Remove the RCU-protected attribute from slot->gmem.file. No need to use RCU
primitives rcu_assign_pointer()/synchronize_rcu() to update this pointer.
- slot->gmem.file is updated in 3 places:
kvm_gmem_bind(), kvm_gmem_unbind(), kvm_gmem_release().
All of them are protected by kvm->slots_lock.
- slot->gmem.file is read in 2 paths:
(1) kvm_gmem_populate
kvm_gmem_get_file
__kvm_gmem_get_pfn
(2) kvm_gmem_get_pfn
kvm_gmem_get_file
__kvm_gmem_get_pfn
Path (1) kvm_gmem_populate() requires holding kvm->slots_lock, so
slot->gmem.file is protected by the kvm->slots_lock in this path.
Path (2) kvm_gmem_get_pfn() does not require holding kvm->slots_lock.
However, it's also not guarded by rcu_read_lock() and rcu_read_unlock().
So synchronize_rcu() in kvm_gmem_unbind()/kvm_gmem_release() actually
will not wait for the readers in kvm_gmem_get_pfn() due to lack of RCU
read-side critical section.
The path (2) kvm_gmem_get_pfn() is safe without RCU protection because:
a) kvm_gmem_bind() is called on a new memslot, before the memslot is
visible to kvm_gmem_get_pfn().
b) kvm->srcu ensures that kvm_gmem_unbind() and freeing of a memslot
occur after the memslot is no longer visible to kvm_gmem_get_pfn().
c) get_file_active() ensures that kvm_gmem_get_pfn() will not access the
stale file if kvm_gmem_release() sets it to NULL. This is because if
kvm_gmem_release() occurs before kvm_gmem_get_pfn(), get_file_active()
will return NULL; if get_file_active() does not return NULL,
kvm_gmem_release() should not occur until after kvm_gmem_get_pfn()
releases the file reference.
Signed-off-by: Yan Zhao <yan.y.zhao@xxxxxxxxx>
---
include/linux/kvm_host.h | 2 +-
virt/kvm/guest_memfd.c | 23 ++++++++++-------------
2 files changed, 11 insertions(+), 14 deletions(-)
diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index c7e4f8be3e17..3c3088a9e336 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -600,7 +600,7 @@ struct kvm_memory_slot {
#ifdef CONFIG_KVM_PRIVATE_MEM
struct {
- struct file __rcu *file;
+ struct file *file;
pgoff_t pgoff;
} gmem;
#endif
diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
index 651c2f08df62..9d9bf3d033bd 100644
--- a/virt/kvm/guest_memfd.c
+++ b/virt/kvm/guest_memfd.c
@@ -267,9 +267,7 @@ static int kvm_gmem_release(struct inode *inode, struct file *file)
filemap_invalidate_lock(inode->i_mapping);
xa_for_each(&gmem->bindings, index, slot)
- rcu_assign_pointer(slot->gmem.file, NULL);
-
- synchronize_rcu();
+ WRITE_ONCE(slot->gmem.file, NULL);
/*
* All in-flight operations are gone and new bindings can be created.
@@ -298,8 +296,7 @@ static inline struct file *kvm_gmem_get_file(struct kvm_memory_slot *slot)
/*
* Do not return slot->gmem.file if it has already been closed;
* there might be some time between the last fput() and when
- * kvm_gmem_release() clears slot->gmem.file, and you do not
- * want to spin in the meanwhile.
+ * kvm_gmem_release() clears slot->gmem.file.
*/
return get_file_active(&slot->gmem.file);
}
@@ -510,11 +507,11 @@ int kvm_gmem_bind(struct kvm *kvm, struct kvm_memory_slot *slot,
}
/*
- * No synchronize_rcu() needed, any in-flight readers are guaranteed to
- * be see either a NULL file or this new file, no need for them to go
- * away.
+ * memslots of flag KVM_MEM_GUEST_MEMFD are immutable to change, so
+ * kvm_gmem_bind() must occur on a new memslot.
+ * Readers are guaranteed to see this new file.
*/
- rcu_assign_pointer(slot->gmem.file, file);
+ WRITE_ONCE(slot->gmem.file, file);
slot->gmem.pgoff = start;
xa_store_range(&gmem->bindings, start, end - 1, slot, GFP_KERNEL);
@@ -550,8 +547,7 @@ void kvm_gmem_unbind(struct kvm_memory_slot *slot)
filemap_invalidate_lock(file->f_mapping);
xa_store_range(&gmem->bindings, start, end - 1, NULL, GFP_KERNEL);
- rcu_assign_pointer(slot->gmem.file, NULL);
- synchronize_rcu();
+ WRITE_ONCE(slot->gmem.file, NULL);
filemap_invalidate_unlock(file->f_mapping);
fput(file);
@@ -563,11 +559,12 @@ static struct folio *__kvm_gmem_get_pfn(struct file *file,
pgoff_t index, kvm_pfn_t *pfn,
bool *is_prepared, int *max_order)
{
+ struct file *gmem_file = READ_ONCE(slot->gmem.file);
struct kvm_gmem *gmem = file->private_data;
struct folio *folio;
- if (file != slot->gmem.file) {
- WARN_ON_ONCE(slot->gmem.file);
+ if (file != gmem_file) {
+ WARN_ON_ONCE(gmem_file);
return ERR_PTR(-EFAULT);
}
--
2.43.2
