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

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Message-ID: <4pjmnp7rvgftqg2l3icju2sduqfwun53ugbkw2pabbegpshvu4@uto2h4so2rgs>
Date: Wed, 26 Nov 2025 10:22:16 -0500
From: "Liam R. Howlett" <Liam.Howlett@...cle.com>
To: Mike Rapoport <rppt@...nel.org>
Cc: linux-mm@...ck.org, 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>,
        Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
        Michal Hocko <mhocko@...e.com>, Nikita Kalyazin <kalyazin@...zon.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

* Mike Rapoport <rppt@...nel.org> [251125 13:39]:
> 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>

Reviewed-by: Liam R. Howlett <Liam.Howlett@...cle.com>

> ---
>  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)
> +{
> +	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
>