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

["David Hildenbrand (Red Hat)" <david@kernel.org>]

On 12/1/25 14:39, Nikita Kalyazin wrote:
> 
> 
> On 30/11/2025 11:18, 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_folio_noalloc() to guest_memfd
>> vm_ops.
>>
>> Reviewed-by: Liam R. Howlett <Liam.Howlett@oracle.com>
>> Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.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)) {
>> +               ret = VM_FAULT_UFFD_MINOR;
>> +               goto out_folio;
>> +       }
> 
> I realised that I might have been wrong in [1] saying that the noalloc
> get folio was ok for our use case.  Unfortunately we rely on a minor
> fault to get generated even when the page is being allocated.  Peter and
> I discussed it originally in [2].  Since we want to populate guest
> memory with the content supplied by userspace on demand, we have to be
> able to intercept the very first access, meaning we either need a minor
> or major UFFD event for that.  We decided to make use of the minor at
> the time.  If we have to preserve the shmem semantics, it forces us to
> implement support for major/UFFDIO_COPY.

If we want missing semantics then likely we should be adding ... missing 
support? :)

-- 
Cheers

David