linux-kernel - Re: [PATCH v3 4/5] guest_memfd: add support for userfaultfd minor mode

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Message-ID: <c3ae6036-df95-4874-ad7a-6077a30ae726@kernel.org>
Date: Mon, 1 Dec 2025 10:12:38 +0100
From: "David Hildenbrand (Red Hat)" <david@...nel.org>
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>, 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>, 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 v3 4/5] guest_memfd: add support for userfaultfd minor
 mode

On 11/30/25 12:18, 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_folio_noalloc() to guest_memfd
> vm_ops.
> 
> Reviewed-by: Liam R. Howlett <Liam.Howlett@...cle.com>
> Signed-off-by: Mike Rapoport (Microsoft) <rppt@...nel.org>
> ---
>   virt/kvm/guest_memfd.c | 33 ++++++++++++++++++++++++++++++++-
>   1 file changed, 32 insertions(+), 1 deletion(-)
> 
> diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
> index ffadc5ee8e04..dca6e373937b 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"
>   
> @@ -359,7 +360,15 @@ static vm_fault_t kvm_gmem_fault_user_mapping(struct vm_fault *vmf)
>   	if (!((u64)inode->i_private & GUEST_MEMFD_FLAG_INIT_SHARED))
>   		return VM_FAULT_SIGBUS;
>   
> -	folio = kvm_gmem_get_folio(inode, vmf->pgoff);
> +	folio = filemap_lock_folio(inode->i_mapping, vmf->pgoff);
> +	if (!IS_ERR_OR_NULL(folio) && userfaultfd_minor(vmf->vma)) {

Can we ever get NULL here?

> +		ret = VM_FAULT_UFFD_MINOR;
> +		goto out_folio;
> +	}
> +
> +	if (PTR_ERR(folio) == -ENOENT)
> +		folio = kvm_gmem_get_folio(inode, vmf->pgoff);

Was briefly wondering what the performance impact of that two-step 
approach is (two lookups in case we have to create it IIUC)

Wouldn't it be better to limit it to the userfaultfd_minor(vmf->vma) case?

if (userfaultfd_minor(vmf->vma)) {
	folio = filemap_lock_folio(inode->i_mapping, vmf->pgoff);
	if (!IS_ERR(folio)) {
		ret = VM_FAULT_UFFD_MINOR;
		goto out_folio;
	}
} else {
	folio = kvm_gmem_get_folio(inode, vmf->pgoff);
}

if (IS_ERR(folio)) {
...

-- 
Cheers

David