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    SubjectRe: linux-next: Tree for Nov 7
    On Mon 13-11-17 22:34:50, Michael Ellerman wrote:
    > Hi Michal,
    >
    > Michal Hocko <mhocko@kernel.org> writes:
    > > On Mon 13-11-17 10:20:06, Michal Hocko wrote:
    > >> [Cc arm and ppc maintainers]
    > >
    > > Hmm, it turned out to be a problem on other architectures as well.
    > > CCing more maintainers. For your reference, we are talking about
    > > http://lkml.kernel.org/r/20171023082608.6167-1-mhocko@kernel.org
    > > which has broken architectures which do apply aligning on the mmap
    > > address hint without MAP_FIXED applied. See below my proposed way
    > > around this issue because I belive that the above patch is quite
    > > valuable on its own to be dropped for all archs.
    >
    > I don't really like your solution sorry :) The fact that you've had to
    > patch seven arches seems like a red flag.
    >
    > I think this is a generic problem with MAP_FIXED, which I've heard
    > userspace folks complain about in the past.

    The thing is that we canno change MAP_FIXED behavior as it is carved in
    stone

    > Currently MAP_FIXED does two things:
    > 1. makes addr not a hint but the required address
    > 2. blasts any existing mapping
    >
    > You want 1) but not 2).

    + fail if there is a clashing range

    > So the right solution IMHO would be to add a new mmap flag to request
    > that behaviour, ie. a fixed address but iff there is nothing already
    > mapped there.
    >
    > I don't know the mm code well enough to know if that's hard for some
    > reason, but it *seems* like it should be doable.

    Yes, I have mentioned that in the previous email but the amount of code
    would be even larger. Basically every arch which reimplements
    arch_get_unmapped_area would have to special case new MAP_FIXED flag to
    do vma lookup. So this was the most simple solution I could come up
    with. If there was a general interest for MAP_FIXED_SAFE then we can
    introduce it later of course. I would just like the hardening merged
    sooner rather than later.
    --
    Michal Hocko
    SUSE Labs

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