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 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.
-- 
Michal Hocko
SUSE Labs

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