Re: [PATCH RFC 15/17] KVM: guest_memfd: implement userfaultfd missing mode

[Nikita Kalyazin <kalyazin@amazon.com>]

On 27/01/2026 19:29, Mike Rapoport wrote:
> From: Nikita Kalyazin <kalyazin@amazon.com>
> 
> userfaultfd missing mode allows populating guest memory with the content
> supplied by userspace on demand.
> 
> Extend guest_memfd implementation of vm_uffd_ops to support MISSING
> mode.
> 
> Signed-off-by: Nikita Kalyazin <kalyazin@amazon.com>
> Co-developed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
> Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
> ---
>   virt/kvm/guest_memfd.c | 60 +++++++++++++++++++++++++++++++++++++++++-
>   1 file changed, 59 insertions(+), 1 deletion(-)
> 
> diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
> index 087e7632bf70..14cca057fc0e 100644
> --- a/virt/kvm/guest_memfd.c
> +++ b/virt/kvm/guest_memfd.c
> @@ -431,6 +431,14 @@ static vm_fault_t kvm_gmem_fault_user_mapping(struct vm_fault *vmf)
>                          ret = VM_FAULT_UFFD_MINOR;
>                          goto out_folio;
>                  }
> +
> +               /*
> +                * Check if userfaultfd is registered in missing mode. If so,
> +                * check if a folio exists in the page cache. If not, return
> +                * VM_FAULT_UFFD_MISSING to trigger the userfaultfd handler.
> +                */
> +               if (userfaultfd_missing(vmf->vma) && IS_ERR_OR_NULL(folio))
> +                       return VM_FAULT_UFFD_MISSING;
>          }
> 
>          /* folio not in the pagecache, try to allocate */
> @@ -507,9 +515,59 @@ static bool kvm_gmem_can_userfault(struct vm_area_struct *vma, vm_flags_t vm_fla
>          return true;
>   }
> 
> +static struct folio *kvm_gmem_folio_alloc(struct vm_area_struct *vma,
> +                                         unsigned long addr)
> +{
> +       struct inode *inode = file_inode(vma->vm_file);
> +       pgoff_t pgoff = linear_page_index(vma, addr);
> +       struct mempolicy *mpol;
> +       struct folio *folio;
> +       gfp_t gfp;
> +
> +       if (unlikely(pgoff >= (i_size_read(inode) >> PAGE_SHIFT)))
> +               return NULL;
> +
> +       gfp = mapping_gfp_mask(inode->i_mapping);
> +       mpol = mpol_shared_policy_lookup(&GMEM_I(inode)->policy, pgoff);
> +       mpol = mpol ?: get_task_policy(current);
> +       folio = folio_alloc_mpol(gfp, 0, mpol, pgoff, numa_node_id());

It looks like folio_alloc_mpol_noprof() and filemap_remove_folio() are 
not actually exported to modules.  Would it be ok to export them similar 
to how it was done in f634f10809ec ("mm/mempolicy: Export memory policy 
symbols")?

diff --git a/mm/filemap.c b/mm/filemap.c
index 3e4579e4b8bb..041c7719e524 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -261,6 +261,7 @@ void filemap_remove_folio(struct folio *folio)

         filemap_free_folio(mapping, folio);
  }
+EXPORT_SYMBOL_FOR_MODULES(filemap_remove_folio, "kvm");

  /*
   * page_cache_delete_batch - delete several folios from page cache
diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index 3d797d47a040..1dbbbb28a36e 100644
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -2442,6 +2442,7 @@ struct folio *folio_alloc_mpol_noprof(gfp_t gfp, 
unsigned int order,
         set_page_refcounted(page);
         return page_rmappable_folio(page);
  }
+EXPORT_SYMBOL_FOR_MODULES(folio_alloc_mpol_noprof, "kvm");

  /**
   * vma_alloc_folio - Allocate a folio for a VMA.

[1] 
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=f634f10809ec3d51d9529dfb0f99bc7cec1b5eff

> +       mpol_cond_put(mpol);
> +
> +       return folio;
> +}
> +
> +static int kvm_gmem_filemap_add(struct folio *folio,
> +                               struct vm_area_struct *vma,
> +                               unsigned long addr)
> +{
> +       struct inode *inode = file_inode(vma->vm_file);
> +       struct address_space *mapping = inode->i_mapping;
> +       pgoff_t pgoff = linear_page_index(vma, addr);
> +       int err;
> +
> +       __folio_set_locked(folio);
> +       err = filemap_add_folio(mapping, folio, pgoff, GFP_KERNEL);
> +       if (err) {
> +               folio_unlock(folio);
> +               return err;
> +       }
> +
> +       return 0;
> +}
> +
> +static void kvm_gmem_filemap_remove(struct folio *folio,
> +                                   struct vm_area_struct *vma)
> +{
> +       filemap_remove_folio(folio);
> +       folio_unlock(folio);
> +}
> +
>   static const struct vm_uffd_ops kvm_gmem_uffd_ops = {
> -       .can_userfault = kvm_gmem_can_userfault,
> +       .can_userfault     = kvm_gmem_can_userfault,
>          .get_folio_noalloc = kvm_gmem_get_folio_noalloc,
> +       .alloc_folio       = kvm_gmem_folio_alloc,
> +       .filemap_add       = kvm_gmem_filemap_add,
> +       .filemap_remove    = kvm_gmem_filemap_remove,
>   };
>   #endif /* CONFIG_USERFAULTFD */
> 
> --
> 2.51.0
>