Re: [RFC PATCH v1 11/37] KVM: guest_memfd: Add support for KVM_SET_MEMORY_ATTRIBUTES

On Tue, Nov 4, 2025 at 1:27 AM Yan Zhao [off-list ref] wrote:

On Fri, Oct 17, 2025 at 01:11:52PM -0700, Ackerley Tng wrote:

quoted

For shared to private conversions, if refcounts on any of the folios
within the range are elevated, fail the conversion with -EAGAIN.

At the point of shared to private conversion, all folios in range are
also unmapped. The filemap_invalidate_lock() is held, so no faulting
can occur. Hence, from that point on, only transient refcounts can be
taken on the folios associated with that guest_memfd.

Hence, it is safe to do the conversion from shared to private.

After conversion is complete, refcounts may become elevated, but that
is fine since users of transient refcounts don't actually access
memory.

For private to shared conversions, there are no refcount checks. any
transient refcounts are expected to drop their refcounts soon. The
conversion process will spin waiting for these transient refcounts to
go away.

Where's the code to spin?

When dealing with 4k pages, I think we don't need to spin waiting for
transient refcounts to drop, that logic will be needed when dealing
with huge folios in order to restructure them while handling
conversion. So the specific part can be safely dropped from the commit
message.