Re: [PATCH v4 03/10] KVM: guest_memfd: Allow host to map guest_memfd() pages

[David Hildenbrand <david@redhat.com>]

On 18.02.25 18:24, Fuad Tabba wrote:
> Add support for mmap() and fault() for guest_memfd backed memory
> in the host for VMs that support in-place conversion between
> shared and private. To that end, this patch adds the ability to
> check whether the VM type supports in-place conversion, and only
> allows mapping its memory if that's the case.
> 
> This behavior is also gated by the configuration option
> KVM_GMEM_SHARED_MEM.
> 
> Signed-off-by: Fuad Tabba <tabba@google.com>
> ---
>   include/linux/kvm_host.h |  11 +++++
>   virt/kvm/guest_memfd.c   | 103 +++++++++++++++++++++++++++++++++++++++
>   2 files changed, 114 insertions(+)
> 
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index 3ad0719bfc4f..f9e8b10a4b09 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -728,6 +728,17 @@ static inline bool kvm_arch_has_private_mem(struct kvm *kvm)
>   }
>   #endif
>   
> +/*
> + * Arch code must define kvm_arch_gmem_supports_shared_mem if support for
> + * private memory is enabled and it supports in-place shared/private conversion.
> + */
> +#if !defined(kvm_arch_gmem_supports_shared_mem) && !IS_ENABLED(CONFIG_KVM_PRIVATE_MEM)
> +static inline bool kvm_arch_gmem_supports_shared_mem(struct kvm *kvm)
> +{
> +	return false;
> +}
> +#endif
> +
>   #ifndef kvm_arch_has_readonly_mem
>   static inline bool kvm_arch_has_readonly_mem(struct kvm *kvm)
>   {
> diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
> index c6f6792bec2a..30b47ff0e6d2 100644
> --- a/virt/kvm/guest_memfd.c
> +++ b/virt/kvm/guest_memfd.c
> @@ -317,9 +317,112 @@ void kvm_gmem_handle_folio_put(struct folio *folio)
>   {
>   	WARN_ONCE(1, "A placeholder that shouldn't trigger. Work in progress.");
>   }
> +
> +static bool kvm_gmem_offset_is_shared(struct file *file, pgoff_t index)
> +{
> +	struct kvm_gmem *gmem = file->private_data;
> +
> +	/* For now, VMs that support shared memory share all their memory. */
> +	return kvm_arch_gmem_supports_shared_mem(gmem->kvm);
> +}
> +
> +static vm_fault_t kvm_gmem_fault(struct vm_fault *vmf)
> +{
> +	struct inode *inode = file_inode(vmf->vma->vm_file);
> +	struct folio *folio;
> +	vm_fault_t ret = VM_FAULT_LOCKED;
> +
> +	filemap_invalidate_lock_shared(inode->i_mapping);
> +
> +	folio = kvm_gmem_get_folio(inode, vmf->pgoff);
> +	if (IS_ERR(folio)) {
> +		switch (PTR_ERR(folio)) {
> +		case -EAGAIN:
> +			ret = VM_FAULT_RETRY;
> +			break;
> +		case -ENOMEM:
> +			ret = VM_FAULT_OOM;
> +			break;
> +		default:
> +			ret = VM_FAULT_SIGBUS;
> +			break;
> +		}
> +		goto out_filemap;
> +	}
> +
> +	if (folio_test_hwpoison(folio)) {
> +		ret = VM_FAULT_HWPOISON;
> +		goto out_folio;
> +	}
> +
> +	/* Must be called with folio lock held, i.e., after kvm_gmem_get_folio() */
> +	if (!kvm_gmem_offset_is_shared(vmf->vma->vm_file, vmf->pgoff)) {
> +		ret = VM_FAULT_SIGBUS;
> +		goto out_folio;
> +	}
> +
> +	/*
> +	 * Only private folios are marked as "guestmem" so far, and we never
> +	 * expect private folios at this point.
> +	 */
> +	if (WARN_ON_ONCE(folio_test_guestmem(folio)))  {
> +		ret = VM_FAULT_SIGBUS;
> +		goto out_folio;
> +	}
> +
> +	/* No support for huge pages. */
> +	if (WARN_ON_ONCE(folio_test_large(folio))) {
> +		ret = VM_FAULT_SIGBUS;
> +		goto out_folio;
> +	}
> +
> +	if (!folio_test_uptodate(folio)) {
> +		clear_highpage(folio_page(folio, 0));
> +		kvm_gmem_mark_prepared(folio);
> +	}

kvm_gmem_get_pfn()->__kvm_gmem_get_pfn() seems to call 
kvm_gmem_prepare_folio() instead.

Could we do the same here?

-- 
Cheers,

David / dhildenb