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

[Christian König <christian.koenig@xxxxxxx>]

Am 02.07.2018 um 14:35 schrieb Michal Hocko:
> On Mon 02-07-18 14:24:29, Christian König wrote:
> > Am 02.07.2018 um 14:20 schrieb Michal Hocko:
> > > On Mon 02-07-18 14:13:42, Christian König wrote:
> > > > Am 02.07.2018 um 13:54 schrieb Michal Hocko:
> > > > > On Mon 02-07-18 11:14:58, Christian König wrote:
> > > > > > Am 27.06.2018 um 09:44 schrieb Michal Hocko:
> > > > > > > This is the v2 of RFC based on the feedback I've received so far. The
> > > > > > > code even compiles as a bonus ;) I haven't runtime tested it yet, mostly
> > > > > > > because I have no idea how.
> > > > > > >
> > > > > > > Any further feedback is highly appreciated of course.
> > > > > > That sounds like it should work and at least the amdgpu changes now look
> > > > > > good to me on first glance.
> > > > > >
> > > > > > Can you split that up further in the usual way? E.g. adding the blockable
> > > > > > flag in one patch and fixing all implementations of the MMU notifier in
> > > > > > follow up patches.
> > > > > But such a code would be broken, no? Ignoring the blockable state will
> > > > > simply lead to lockups until the fixup parts get applied.
> > > > Well to still be bisect-able you only need to get the interface change in
> > > > first with fixing the function signature of the implementations.
> > > That would only work if those functions return -AGAIN unconditionally.
> > > Otherwise they would pretend to not block while that would be obviously
> > > incorrect. This doesn't sound correct to me.
> > >
> > > > Then add all the new code to the implementations and last start to actually
> > > > use the new interface.
> > > >
> > > > That is a pattern we use regularly and I think it's good practice to do
> > > > this.
> > > But we do rely on the proper blockable handling.
> > Yeah, but you could add the handling only after you have all the
> > implementations in place. Don't you?
> Yeah, but then I would be adding a code with no user. And I really
> prefer to no do so because then the code is harder to argue about.
>
> > > > > Is the split up really worth it? I was thinking about that but had hard
> > > > > times to end up with something that would be bisectable. Well, except
> > > > > for returning -EBUSY until all notifiers are implemented. Which I found
> > > > > confusing.
> > > > It at least makes reviewing changes much easier, cause as driver maintainer
> > > > I can concentrate on the stuff only related to me.
> > > >
> > > > Additional to that when you cause some unrelated side effect in a driver we
> > > > can much easier pinpoint the actual change later on when the patch is
> > > > smaller.
> > > >
> > > > > > This way I'm pretty sure Felix and I can give an rb on the amdgpu/amdkfd
> > > > > > changes.
> > > > > If you are worried to give r-b only for those then this can be done even
> > > > > for larger patches. Just make your Reviewd-by more specific
> > > > > R-b: name # For BLA BLA
> > > > Yeah, possible alternative but more work for me when I review it :)
> > > I definitely do not want to add more work to reviewers and I completely
> > > see how massive "flag days" like these are not popular but I really
> > > didn't find a reasonable way around that would be both correct and
> > > wouldn't add much more churn on the way. So if you really insist then I
> > > would really appreciate a hint on the way to achive the same without any
> > > above downsides.
> > Well, I don't insist on this. It's just from my point of view that this
> > patch doesn't needs to be one patch, but could be split up.
> Well, if there are more people with the same concern I can try to do
> that. But if your only concern is to focus on your particular part then
> I guess it would be easier both for you and me to simply apply the patch
> and use git show $files_for_your_subystem on your end. I have put the
> patch to attempts/oom-vs-mmu-notifiers branch to my tree at
> git://git.kernel.org/pub/scm/linux/kernel/git/mhocko/mm.git
Not wanting to block something as important as this, so feel free to add 
an Acked-by: Christian König <christian.koenig@xxxxxxx> to the patch.
Let's rather face the next topic: Any idea how to runtime test this?

I mean I can rather easily provide a test which crashes an AMD GPU, 
which in turn then would mean that the MMU notifier would block forever 
without this patch.
But do you know a way to let the OOM killer kill a specific process?

Regards,
Christian.

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