Re: [PATCH kvm-next V11 5/7] KVM: guest_memfd: Add slab-allocated inode cache

[Sean Christopherson <seanjc@google.com>]

On Thu, Sep 25, 2025, Sean Christopherson wrote:
> On Wed, Aug 27, 2025, Shivank Garg wrote:
> > Add dedicated inode structure (kvm_gmem_inode_info) and slab-allocated
> > inode cache for guest memory backing, similar to how shmem handles inodes.
> > 
> > This adds the necessary allocation/destruction functions and prepares
> > for upcoming guest_memfd NUMA policy support changes.
> > 
> > Signed-off-by: Shivank Garg <shivankg@amd.com>
> > ---
> >  virt/kvm/guest_memfd.c | 70 ++++++++++++++++++++++++++++++++++++++++--
> >  1 file changed, 68 insertions(+), 2 deletions(-)
> > 
> > diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
> > index 6c66a0974055..356947d36a47 100644
> > --- a/virt/kvm/guest_memfd.c
> > +++ b/virt/kvm/guest_memfd.c
> > @@ -17,6 +17,15 @@ struct kvm_gmem {
> >  	struct list_head entry;
> >  };
> >  
> > +struct kvm_gmem_inode_info {
> 
> What about naming this simply gmem_inode?

Heh, after looking through other filesystems, they're fairly even on appending
_info or not.  My vote is definitely for gmem_inode.

Before we accumulate more inode usage, e.g. for in-place conversion (which is
actually why I started looking at this code), I think we should also settle on
naming for gmem_file and gmem_inode variables.

As below, "struct kvm_gmem *gmem" gets quite confusing once inodes are in the
picture, especially since that structure isn't _the_ gmem instance, rather it's
a VM's view of that gmem instance.  And on the other side, "info" for the inode
is a bit imprecise, e.g. doesn't immediately make me think of inodes.

A few ideas:

 (a)
   struct gmem_inode *gmem;
   struct gmem_file *f;

 (b)
   struct gmem_inode *gi;
   struct gmem_file *f;

 (c)
   struct gmem_inode *gi;
   struct gmem_file *gf;

 (d)
   struct gmem_inode *gmem_i;
   struct gmem_file *gmem_f;


I think my would be for (a) or (b).  Option (c) seems like it would be hard to
visually differentiate between "gi" and "gf", and gmem_{i,f} are a bit verbose
IMO.

> > +	struct inode vfs_inode;
> > +};
> > +
> > +static inline struct kvm_gmem_inode_info *KVM_GMEM_I(struct inode *inode)
> 
> And then GMEM_I()?
> 
> And then (in a later follow-up if we target this for 6.18, or as a prep patch if
> we push this out to 6.19), rename kvm_gmem to gmem_file?
> 
> That would make guest_memfd look a bit more like other filesystems, and I don't
> see a need to preface the local structures and helpers with "kvm_", e.g. GMEM_I()
> is analogous to x86's to_vmx() and to_svm().
> 
> As for renaming kvm_gmem => gmem_file, I wandered back into this code via Ackerley's
> in-place conversion series, and it took me a good long while to remember the roles
> of files vs. inodes in gmem.  That's probably a sign that the code needs clarification
> given that I wrote the original code.  :-)
> 
> Leveraging an old discussion[*], my thought is to get to this:
> 
> /*
>  * A guest_memfd instance can be associated multiple VMs, each with its own
>  * "view" of the underlying physical memory.
>  *
>  * The gmem's inode is effectively the raw underlying physical storage, and is
>  * used to track properties of the physical memory, while each gmem file is
>  * effectively a single VM's view of that storage, and is used to track assets
>  * specific to its associated VM, e.g. memslots=>gmem bindings.
>  */
> struct gmem_file {
> 	struct kvm *kvm;
> 	struct xarray bindings;
> 	struct list_head entry;
> };
> 
> struct gmem_inode {
> 	struct shared_policy policy;
> 	struct inode vfs_inode;
> };
> 
> [*] https://lore.kernel.org/all/ZLGiEfJZTyl7M8mS@google.com