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

From: Christian KÃnig
Date: Fri Aug 24 2018 - 09:10:16 EST

Am 24.08.2018 um 15:01 schrieb Michal Hocko:
On Fri 24-08-18 14:52:26, Christian KÃnig 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.

With avoiding allocating memory in the write lock path I don't see an issue any more with that.

All what the write lock path does now is adding items to a linked lists, arrays etc....

So there is no more blocking involved here and the read lock side should be able to grab the lock immediately.