[cnt].max_db_size,
nscd/connections.c-                                 PROT_READ | PROT_WRITE,
nscd/connections.c-                                 MAP_SHARED, fd, 0))
nscd/connections.c:               || (mem = mmap (NULL, dbs[cnt].max_db_size,
nscd/connections.c-                               PROT_READ | PROT_WRITE,
nscd/connections.c-                               MAP_SHARED, fd, 0)) == MAP_FAILED)
nscd/nscd_helper.c:  void *mapping = __mmap (NULL, mapsize, PROT_READ, MAP_SHARED, mapfd, 0);

No fallback for MAP_FAILED.

nss/makedb.c:  const struct nss_db_header *header = mmap (NULL, st.st_size, PROT_READ,
nss/makedb.c-                                      MAP_PRIVATE|MAP_POPULATE, fd, 0);
nss/nss_db/db-open.c:   mapping->header = mmap (NULL, header.allocate, PROT_READ,
nss/nss_db/db-open.c-                           MAP_PRIVATE, fd, 0);

No fallback for MAP_FAILED.

posix/tst-mmap.c:  ptr = mmap (NULL, 1000, PROT_READ, MAP_SHARED, fd, ps);
posix/tst-mmap.c:  ptr = mmap64 (NULL, 1000, PROT_READ, MAP_SHARED, fd, ps);
rt/tst-mqueue3.c:  void *mem = mmap (NULL, ps, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
rt/tst-mqueue5.c:  void *mem = mmap (NULL, ps, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
rt/tst-shm.c:  mem = mmap (NULL, 4000, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
stdio-common/tst-fmemopen.c:  if ((mmap_data = (char *) mmap (NULL, fs.st_size, PROT_READ,
stdio-common/tst-fmemopen.c-                            MAP_SHARED, fd, 0)) == MAP_FAILED)

No fallback for MAP_FAILED.

io_map Failure

This is the libnetfs: io map issue.

tschwinge's current plan is to make the following cases do the same (if that is possible); probably by introducing a generic mmap_or_read function, that first tries mmap (and that will succeed on Linux-based systems and also on Hurd-based, if it's backed by libdiskfs), and if that fails tries mmap on anonymous memory and then fills it by reading the required data. This is also what the ?exec server is doing (and is the reason that the ./true invocation on libnetfs: io map works, to my understanding): see exec.c:prepare, if io_map fails, e->filemap == MACH_PORT_NULL; then exec.c:map (as invoked from exec.c:load_section, exec.c:check_elf, exec.c:do_exec, or hashexec.c:check_hashbang) will use io_read instead.

Doing so potentially means reading in a lot of unused data -- but we probably can't do any better?

In parallel (or even alternatively?), it should be researched how Linux (or any other kernel) implements mmap on NFS and similar file systems, and then implement the same in libnetfs and/or nfs, etc.

Here, also probably the whole mapping region has to be read (bug-hurd list archive) at mmap time. Then, only MAP_PRIVATE (or rather: MAP_COPY) is possible, but not MAP_SHARED.