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

[Christian König <ckoenig.leichtzumerken@xxxxxxxxx>]

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.

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