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

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Date:   Mon, 2 Jul 2018 14:35:21 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Christian König <christian.koenig@....com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        "David (ChunMing) Zhou" <David1.Zhou@....com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Alex Deucher <alexander.deucher@....com>,
        David Airlie <airlied@...ux.ie>,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>,
        Doug Ledford <dledford@...hat.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Mike Marciniszyn <mike.marciniszyn@...el.com>,
        Dennis Dalessandro <dennis.dalessandro@...el.com>,
        Sudeep Dutt <sudeep.dutt@...el.com>,
        Ashutosh Dixit <ashutosh.dixit@...el.com>,
        Dimitri Sivanich <sivanich@....com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>,
        Jérôme Glisse <jglisse@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>, kvm@...r.kernel.org,
        amd-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
        intel-gfx@...ts.freedesktop.org, linux-rdma@...r.kernel.org,
        xen-devel@...ts.xenproject.org, linux-mm@...ck.org,
        David Rientjes <rientjes@...gle.com>,
        Felix Kuehling <felix.kuehling@....com>
Subject: Re: [RFC PATCH] mm, oom: distinguish blockable mode for mmu notifiers

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
-- 
Michal Hocko
SUSE Labs