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

[Christian KÃnig]

Am 02.07.2018 um 14:20 schrieb Michal Hocko:
> On Mon 02-07-18 14:13:42, Christian KÃnig wrote:
> > Am 02.07.2018 um 13:54 schrieb Michal Hocko:
> > > On Mon 02-07-18 11:14:58, Christian KÃnig 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?

> > > 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.

Could be that I just don't know the code or the consequences of adding 
that well enough to really judge.

Christian.