[PATCH v6 03/10] KVM: guest_memfd: Handle kvm_gmem_handle_folio_put() for KVM as a module

[Fuad Tabba <tabba@google.com>]

In some architectures, KVM could be defined as a module. If there is a
pending folio_put() while KVM is unloaded, the system could crash. By
having a helper check for that and call the function only if it's
available, we are able to handle that case more gracefully.

Signed-off-by: Fuad Tabba <tabba@google.com>

---

This patch could be squashed with the previous one of the maintainers
think it would be better.
---
 include/linux/kvm_host.h |  5 +----
 mm/swap.c                | 20 +++++++++++++++++++-
 virt/kvm/guest_memfd.c   |  8 ++++++++
 3 files changed, 28 insertions(+), 5 deletions(-)

diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 7788e3625f6d..3ad0719bfc4f 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -2572,10 +2572,7 @@ long kvm_arch_vcpu_pre_fault_memory(struct kvm_vcpu *vcpu,
 #endif
 
 #ifdef CONFIG_KVM_GMEM_SHARED_MEM
-static inline void kvm_gmem_handle_folio_put(struct folio *folio)
-{
-	WARN_ONCE(1, "A placeholder that shouldn't trigger. Work in progress.");
-}
+void kvm_gmem_handle_folio_put(struct folio *folio);
 #endif
 
 #endif
diff --git a/mm/swap.c b/mm/swap.c
index 241880a46358..27dfd75536c8 100644
--- a/mm/swap.c
+++ b/mm/swap.c
@@ -98,6 +98,24 @@ static void page_cache_release(struct folio *folio)
 		unlock_page_lruvec_irqrestore(lruvec, flags);
 }
 
+#ifdef CONFIG_KVM_GMEM_SHARED_MEM
+static void gmem_folio_put(struct folio *folio)
+{
+#if IS_MODULE(CONFIG_KVM)
+	void (*fn)(struct folio *folio);
+
+	fn = symbol_get(kvm_gmem_handle_folio_put);
+	if (WARN_ON_ONCE(!fn))
+		return;
+
+	fn(folio);
+	symbol_put(kvm_gmem_handle_folio_put);
+#else
+	kvm_gmem_handle_folio_put(folio);
+#endif
+}
+#endif
+
 static void free_typed_folio(struct folio *folio)
 {
 	switch (folio_get_type(folio)) {
@@ -108,7 +126,7 @@ static void free_typed_folio(struct folio *folio)
 #endif
 #ifdef CONFIG_KVM_GMEM_SHARED_MEM
 	case PGTY_guestmem:
-		kvm_gmem_handle_folio_put(folio);
+		gmem_folio_put(folio);
 		return;
 #endif
 	default:
diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
index b2aa6bf24d3a..5fc414becae5 100644
--- a/virt/kvm/guest_memfd.c
+++ b/virt/kvm/guest_memfd.c
@@ -13,6 +13,14 @@ struct kvm_gmem {
 	struct list_head entry;
 };
 
+#ifdef CONFIG_KVM_GMEM_SHARED_MEM
+void kvm_gmem_handle_folio_put(struct folio *folio)
+{
+	WARN_ONCE(1, "A placeholder that shouldn't trigger. Work in progress.");
+}
+EXPORT_SYMBOL_GPL(kvm_gmem_handle_folio_put);
+#endif /* CONFIG_KVM_GMEM_SHARED_MEM */
+
 /**
  * folio_file_pfn - like folio_file_page, but return a pfn.
  * @folio: The folio which contains this index.
-- 
2.49.0.rc0.332.g42c0ae87b1-goog