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

[Christian König <ckoenig.leichtzumerken@xxxxxxxxx>]

Am 24.08.2018 um 15:24 schrieb Michal Hocko:
> On Fri 24-08-18 15:10:08, Christian König wrote:
> > 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....
> 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.
Christian.

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