Re: [PATCH v7 00/14] KVM: mm: fd-based approach for supporting KVM guest private memory

From: Gupta, Pankaj
Date: Thu Jul 14 2022 - 01:08:27 EST

This is the v7 of this series which tries to implement the fd-based KVM
guest private memory. The patches are based on latest kvm/queue branch

     b9b71f43683a (kvm/queue) KVM: x86/mmu: Buffer nested MMU
split_desc_cache only by default capacity

In general this patch series introduce fd-based memslot which provides
guest memory through memory file descriptor fd[offset,size] instead of
hva/size. The fd can be created from a supported memory filesystem
like tmpfs/hugetlbfs etc. which we refer as memory backing store. KVM

Thinking a bit, As host side fd on tmpfs or shmem will store memory on host
page cache instead of mapping pages into userspace address space. Can we hit
double (un-coordinated) page cache problem with this when guest page cache
is also used?

This is my understanding: in host it will be indeed in page cache (in
current shmem implementation) but that's just the way it allocates and
provides the physical memory for the guest. In guest, guest OS will not
see this fd (absolutely), it only sees guest memory, on top of which it
can build its own page cache system for its own file-mapped content but
that is unrelated to host page cache.

yes. If guest fills its page cache with file backed memory, this at host
side(on shmem fd backend) will also fill the host page cache fast. This can
have an impact on performance of guest VM's if host goes to memory pressure
situation sooner. Or else we end up utilizing way less System RAM.

(Currently), the file backed guest private memory is long-term pinned
and not reclaimable, it's in page cache anyway once we allocated it for
guest. This does not depend on how guest use it (e.g. use it for guest
page cache or not).

Even if host shmem backed memory always be always un-reclaimable, we end up utilizing double RAM (both in guest & host page cache) for guest disk accesses?

Answering my own question:

We wont use double RAM, just view of guest & host structures would change as per the code path taken. If we we don't care about reclaim situations we should be good, else we have to think something to coordinate page cache between guest & host (that could be an optimization for later).