• Pavel Emelyanov's avatar
    aio: Make it possible to remap aio ring · e4a0d3e7
    Pavel Emelyanov authored
    There are actually two issues this patch addresses. Let me start with
    the one I tried to solve in the beginning.
    
    So, in the checkpoint-restore project (criu) we try to dump tasks'
    state and restore one back exactly as it was. One of the tasks' state
    bits is rings set up with io_setup() call. There's (almost) no problems
    in dumping them, there's a problem restoring them -- if I dump a task
    with aio ring originally mapped at address A, I want to restore one
    back at exactly the same address A. Unfortunately, the io_setup() does
    not allow for that -- it mmaps the ring at whatever place mm finds
    appropriate (it calls do_mmap_pgoff() with zero address and without
    the MAP_FIXED flag).
    
    To make restore possible I'm going to mremap() the freshly created ring
    into the address A (under which it was seen before dump). The problem is
    that the ring's virtual address is passed back to the user-space as the
    context ID and this ID is then used as search key by all the other io_foo()
    calls. Reworking this ID to be just some integer doesn't seem to work, as
    this value is already used by libaio as a pointer using which this library
    accesses memory for aio meta-data.
    
    So, to make restore work we need to make sure that
    
    a) ring is mapped at desired virtual address
    b) kioctx->user_id matches this value
    
    Having said that, the patch makes mremap() on aio region update the
    kioctx's user_id and mmap_base values.
    
    Here appears the 2nd issue I mentioned in the beginning of this mail.
    If (regardless of the C/R dances I do) someone creates an io context
    with io_setup(), then mremap()-s the ring and then destroys the context,
    the kill_ioctx() routine will call munmap() on wrong (old) address.
    This will result in a) aio ring remaining in memory and b) some other
    vma get unexpectedly unmapped.
    
    What do you think?
    Signed-off-by: default avatarPavel Emelyanov <xemul@parallels.com>
    Acked-by: default avatarDmitry Monakhov <dmonakhov@openvz.org>
    Signed-off-by: default avatarBenjamin LaHaise <bcrl@kvack.org>
    e4a0d3e7
mremap.c 15.2 KB