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

[Chao Peng]

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.

> 
> > 
> > It will be used for KVM to use a file descriptor as the guest memory
> > backing store and KVM will use this memfile_notifier interface to
> > interact with memory file subsystems. In the future there might be other
> > consumers (e.g. VFIO with encrypted device memory).
> > 
> > It consists below components:
> >  - memfile_backing_store: Each supported memory file subsystem can be
> >    implemented as a memory backing store which bookmarks memory and
> >    provides callbacks for other kernel systems (memfile_notifier
> >    consumers) to interact with.
> >  - memfile_notifier: memfile_notifier consumers defines callbacks and
> >    associate them to a file using memfile_register_notifier().
> >  - memfile_node: A memfile_node is associated with the file (inode) from
> >    the backing store and includes feature flags and a list of registered
> >    memfile_notifier for notifying.
> > 
> > In KVM usages, userspace is in charge of guest memory lifecycle: it first
> > allocates pages in memory backing store and then passes the fd to KVM and
> > lets KVM register memory slot to memory backing store via
> > memfile_register_notifier.
> 
> Can we add documentation/description in any form how the different
> functions exposed in linux/memfile_notifier.h are supposed to be used?

Yeah, code comments can be added.

> 
> Staring at memfile_node_set_flags() and memfile_notifier_invalidate()
> it's not immediately clear to me who's supposed to call that and under
> which conditions.

I will also amend the commit message.

Chao
> 
> -- 
> Thanks,
> 
> David / dhildenb