[RFC PATCH v4 03/14] KVM: guest_memfd: Introduce kvm_gmem_get_pfn_locked(), which retains the folio lock

Fuad Tabba
2024-12-13T16:48:21Z

Create a new variant of kvm_gmem_get_pfn(), which retains the
folio lock if it returns successfully. This is needed in
subsequent patches in order to protect against races when
checking whether a folio can be mapped by the host.

Signed-off-by: Fuad Tabba <tabba@google.com>
---
include/linux/kvm_host.h | 11 +++++++++++
virt/kvm/guest_memfd.c | 27 ++++++++++++++++++++-------
2 files changed, 31 insertions(+), 7 deletions(-)

diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 401439bb21e3..cda3ed4c3c27 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -2500,6 +2500,9 @@ static inline bool kvm_mem_is_private(struct kvm *kvm, gfn_t gfn)
int kvm_gmem_get_pfn(struct kvm *kvm, struct kvm_memory_slot *slot,
gfn_t gfn, kvm_pfn_t *pfn, struct page **page,
int *max_order);
+int kvm_gmem_get_pfn_locked(struct kvm *kvm, struct kvm_memory_slot *slot,
+ gfn_t gfn, kvm_pfn_t *pfn, struct page **page,
+ int *max_order);
#else
static inline int kvm_gmem_get_pfn(struct kvm *kvm,
struct kvm_memory_slot *slot, gfn_t gfn,
@@ -2509,6 +2512,14 @@ static inline int kvm_gmem_get_pfn(struct kvm *kvm,
KVM_BUG_ON(1, kvm);
return -EIO;
}
+static inline int kvm_gmem_get_pfn_locked(struct kvm *kvm,
+ struct kvm_memory_slot *slot,
+ gfn_t gfn, kvm_pfn_t *pfn,
+ struct page **page, int *max_order)
+{
+ KVM_BUG_ON(1, kvm);
+ return -EIO;
+}
#endif /* CONFIG_KVM_PRIVATE_MEM */

#ifdef CONFIG_HAVE_KVM_ARCH_GMEM_PREPARE
diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
index 198554b1f0b5..6453658d2650 100644
--- a/virt/kvm/guest_memfd.c
+++ b/virt/kvm/guest_memfd.c
@@ -672,9 +672,9 @@ static struct folio *__kvm_gmem_get_pfn(struct file *file,
return folio;
}

-int kvm_gmem_get_pfn(struct kvm *kvm, struct kvm_memory_slot *slot,
- gfn_t gfn, kvm_pfn_t *pfn, struct page **page,
- int *max_order)
+int kvm_gmem_get_pfn_locked(struct kvm *kvm, struct kvm_memory_slot *slot,
+ gfn_t gfn, kvm_pfn_t *pfn, struct page **page,
+ int *max_order)
{
pgoff_t index = kvm_gmem_get_index(slot, gfn);
struct file *file = kvm_gmem_get_file(slot);
@@ -694,17 +694,30 @@ int kvm_gmem_get_pfn(struct kvm *kvm, struct kvm_memory_slot *slot,
if (!is_prepared)
r = kvm_gmem_prepare_folio(kvm, slot, gfn, folio);

- folio_unlock(folio);
-
- if (!r)
+ if (!r) {
*page = folio_file_page(folio, index);
- else
+ } else {
+ folio_unlock(folio);
folio_put(folio);
+ }

out:
fput(file);
return r;
}
+EXPORT_SYMBOL_GPL(kvm_gmem_get_pfn_locked);
+
+int kvm_gmem_get_pfn(struct kvm *kvm, struct kvm_memory_slot *slot,
+ gfn_t gfn, kvm_pfn_t *pfn, struct page **page,
+ int *max_order)
+{
+ int r = kvm_gmem_get_pfn_locked(kvm, slot, gfn, pfn, page, max_order);
+
+ if (!r)
+ unlock_page(*page);
+
+ return r;
+}
EXPORT_SYMBOL_GPL(kvm_gmem_get_pfn);

#ifdef CONFIG_KVM_GENERIC_PRIVATE_MEM
--
2.47.1.613.gc27f4b7a9f-goog