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    SubjectRe: [PATCH v7 3/6] mm: Introduce VM_LOCKONFAULT
    On Fri 21-08-15 14:31:32, Eric B Munson wrote:
    [...]
    > I am in the middle of implementing lock on fault this way, but I cannot
    > see how we will hanlde mremap of a lock on fault region. Say we have
    > the following:
    >
    > addr = mmap(len, MAP_ANONYMOUS, ...);
    > mlock(addr, len, MLOCK_ONFAULT);
    > ...
    > mremap(addr, len, 2 * len, ...)
    >
    > There is no way for mremap to know that the area being remapped was lock
    > on fault so it will be locked and prefaulted by remap. How can we avoid
    > this without tracking per vma if it was locked with lock or lock on
    > fault?

    Yes mremap is a problem and it is very much similar to mmap(MAP_LOCKED).
    It doesn't guarantee the full mlock semantic because it leaves partially
    populated ranges behind without reporting any error.

    Considering the current behavior I do not thing it would be terrible
    thing to do what Konstantin was suggesting and populate only the full
    ranges in a best effort mode (it is done so anyway) and document the
    behavior properly.
    "
    If the memory segment specified by old_address and old_size is
    locked (using mlock(2) or similar), then this lock is maintained
    when the segment is resized and/or relocated. As a consequence,
    the amount of memory locked by the process may change.

    If the range is already fully populated and the range is
    enlarged the new range is attempted to be fully populated
    as well to preserve the full mlock semantic but there is no
    guarantee this will succeed. Partially populated (e.g. created by
    mlock(MLOCK_ONFAULT)) ranges do not have the full mlock semantic
    so they are not populated on resize.
    "

    So what we have as a result is that partially populated ranges are
    preserved and fully populated ones work in the best effort mode the same
    way as they are now.

    Does that sound at least remotely reasonably?


    --
    Michal Hocko
    SUSE Labs


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