Re: [PATCH v12 11/45] KVM: guest_memfd: Invalidate both SHARED and PRIVATE mappings for in-place conversions

[Binbin Wu]

On 8/31/2026 8:25 AM, Ackerley Tng via B4 Relay wrote:
> From: Sean Christopherson <seanjc@xxxxxxxxxx>
> 
> When removing one or more folios from a guest_memfd instance, invalidate
> both SHARED and PRIVATE mappings if in-place conversion is enabled, because
> stating the obvious, KVM needs to ensure that all mappings to the folio(s)
> are dropped.
> 
> Opportunistically rename the helper to capture that it returns a filter for
> all gfns in anticipation of zapping only the previous mapping types on
> conversion.  I.e. when doing in-place conversion to PRIVATE, only SHARED
> mappings need to be zapped (ignoring that KVM would ideally not invalidate
> ranges whose attributes aren't changing in the first place).
> 
> Note, precisely zapping only the possible mapping types when in-place
> conversion is disabled is important for functional correctness, not just
> for performance.  Specifically, if KVM zaps both when SHARED vs. PRIVATE is
> tracked per-VM, then a PUNCH_HOLE operation on a PRIVATE guest_memfd will
> incorrectly zap SHARED mappings that have nothing to do with that gmem
> instance (because they're mapped via a VMA, not a gmem fd).
> 
> The incorrect over-zapping of SHARED memory that doesn't belong to the gmem
> fd requesting the zapping will be resolved in a later patch, where, if
> in-place conversion is enabled, KVM will use both shared and private memory
> from the guest_memfd. If both shared and private memory are from the
> guest_memfd, invalidation will only zap memory belonging to the given gmem
> instance.
> 
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> Signed-off-by: Ackerley Tng <ackerleytng@xxxxxxxxxx>

Reviewed-by: Binbin Wu <binbin.wu@xxxxxxxxxxxxxxx>