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

[Nikita Kalyazin <kalyazin@amazon.com>]

On 01/12/2025 15:54, David Hildenbrand (Red Hat) wrote:
> 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? :)

I believe I found the precise point where we convinced ourselves that 
minor support was sufficient: [1].  If at this moment we don't find that 
reasoning valid anymore, then indeed implementing missing is the only 
option.

[1] https://lore.kernel.org/kvm/Z9GsIDVYWoV8d8-C@x1.local

> 
> -- 
> Cheers
> 
> David