[PATCH RFC v2 2/6] KVM: guest_memfd: Directly allocate folios with filemap_alloc_folio()

[Ackerley Tng <ackerleytng@google.com>]

__filemap_get_folio_mpol() is parametrized by a bunch of GFP flags, which
adds complexity for the reader. Since guest_memfd doesn't meaningfully use
any of the other FGP flags, undo that complexity by directly calling
filemap_alloc_folio().

Directly calling filemap_alloc_folio() also allows the order of 0 to be
explicitly specified, which is the only order guest_memfd supports. This is
easier to understand, and removes the chance of anything else being able to
unintentionally influence allocated folio size.

Signed-off-by: Ackerley Tng <ackerleytng@google.com>
---
 virt/kvm/guest_memfd.c | 51 +++++++++++++++++++++++++++++++++++---------------
 1 file changed, 36 insertions(+), 15 deletions(-)

diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
index 2df27b6443115..2488d7b8f2b0d 100644
--- a/virt/kvm/guest_memfd.c
+++ b/virt/kvm/guest_memfd.c
@@ -107,6 +107,39 @@ 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(struct inode *inode, pgoff_t index)
+{
+	/* TODO: Support huge pages. */
+	struct mempolicy *policy;
+	struct folio *folio;
+	gfp_t gfp;
+	int ret;
+
+	/*
+	 * Fast-path: See if folio is already present in mapping to avoid
+	 * policy_lookup.
+	 */
+	folio = filemap_lock_folio(inode->i_mapping, index);
+	if (!IS_ERR(folio))
+		return folio;
+
+	gfp = mapping_gfp_mask(inode->i_mapping);
+
+	policy = mpol_shared_policy_lookup(&GMEM_I(inode)->policy, index);
+	folio = filemap_alloc_folio(gfp, 0, policy);
+	mpol_cond_put(policy);
+	if (!folio)
+		return ERR_PTR(-ENOMEM);
+
+	ret = filemap_add_folio(inode->i_mapping, folio, index, gfp);
+	if (ret) {
+		folio_put(folio);
+		return ERR_PTR(ret);
+	}
+
+	return folio;
+}
+
 /*
  * 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.
@@ -118,23 +151,11 @@ static int kvm_gmem_prepare_folio(struct kvm *kvm, struct kvm_memory_slot *slot,
  */
 static struct folio *kvm_gmem_get_folio(struct inode *inode, pgoff_t index)
 {
-	/* TODO: Support huge pages. */
-	struct mempolicy *policy;
 	struct folio *folio;
 
-	/*
-	 * Fast-path: See if folio is already present in mapping to avoid
-	 * policy_lookup.
-	 */
-	folio = filemap_lock_folio(inode->i_mapping, index);
-	if (!IS_ERR(folio))
-		return folio;
-
-	policy = mpol_shared_policy_lookup(&GMEM_I(inode)->policy, index);
-	folio = __filemap_get_folio_mpol(inode->i_mapping, index,
-					 FGP_LOCK | FGP_CREAT,
-					 mapping_gfp_mask(inode->i_mapping), policy);
-	mpol_cond_put(policy);
+	do {
+		folio = __kvm_gmem_get_folio(inode, index);
+	} while (PTR_ERR(folio) == -EEXIST);
 
 	/*
 	 * External interfaces like kvm_gmem_get_pfn() support dealing

-- 
2.53.0.414.gf7e9f6c205-goog