Re: [PATCH v2 4/5] guest_memfd: add support for userfaultfd minor mode

Nikita Kalyazin
2025-11-26T16:49:56Z

On 25/11/2025 18:38, Mike Rapoport wrote:
> From: "Mike Rapoport (Microsoft)" <rppt@kernel.org>
>
> userfaultfd notifications about minor page faults used for live migration
> and snapshotting of VMs with memory backed by shared hugetlbfs or tmpfs
> mappings as described in detail in commit 7677f7fd8be7 ("userfaultfd: add
> minor fault registration mode").
>
> To use the same mechanism for VMs that use guest_memfd to map their memory,
> guest_memfd should support userfaultfd minor mode.
>
> Extend ->fault() method of guest_memfd with ability to notify core page
> fault handler that a page fault requires handle_userfault(VM_UFFD_MINOR) to
> complete and add implementation of ->get_shared_folio() to guest_memfd
> vm_ops.
>
> Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
> ---
> virt/kvm/guest_memfd.c | 28 ++++++++++++++++++++++++++++
> 1 file changed, 28 insertions(+)
>
> diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
> index ffadc5ee8e04..2a2b076293f9 100644
> --- a/virt/kvm/guest_memfd.c
> +++ b/virt/kvm/guest_memfd.c
> @@ -4,6 +4,7 @@
> #include <linux/kvm_host.h>
> #include <linux/pagemap.h>
> #include <linux/anon_inodes.h>
> +#include <linux/userfaultfd_k.h>
>
> #include "kvm_mm.h"
>
> @@ -369,6 +370,12 @@ static vm_fault_t kvm_gmem_fault_user_mapping(struct vm_fault *vmf)
> return vmf_error(err);
> }
>
> + if (userfaultfd_minor(vmf->vma)) {
> + folio_unlock(folio);
> + folio_put(folio);
> + return VM_FAULT_UFFD_MINOR;
> + }
> +
> if (WARN_ON_ONCE(folio_test_large(folio))) {
> ret = VM_FAULT_SIGBUS;
> goto out_folio;
> @@ -390,8 +397,29 @@ static vm_fault_t kvm_gmem_fault_user_mapping(struct vm_fault *vmf)
> return ret;
> }
>
> +#ifdef CONFIG_USERFAULTFD
> +static struct folio *kvm_gmem_get_folio(struct inode *inode, pgoff_t pgoff)

We have to name it differently, otherwise it clashes with the existing
one in this file.

> +{
> + struct folio *folio;
> +
> + folio = kvm_gmem_get_folio(inode, pgoff);

^^

> + if (IS_ERR_OR_NULL(folio))
> + return folio;
> +
> + if (!folio_test_uptodate(folio)) {
> + clear_highpage(folio_page(folio, 0));
> + kvm_gmem_mark_prepared(folio);
> + }
> +
> + return folio;
> +}
> +#endif
> +
> static const struct vm_operations_struct kvm_gmem_vm_ops = {
> .fault = kvm_gmem_fault_user_mapping,
> +#ifdef CONFIG_USERFAULTFD
> + .get_folio = kvm_gmem_get_folio,
> +#endif
> };
>
> static int kvm_gmem_mmap(struct file *file, struct vm_area_struct *vma)
> --
> 2.50.1
>