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    SubjectRe: [PATCH v7 3/6] mm: Introduce VM_LOCKONFAULT
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    On Mon, Aug 24, 2015 at 4:30 PM, Vlastimil Babka <vbabka@suse.cz> wrote:
    > On 08/24/2015 12:17 PM, Konstantin Khlebnikov 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?
    >>
    >>
    >> remap can count filled ptes and prefault only completely populated areas.
    >
    >
    > Does (and should) mremap really prefault non-present pages? Shouldn't it
    > just prepare the page tables and that's it?

    As I see mremap prefaults pages when it extends mlocked area.

    Also quote from manpage
    : 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.

    >
    >> There might be a problem after failed populate: remap will handle them
    >> as lock on fault. In this case we can fill ptes with swap-like non-present
    >> entries to remember that fact and count them as should-be-locked pages.
    >
    >
    > I don't think we should strive to have mremap try to fix the inherent
    > unreliability of mmap (MAP_POPULATE)?

    I don't think so. MAP_POPULATE works only when mmap happens.
    Flag MREMAP_POPULATE might be a good idea. Just for symmetry.


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