+ David's new email
On 04/03/2026 17:12, Nikita Kalyazin wrote:
On 27/01/2026 19:29, Mike Rapoport wrote:
From: Nikita Kalyazin <[email protected]>
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 <[email protected]>
Co-developed-by: Mike Rapoport (Microsoft) <[email protected]>
Signed-off-by: Mike Rapoport (Microsoft) <[email protected]>
---
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
