• Badari Pulavarty's avatar
    [PATCH] madvise(MADV_REMOVE): remove pages from tmpfs shm backing store · f6b3ec23
    Badari Pulavarty authored
    Here is the patch to implement madvise(MADV_REMOVE) - which frees up a
    given range of pages & its associated backing store.  Current
    implementation supports only shmfs/tmpfs and other filesystems return
    "Some app allocates large tmpfs files, then when some task quits and some
    client disconnect, some memory can be released.  However the only way to
    release tmpfs-swap is to MADV_REMOVE". - Andrea Arcangeli
    Databases want to use this feature to drop a section of their bufferpool
    (shared memory segments) - without writing back to disk/swap space.
    This feature is also useful for supporting hot-plug memory on UML.
    Concerns raised by Andrew Morton:
    - "We have no plan for holepunching!  If we _do_ have such a plan (or
      might in the future) then what would the API look like?  I think
      sys_holepunch(fd, start, len), so we should start out with that."
    - Using madvise is very weird, because people will ask "why do I need to
      mmap my file before I can stick a hole in it?"
    - None of the other madvise operations call into the filesystem in this
      manner.  A broad question is: is this capability an MM operation or a
      filesytem operation?  truncate, for example, is a filesystem operation
      which sometimes has MM side-effects.  madvise is an mm operation and with
      this patch, it gains FS side-effects, only they're really, really
      significant ones."
    - Andrea suggested the fs operation too but then it's more efficient to
      have it as a mm operation with fs side effects, because they don't
      immediatly know fd and physical offset of the range.  It's possible to
      fixup in userland and to use the fs operation but it's more expensive,
      the vmas are already in the kernel and we can use them.
    Short term plan &  Future Direction:
    - We seem to need this interface only for shmfs/tmpfs files in the short
      term.  We have to add hooks into the filesystem for correctness and
      completeness.  This is what this patch does.
    - In the future, plan is to support both fs and mmap apis also.  This
      also involves (other) filesystem specific functions to be implemented.
    - Current patch doesn't support VM_NONLINEAR - which can be addressed in
      the future.
    Signed-off-by: default avatarBadari Pulavarty <pbadari@us.ibm.com>
    Cc: Hugh Dickins <hugh@veritas.com>
    Cc: Andrea Arcangeli <andrea@suse.de>
    Cc: Michael Kerrisk <mtk-manpages@gmx.net>
    Cc: Ulrich Drepper <drepper@redhat.com>
    Signed-off-by: default avatarAndrew Morton <akpm@osdl.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@osdl.org>
mman.h 2.77 KB