Re: [PATCH v5 6/6] KVM: guest_memfd: Drop superfluous WRITE_ONCE() when binding a memslot

[Yan Zhao]

On Mon, Sep 21, 2026 at 05:13:32PM -0700, Sean Christopherson wrote:
> Drop the superfluous WRITE_ONCE() when setting a memslot's guest_memfd file
> during initial binding, as the memslot *must* be inactive and unreachable.
> The superfluous WRITE_ONCE() was added by commit 67b43038ce14 ("KVM:
> guest_memfd: Remove RCU-protected attribute from slot->gmem.file") to
> maintain rough "parity" with the existing rcu_assign_pointer(), not
> realizing that the only reason rcu_assign_pointer() was used was to make
> sparse and other checkers happy.
> 
> Cc: Yan Zhao <yan.y.zhao@xxxxxxxxx>
> Reviewed-by: David Hildenbrand (Arm) <david@xxxxxxxxxx>
> Reviewed-by: Ackerley Tng <ackerleytng@xxxxxxxxxx>
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> ---
>  virt/kvm/guest_memfd.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
> index 80932f4ec4a3..826d26036926 100644
> --- a/virt/kvm/guest_memfd.c
> +++ b/virt/kvm/guest_memfd.c
> @@ -677,7 +677,7 @@ int kvm_gmem_prepare_memory_region(struct kvm *kvm, struct kvm_memory_slot *slot
>  	 * kvm_gmem_bind() must occur on a new memslot.  Because the memslot
>  	 * is not visible yet, kvm_gmem_get_pfn() is guaranteed to see the file.
>  	 */
> -	WRITE_ONCE(slot->gmem.file, file);
> +	slot->gmem.file = file;
>  	slot->gmem.pgoff = offset >> PAGE_SHIFT;
>  	if (kvm_gmem_supports_mmap(inode))
>  		slot->flags |= KVM_MEMSLOT_GMEM_ONLY;
> 
Thanks for the fix.
Reviewed-by: Yan Zhao <yan.y.zhao@xxxxxxxxx>

BTW: some questions regarding read/write to slot->gmem.file:

The WRITE_ONCE() in kvm_gmem_unbind() and kvm_gmem_release() are also invoked
when the memslot is inactive and unreachable -- are they also superfluous?

Do we need the READ_ONCE() in __kvm_gmem_get_pfn(), considering that other slot
fields (e.g., slot->gmem.pgoff) are read without READ_ONCE()?