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

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Message-ID: <bafb0c9e-9ce6-4294-b1d6-e32c41635add@amazon.com>
Date: Wed, 26 Nov 2025 16:49:31 +0000
From: Nikita Kalyazin <kalyazin@...zon.com>
To: Mike Rapoport <rppt@...nel.org>, <linux-mm@...ck.org>
CC: Andrea Arcangeli <aarcange@...hat.com>, Andrew Morton
	<akpm@...ux-foundation.org>, Axel Rasmussen <axelrasmussen@...gle.com>,
	Baolin Wang <baolin.wang@...ux.alibaba.com>, David Hildenbrand
	<david@...hat.com>, Hugh Dickins <hughd@...gle.com>, James Houghton
	<jthoughton@...gle.com>, "Liam R. Howlett" <Liam.Howlett@...cle.com>,
	"Lorenzo Stoakes" <lorenzo.stoakes@...cle.com>, Michal Hocko
	<mhocko@...e.com>, "Paolo Bonzini" <pbonzini@...hat.com>, Peter Xu
	<peterx@...hat.com>, "Sean Christopherson" <seanjc@...gle.com>, Shuah Khan
	<shuah@...nel.org>, "Suren Baghdasaryan" <surenb@...gle.com>, Vlastimil Babka
	<vbabka@...e.cz>, <linux-kernel@...r.kernel.org>, <kvm@...r.kernel.org>,
	<linux-kselftest@...r.kernel.org>
Subject: Re: [PATCH v2 4/5] guest_memfd: add support for userfaultfd minor
 mode



On 25/11/2025 18:38, Mike Rapoport wrote:
> From: "Mike Rapoport (Microsoft)" <rppt@...nel.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@...nel.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
>