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?

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?

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.

-- 
Thanks,

David / dhildenb