[PATCH v2 13/15] KVM: guest_memfd: implement userfaultfd operations

[Mike Rapoport <rppt@kernel.org>]

From: Nikita Kalyazin <kalyazin@amazon.com>

userfaultfd notifications about page faults used for live migration
and snapshotting of VMs.

MISSING mode allows post-copy live migration and MINOR mode allows
optimization for post-copy live migration for VMs backed with shared
hugetlbfs or tmpfs mappings as described in detail in commit
7677f7fd8be7 ("userfaultfd: add minor fault registration mode").

To use the same mechanisms for VMs that use guest_memfd to map their
memory, guest_memfd should support userfaultfd operations.

Add implementation of vm_uffd_ops to guest_memfd.

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>
---
 mm/filemap.c           |  1 +
 virt/kvm/guest_memfd.c | 84 +++++++++++++++++++++++++++++++++++++++++-
 2 files changed, 83 insertions(+), 2 deletions(-)

diff --git a/mm/filemap.c b/mm/filemap.c
index 6cd7974d4ada..19dfcebcd23f 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -262,6 +262,7 @@ 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/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
index 017d84a7adf3..46582feeed75 100644
--- a/virt/kvm/guest_memfd.c
+++ b/virt/kvm/guest_memfd.c
@@ -7,6 +7,7 @@
 #include <linux/mempolicy.h>
 #include <linux/pseudo_fs.h>
 #include <linux/pagemap.h>
+#include <linux/userfaultfd_k.h>
 
 #include "kvm_mm.h"
 
@@ -107,6 +108,12 @@ static int kvm_gmem_prepare_folio(struct kvm *kvm, struct kvm_memory_slot *slot,
 	return __kvm_gmem_prepare_folio(kvm, slot, index, folio);
 }
 
+static struct folio *kvm_gmem_get_folio_noalloc(struct inode *inode, pgoff_t pgoff)
+{
+	return __filemap_get_folio(inode->i_mapping, pgoff,
+				   FGP_LOCK | FGP_ACCESSED, 0);
+}
+
 /*
  * Returns a locked folio on success.  The caller is responsible for
  * setting the up-to-date flag before the memory is mapped into the guest.
@@ -126,8 +133,7 @@ static struct folio *kvm_gmem_get_folio(struct inode *inode, pgoff_t index)
 	 * Fast-path: See if folio is already present in mapping to avoid
 	 * policy_lookup.
 	 */
-	folio = __filemap_get_folio(inode->i_mapping, index,
-				    FGP_LOCK | FGP_ACCESSED, 0);
+	folio = kvm_gmem_get_folio_noalloc(inode, index);
 	if (!IS_ERR(folio))
 		return folio;
 
@@ -457,12 +463,86 @@ static struct mempolicy *kvm_gmem_get_policy(struct vm_area_struct *vma,
 }
 #endif /* CONFIG_NUMA */
 
+#ifdef CONFIG_USERFAULTFD
+static bool kvm_gmem_can_userfault(struct vm_area_struct *vma, vm_flags_t vm_flags)
+{
+	struct inode *inode = file_inode(vma->vm_file);
+
+	/*
+	 * Only support userfaultfd for guest_memfd with INIT_SHARED flag.
+	 * This ensures the memory can be mapped to userspace.
+	 */
+	if (!(GMEM_I(inode)->flags & GUEST_MEMFD_FLAG_INIT_SHARED))
+		return false;
+
+	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 = filemap_alloc_folio(gfp, 0, mpol);
+	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,
+	.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 */
+
 static const struct vm_operations_struct kvm_gmem_vm_ops = {
 	.fault		= kvm_gmem_fault_user_mapping,
 #ifdef CONFIG_NUMA
 	.get_policy	= kvm_gmem_get_policy,
 	.set_policy	= kvm_gmem_set_policy,
 #endif
+#ifdef CONFIG_USERFAULTFD
+	.uffd_ops	= &kvm_gmem_uffd_ops,
+#endif
 };
 
 static int kvm_gmem_mmap(struct file *file, struct vm_area_struct *vma)
-- 
2.51.0