Re: [PATCH v9 01/41] KVM: guest_memfd: Introduce per-gmem attributes, use to guard user mappings

[Ackerley Tng]

Sean Christopherson <seanjc@xxxxxxxxxx> writes:

> On Fri, Jul 31, 2026, Xiaoyao Li wrote:
>> On 7/29/2026 8:35 AM, Ackerley Tng via B4 Relay wrote:
>> > From: Sean Christopherson<seanjc@xxxxxxxxxx>
>> >
>> > Start plumbing in guest_memfd support for in-place private<=>shared
>> > conversions by tracking attributes via a maple tree.  KVM currently tracks
>> > private vs. shared attributes on a per-VM basis, which made sense when a
>> > guest_memfd_only_ supported private memory, but tracking per-VM simply
>> > can't work for in-place conversions as the shared/private status of a given
>> > page needs to be per-gmem_inode, not per-VM.
>> >
>> > Use the filemap invalidation lock to protect the maple tree, as taking the
>> > lock for read when faulting in memory (for userspace or the guest) isn't
>> > expected to result in meaningful contention, and using a separate lock
>> > would add significant complexity (avoiding deadlock is quite difficult).
>> >
>> > Co-developed-by: Vishal Annapurve<vannapurve@xxxxxxxxxx>
>> > Signed-off-by: Vishal Annapurve<vannapurve@xxxxxxxxxx>
>> > Co-developed-by: Fuad Tabba<tabba@xxxxxxxxxx>
>> > Signed-off-by: Fuad Tabba<tabba@xxxxxxxxxx>
>> > Signed-off-by: Sean Christopherson<seanjc@xxxxxxxxxx>
>> > Tested-by: Shivank Garg<shivankg@xxxxxxx>
>> > Co-developed-by: Ackerley Tng<ackerleytng@xxxxxxxxxx>
>> > Signed-off-by: Ackerley Tng<ackerleytng@xxxxxxxxxx>
>>
>> Reviewed-by: Xiaoyao Li <xiaoyao.li@xxxxxxxxx>
>>
>> <...>
>>
>> >   static __always_inline struct gmem_inode *GMEM_I(struct inode *inode)
>> > @@ -60,9 +68,25 @@ static pgoff_t kvm_gmem_get_index(struct kvm_memory_slot *slot, gfn_t gfn)
>> >   	return gfn - slot->base_gfn + slot->gmem.pgoff;
>> >   }
>> > +static u64 kvm_gmem_interpret_entry(struct inode *inode, void *entry)
>>
>> Ummm, I'm not sure on the name. It doesn't look like KVM's style?
>>
>> Maybe, just kvm_gmem_get_attributes()?
>
> +1000, "interpret_entry" is extremely vague.

Was a little stuck with naming, my frame of mind was that "entry" is the
attributes, so I can't get attributes from attributes. After you pointed
this out, getting attributes out of an entry makes sense.

Will rename to kvm_gmem_get_attributes().