[PATCH RFC v3 01/88] KVM: guest_memfd: Split folio allocation and insertion in kvm_gmem_get_folio

[Ackerley Tng via B4 Relay <devnull+ackerleytng.google.com@kernel.org>]

From: Ackerley Tng <ackerleytng@google.com>

Separate __filemap_get_folio_mpol() call in kvm_gmem_get_folio() into two
distinct steps:

1. Allocating a folio via filemap_alloc_folio() with the NUMA policy.
2. Inserting the allocated folio into the inode filemap.

Handle -EEXIST from filemap_add_folio() by retrying filemap_lock_folio()
safely to manage concurrent insertions.

Decouple folio allocation from filemap insertion to prepare for supporting
other memory providers.

No functional change intended.

Signed-off-by: Ackerley Tng <ackerleytng@google.com>
---
 virt/kvm/guest_memfd.c | 19 +++++++++++++++----
 1 file changed, 15 insertions(+), 4 deletions(-)

diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
index 93f3d29d5751a..f865fa0e46840 100644
--- a/virt/kvm/guest_memfd.c
+++ b/virt/kvm/guest_memfd.c
@@ -131,7 +131,9 @@ static struct folio *kvm_gmem_get_folio(struct inode *inode, pgoff_t index)
 	/* TODO: Support huge pages. */
 	struct mempolicy *policy;
 	struct folio *folio;
+	int r;
 
+retry:
 	/*
 	 * Fast-path: See if folio is already present in mapping to avoid
 	 * policy_lookup.
@@ -141,10 +143,19 @@ static struct folio *kvm_gmem_get_folio(struct inode *inode, pgoff_t index)
 		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);
+	folio = filemap_alloc_folio(mapping_gfp_mask(inode->i_mapping), 0,
+				    policy);
 	mpol_cond_put(policy);
+	if (!folio)
+		return ERR_PTR(-ENOMEM);
+
+	r = filemap_add_folio(inode->i_mapping, folio, index, GFP_KERNEL);
+	if (r) {
+		folio_put(folio);
+		if (r == -EEXIST)
+			goto retry;
+		return ERR_PTR(r);
+	}
 
 	/*
 	 * External interfaces like kvm_gmem_get_pfn() support dealing
@@ -153,7 +164,7 @@ static struct folio *kvm_gmem_get_folio(struct inode *inode, pgoff_t index)
 	 * to be updated for anything otherwise (e.g. page-clearing
 	 * operations).
 	 */
-	WARN_ON_ONCE(!IS_ERR(folio) && folio_order(folio));
+	WARN_ON_ONCE(folio_order(folio));
 
 	return folio;
 }

-- 
2.56.0.455.g6c5a503b38-goog