Re: [PATCH v3 4/6] KVM: guest_memfd: add support for userfaultfd minor

[Peter Xu]

On Fri, Apr 04, 2025 at 03:43:50PM +0000, Nikita Kalyazin wrote:
> Add support for sending a pagefault event if userfaultfd is registered.
> Only page minor event is currently supported.
> 
> Signed-off-by: Nikita Kalyazin <kalya...@amazon.com>
> ---
>  virt/kvm/guest_memfd.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
> index fbf89e643add..096d89e7282d 100644
> --- a/virt/kvm/guest_memfd.c
> +++ b/virt/kvm/guest_memfd.c
> @@ -4,6 +4,9 @@
>  #include <linux/kvm_host.h>
>  #include <linux/pagemap.h>
>  #include <linux/anon_inodes.h>
> +#ifdef CONFIG_KVM_PRIVATE_MEM
> +#include <linux/userfaultfd_k.h>
> +#endif /* CONFIG_KVM_PRIVATE_MEM */
>  
>  #include "kvm_mm.h"
>  
> @@ -380,6 +383,13 @@ static vm_fault_t kvm_gmem_fault(struct vm_fault *vmf)
>               kvm_gmem_mark_prepared(folio);
>       }
>  
> +     if (userfaultfd_minor(vmf->vma) &&
> +         !(vmf->flags & FAULT_FLAG_USERFAULT_CONTINUE)) {
> +             folio_unlock(folio);
> +             filemap_invalidate_unlock_shared(inode->i_mapping);
> +             return handle_userfault(vmf, VM_UFFD_MINOR);
> +     }
> +

Hmm, does guest-memfd (when with your current approach) at least needs to
define the new can_userfault() hook?

Meanwhile, we have some hard-coded lines so far, like:

mfill_atomic():
        if (!vma_is_shmem(dst_vma) &&
            uffd_flags_mode_is(flags, MFILL_ATOMIC_CONTINUE))
                goto out_unlock;

I thought it would fail guest-memfd already on a CONTINUE request, and it
doesn't seem to be touched yet in this series.

I'm not yet sure how the test worked out without hitting things like it.
Highly likely I missed something.  Some explanations would be welcomed.. 

Thanks,

>       vmf->page = folio_file_page(folio, vmf->pgoff);
>  
>  out_folio:
> -- 
> 2.47.1
> 

-- 
Peter Xu