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@xxxxxxxxxx> 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
