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

Christian König
2018-08-24T13:10:08Z

Am 24.08.2018 um 15:01 schrieb Michal Hocko:
> 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.

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.

Christian.