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    SubjectRe: [PATCH] mm: disable `vm.max_map_count' sysctl limit
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    On 11/27/2017 11:52 AM, Michal Hocko wrote:
    > On Mon 27-11-17 20:18:00, Mikael Pettersson wrote:
    >> On Mon, Nov 27, 2017 at 11:12 AM, Michal Hocko <mhocko@kernel.org> wrote:
    >>>> I've kept the kernel tunable to not break the API towards user-space,
    >>>> but it's a no-op now. Also the distinction between split_vma() and
    >>>> __split_vma() disappears, so they are merged.
    >>>
    >>> Could you be more explicit about _why_ we need to remove this tunable?
    >>> I am not saying I disagree, the removal simplifies the code but I do not
    >>> really see any justification here.
    >>
    >> In principle you don't "need" to, as those that know about it can bump it
    >> to some insanely high value and get on with life. Meanwhile those that don't
    >> (and I was one of them until fairly recently, and I'm no newcomer to Unix or
    >> Linux) get to scratch their heads and wonder why the kernel says ENOMEM
    >> when one has loads of free RAM.
    >
    > I agree that our error reporting is more than suboptimal in this regard.
    > These are all historical mistakes and we have much more of those. The
    > thing is that we have means to debug these issues (check
    > /proc/<pid>/maps e.g.).
    >
    >> But what _is_ the justification for having this arbitrary limit?
    >> There might have been historical reasons, but at least ELF core dumps
    >> are no longer a problem.
    >
    > Andi has already mentioned the the resource consumption. You can create
    > a lot of unreclaimable memory and there should be some cap. Whether our
    > default is good is questionable. Whether we can remove it altogether is
    > a different thing.
    >
    > As I've said I am not a great fan of the limit but "I've just notice it
    > breaks on me" doesn't sound like a very good justification. You still
    > have an option to increase it. Considering we do not have too many
    > reports suggests that this is not such a big deal for most users.
    >

    Let me add a belated report, then: we ran into this limit while implementing
    an early version of Unified Memory[1], back in 2013. The implementation
    at the time depended on tracking that assumed "one allocation == one vma".
    So, with only 64K vmas, we quickly ran out, and changed the design to work
    around that. (And later, the design was *completely* changed to use a separate
    tracking system altogether).

    The existing limit seems rather too low, at least from my perspective. Maybe
    it would be better, if expressed as a function of RAM size?


    [1] https://devblogs.nvidia.com/parallelforall/unified-memory-in-cuda-6/

    This is a way to automatically (via page faulting) migrate memory
    between CPUs and devices (GPUs, here). This is before HMM, of course.

    thanks,
    John Hubbard


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