Re: [PATCH] mm, oom: distinguish blockable mode for mmu notifiers

From: Michal Hocko
Date: Fri Aug 24 2018 - 09:01:41 EST

On Fri 24-08-18 14:52:26, Christian König wrote:
> Am 24.08.2018 um 14:33 schrieb Michal Hocko:
> > Thiking about it some more, I can imagine that a notifier callback which
> > performs an allocation might trigger a memory reclaim and that in turn
> > might trigger a notifier to be invoked and recurse. But notifier
> > shouldn't really allocate memory. They are called from deep MM code
> > paths and this would be extremely deadlock prone. Maybe Jerome can come
> > up some more realistic scenario. If not then I would propose to simplify
> > the locking here. We have lockdep to catch self deadlocks and it is
> > always better to handle a specific issue rather than having a code
> > without a clear indication how it can recurse.
> Well I agree that we should probably fix that, but I have some concerns to
> remove the existing workaround.
> See we added that to get rid of a real problem in a customer environment and
> I don't want to that to show up again.

It would really help to know more about that case and fix it properly
rather than workaround it like this. Anyway, let me think how to handle
the non-blocking notifier invocation then. I was not able to come up
with anything remotely sane yet.
Michal Hocko