Re: [PATCH 0/10 -v4] Handle oom bypass more gracefully

On Tue 14-06-16 22:17:40, Oleg Nesterov wrote:

On 06/13, Michal Hocko wrote:

quoted

On Mon 13-06-16 13:23:48, Michal Hocko wrote:

quoted

On Thu 09-06-16 13:52:07, Michal Hocko wrote:

quoted

I would like to explore ways how to remove kthreads (use_mm) special
case. It shouldn't be that hard, we just have to teach the page fault
handler to recognize oom victim mm and enforce EFAULT for kthreads
which have borrowed that mm.

So I was trying to come up with solution for this which would require to
hook into the pagefault an enforce EFAULT when the mm is being reaped
by the oom_repaer. Not hard but then I have checked the current users
and none of them is really needing to read from the userspace (aka
copy_from_user/get_user). So we actually do not need to do anything
special.

As pointed out by Tetsuo [1] vhost does realy on copy_from_user.

Tetsuo, Michal, but do we really care?

I have no idea what vhost does, but obviously this should not lead to kernel
crash or something like this, otherwise it should be fixed. If we are going
to kill the owner of dev->mm anyway, why should we worry about vhost_worker()
which can fail to access this ->mm after that?

This needs a deeper investigation. It relies on some state flags copied
from the userspace. I suspect it might misbehave but let's leave this
alone for a while. It is more complicated than I expected.

-- 
Michal Hocko
SUSE Labs

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