[RFC PATCH v4 05/25] KVM: guest_memfd: Split population by backing type

["Aneesh Kumar K.V (Arm)" <aneesh.kumar@kernel.org>]

Preallocation obtains cached folios for folio backing and walks provider
PFN ranges for RAM PFN backing. Both use the same range validation and
shared inode invalidate lock.

Move the two walks into backing-specific populate() operations. Keep
size checks and locking in kvm_gmem_allocate(), preserving signal
handling, folio references and PFN range stepping.

Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Sean Christopherson <seanjc@google.com>
Cc: David Hildenbrand <david@kernel.org>
Cc: kvm@vger.kernel.org
Cc: patches@lists.linux.dev
Signed-off-by: Aneesh Kumar K.V (Arm) <aneesh.kumar@kernel.org>
---
 virt/kvm/guest_memfd.c | 101 +++++++++++++++++++++++++----------------
 1 file changed, 61 insertions(+), 40 deletions(-)

diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
index 4280c3b740ef..5a4da27f0434 100644
--- a/virt/kvm/guest_memfd.c
+++ b/virt/kvm/guest_memfd.c
@@ -32,6 +32,7 @@ struct gmem_file {
 };
 
 struct kvm_gmem_backing_ops {
+	int (*populate)(struct inode *inode, pgoff_t start, pgoff_t end);
 	int (*revoke)(struct inode *inode, pgoff_t start, pgoff_t end,
 		const unsigned long *pfn, bool put_pfn_range);
 	/*
@@ -578,46 +579,11 @@ int kvm_gmem_revoke(struct inode *inode, pgoff_t start, size_t nr_pages,
 }
 EXPORT_SYMBOL_GPL(kvm_gmem_revoke);
 
-static long kvm_gmem_allocate(struct inode *inode, loff_t offset, loff_t len)
+static int kvm_gmem_folio_populate(struct inode *inode,
+		pgoff_t start, pgoff_t end)
 {
-	struct address_space *mapping = inode->i_mapping;
-	struct gmem_inode *gi = GMEM_I(inode);
-	pgoff_t start, index, end;
-	int r;
-
-	/* Dedicated guest is immutable by default. */
-	if (offset + len > i_size_read(inode))
-		return -EINVAL;
-
-	filemap_invalidate_lock_shared(mapping);
-
-	start = offset >> PAGE_SHIFT;
-	end = (offset + len) >> PAGE_SHIFT;
-
-	r = 0;
-	if (kvm_gmem_is_pfn_mode(gi)) {
-		for (index = start; index < end; ) {
-			pgoff_t next_boundary;
-			pgoff_t max_nr_pages;
-			int max_order = 0;
-			kvm_pfn_t pfn;
-
-			if (signal_pending(current)) {
-				r = -EINTR;
-				break;
-			}
-
-			r = gmem_provider_get_pfn(gi, index, &pfn, &max_order);
-			if (r)
-				break;
-
-			max_nr_pages = 1UL << max_order;
-			next_boundary = ALIGN_DOWN(index + max_nr_pages, max_nr_pages);
-			index += min_t(size_t, next_boundary - index, end - index);
-			cond_resched();
-		}
-		goto out_unlock;
-	}
+	pgoff_t index;
+	int r = 0;
 
 	for (index = start; index < end; ) {
 		struct folio *folio;
@@ -645,7 +611,60 @@ static long kvm_gmem_allocate(struct inode *inode, loff_t offset, loff_t len)
 		cond_resched();
 	}
 
-out_unlock:
+	return r;
+}
+
+static int kvm_gmem_ram_pfn_populate(struct inode *inode,
+		pgoff_t start, pgoff_t end)
+{
+	struct gmem_inode *gi = GMEM_I(inode);
+	pgoff_t index;
+	int r = 0;
+
+	for (index = start; index < end; ) {
+		pgoff_t next_boundary;
+		pgoff_t max_nr_pages;
+		int max_order = 0;
+		kvm_pfn_t pfn;
+
+		if (signal_pending(current)) {
+			r = -EINTR;
+			break;
+		}
+
+		r = gmem_provider_get_pfn(gi, index, &pfn, &max_order);
+		if (r)
+			break;
+
+		max_nr_pages = 1UL << max_order;
+		next_boundary = ALIGN_DOWN(index + max_nr_pages, max_nr_pages);
+		index += min_t(size_t, next_boundary - index, end - index);
+		cond_resched();
+	}
+
+	return r;
+}
+
+static long kvm_gmem_allocate(struct inode *inode, loff_t offset, loff_t len)
+{
+	struct address_space *mapping = inode->i_mapping;
+	const struct kvm_gmem_backing_ops *ops = GMEM_I(inode)->backing_ops;
+	pgoff_t start, end;
+	int r;
+
+	if (!ops->populate)
+		return -EOPNOTSUPP;
+
+	/* Dedicated guest is immutable by default. */
+	if (offset + len > i_size_read(inode))
+		return -EINVAL;
+
+	filemap_invalidate_lock_shared(mapping);
+
+	start = offset >> PAGE_SHIFT;
+	end = (offset + len) >> PAGE_SHIFT;
+	r = ops->populate(inode, start, end);
+
 	filemap_invalidate_unlock_shared(mapping);
 
 	return r;
@@ -1122,6 +1141,7 @@ static int kvm_gmem_ram_prepare_pfn(struct kvm *kvm, gfn_t gfn,
 }
 
 static const struct kvm_gmem_backing_ops kvm_gmem_folio_backing_ops = {
+	.populate = kvm_gmem_folio_populate,
 	.revoke = kvm_gmem_folio_revoke,
 	.prepare_attributes = kvm_gmem_folio_prepare_attributes,
 	.convert_attributes = kvm_gmem_folio_convert_attributes,
@@ -1129,6 +1149,7 @@ static const struct kvm_gmem_backing_ops kvm_gmem_folio_backing_ops = {
 };
 
 static const struct kvm_gmem_backing_ops kvm_gmem_ram_pfn_backing_ops = {
+	.populate = kvm_gmem_ram_pfn_populate,
 	.revoke = kvm_gmem_ram_pfn_revoke,
 	.prepare_attributes = kvm_gmem_ram_pfn_prepare_attributes,
 	.convert_attributes = kvm_gmem_ram_pfn_convert_attributes,
-- 
2.43.0