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Date:   Wed, 10 Aug 2022 14:38:43 +0000
From:   Sean Christopherson <seanjc@google.com>
To:     David Hildenbrand <david@redhat.com>
Cc:     Chao Peng <chao.p.peng@linux.intel.com>, kvm@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
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        Paolo Bonzini <pbonzini@redhat.com>,
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        Jim Mattson <jmattson@google.com>,
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        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
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        Hugh Dickins <hughd@google.com>,
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        Andrew Morton <akpm@linux-foundation.org>,
        Shuah Khan <shuah@kernel.org>, Mike Rapoport <rppt@kernel.org>,
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        Muchun Song <songmuchun@bytedance.com>,
        Will Deacon <will@kernel.org>
Subject: Re: [PATCH v7 03/14] mm: Introduce memfile_notifier
Message-ID: <YvPC87FMgF7uac7z@google.com>
References: <20220706082016.2603916-1-chao.p.peng@linux.intel.com>
 <20220706082016.2603916-4-chao.p.peng@linux.intel.com>
 <13394075-fca0-6f2b-92a2-f1291fcec9a3@redhat.com>
 <20220810092232.GC862421@chaop.bj.intel.com>
 <00f1aa03-bc82-ffce-569b-e2d5c459992c@redhat.com>
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+Will

On Wed, Aug 10, 2022, David Hildenbrand wrote:
> On 10.08.22 11:22, Chao Peng wrote:
> > On Fri, Aug 05, 2022 at 03:22:58PM +0200, David Hildenbrand wrote:
> >> On 06.07.22 10:20, Chao Peng wrote:
> >>> This patch introduces memfile_notifier facility so existing memory file
> >>> subsystems (e.g. tmpfs/hugetlbfs) can provide memory pages to allow a
> >>> third kernel component to make use of memory bookmarked in the memory
> >>> file and gets notified when the pages in the memory file become
> >>> invalidated.
> >>
> >> Stupid question, but why is this called "memfile_notifier" and not
> >> "memfd_notifier". We're only dealing with memfd's after all ... which
> >> are anonymous files essentially. Or what am I missing? Are there any
> >> other plans for fs than plain memfd support that I am not aware of?
> > 
> > There were some discussions on this in v3.
> >   https://lkml.org/lkml/2021/12/28/484
> > Sean commented it's OK to abstract it from memfd but he also wants the
> > kAPI (name) should not bind to memfd to make room for future non-memfd
> > usages.
> 
> Sorry, but how is "memfile" any better? memfd abstracted to memfile?! :)

FWIW, I don't really like the memfile name either.

> I understand Sean's suggestion about abstracting, but if the new name
> makes it harder to grasp and there isn't really an alternative to memfd
> in sight, I'm not so sure I enjoy the tried abstraction here.

ARM's pKVM implementation is potentially (hopefully) going to switch to this API
(as a consumer) sooner than later.  If they anticipate being able to use memfd,
then there's unlikely to be a second backing type any time soon.

Quentin, Will?
 
> Otherwise we'd have to get creative now and discuss something like
> "file_population_notifer" or "mapping_population_notifer" and I am not
> sure that our time is well spent doing so right now.
> 
> ... as this is kernel-internal, we can always adjust the name as we
> please later, once we *actually* now what the abstraction should be.
> Until then I'd suggest to KIS and soft-glue this to memfd.
> 
> Or am I missing something important?

I don't think you're missing anything.  I'd still prefer a name that doesn't couple
KVM to memfd, but it's not a sticking point, and I've never been able to come up
with a better name...

With a little bit of cleverness I think we can keep the coupling in KVM to a
minimum, which is what I really care about.