Re: [PATCH v7 03/14] mm: Introduce memfile_notifier

Sean Christopherson
2022-08-10T14:39:27Z

+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.