[PATCH RFC 10/12] KVM: guest_memfd: Clarify comment about gmem.file vs kvm->srcu

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

From: Ackerley Tng <ackerleytng@google.com>

Clarify the existing comment about synchronize_srcu() and
kvm_gmem_get_pfn() to provide further context. Explain which
synchronize_srcu() prevents races with how kvm_gmem_get_pfn() is used.

Also point reader to documentation for better understanding.

Signed-off-by: Ackerley Tng <ackerleytng@google.com>
---
 virt/kvm/guest_memfd.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
index 69c9d6d546b28..f2218db0af980 100644
--- a/virt/kvm/guest_memfd.c
+++ b/virt/kvm/guest_memfd.c
@@ -711,8 +711,13 @@ static void __kvm_gmem_unbind(struct kvm_memory_slot *slot, struct gmem_file *f)
 	xa_store_range(&f->bindings, start, end - 1, NULL, GFP_KERNEL);
 
 	/*
-	 * synchronize_srcu(&kvm->srcu) ensured that kvm_gmem_get_pfn()
-	 * cannot see this memslot.
+	 * This is called when memslots are updated, after the old
+	 * memslot container is no longer in
+	 * use. synchronize_srcu(&kvm->srcu) was called there, so
+	 * kvm_gmem_get_pfn() from KVM's guest fault handling cannot
+	 * see this memslot. See Documentation/virt/kvm/locking.rst
+	 * for more information about kvm->srcu and the memslots
+	 * container.
 	 */
 	WRITE_ONCE(slot->gmem.file, NULL);
 }

-- 
2.54.0.823.g6e5bcc1fc9-goog