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

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Message-ID: <02d1d52c-f534-f899-a18c-a3169123ac7c@amd.com>
Date:   Mon, 2 Jul 2018 14:24:29 +0200
From:   Christian König <christian.koenig@....com>
To:     Michal Hocko <mhocko@...nel.org>
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

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?

>>> 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.