Re: [RFC PATCH 5/6] vhost, mm: make sure that oom_reaper doesn't reap memory read by vhost

[Michal Hocko <mhocko@kernel.org>]

On Thu 07-07-16 10:39:32, Michal Hocko wrote:
> On Sun 03-07-16 17:09:04, Michael S. Tsirkin wrote:
> [...]
> > Having said all that, how about we just add some kind of per-mm
> > notifier list, and let vhost know that owner is going away so
> > it should stop looking at memory?
> 
> But this would have to be a synchronous operation from the oom killer,
> no? I would really like to reduce the number of external dependencies
> from the oom killer paths as much as possible. This is the whole point
> of these patches. If we have a notification mechanism, what would
> guarantee that the oom killer would make a forward progress if the
> notified end cannot continue (wait for a lock etc...)?
> 
> I do realize that a test per each memory access is not welcome that
> much. An alternative would be to hook the check into the page fault
> handler because then the overhead would be reduced only to the slowpath
> (from the copy_from_user POV). But then also non use_mm users would have
> to pay the price which is even less attractive.
> 
> Another alternative would be disabling pagefaults when copying from the
> userspace. This would require that the memory is prefault when used
> which might be a problem for the current implementation.

ping Michael... I would like to pursue this again and have something for
4.9 ideally.
-- 
Michal Hocko
SUSE Labs

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