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

[Michal Hocko <mhocko@kernel.org>]

On Fri 24-08-18 15:44:03, Christian Konig wrote:
> Am 24.08.2018 um 15:40 schrieb Michal Hocko:
> > On Fri 24-08-18 15:28:33, Christian Konig wrote:
> > > Am 24.08.2018 um 15:24 schrieb Michal Hocko:
> > > > On Fri 24-08-18 15:10:08, Christian Konig wrote:
> > > > > Am 24.08.2018 um 15:01 schrieb Michal Hocko:
> > > > > > On Fri 24-08-18 14:52:26, Christian Konig 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....
> > > > Can we change it to non-sleepable lock then?
> > > No, the write side doesn't sleep any more, but the read side does.
> > > 
> > > See amdgpu_mn_invalidate_node() and that is where you actually need to
> > > handle the non-blocking flag correctly.
> > Ohh, right you are. We already handle that by bailing out before calling
> > amdgpu_mn_invalidate_node in !blockable mode.
> 
> Yeah, that is sufficient.
> 
> It could be improved because we have something like 90% chance that
> amdgpu_mn_invalidate_node() actually doesn't need to do anything.
> 
> But I can take care of that when the patch set has landed.
> 
> > So does this looks good to you?
> 
> Yeah, that looks perfect to me. Reviewed-by: Christian Konig
> <christian.koenig@amd.com>

Cool! Thanks for your guidance and patience with me. Here is the full
patch. Feel free to take it and route per your preference.