Re: [RFC 2/2] kvm: retry nx_huge_page_recovery_thread creation

[Sean Christopherson <seanjc@google.com>]

On Tue, Feb 25, 2025, Keith Busch wrote:
> From: Keith Busch <kbusch@kernel.org>
> 
> A VMM may send a signal to its threads while they've entered KVM_RUN. If
> that thread happens to be trying to make the huge page recovery vhost
> task, then it fails with -ERESTARTNOINTR. We need to retry if that
> happens, so we can't use call_once anymore. Replace it with a simple
> mutex and return the appropriate error instead of defaulting to ENOMEM.
> 
> One downside is that everyone will retry if it fails, which can cause
> some additional pressure on low memory situations. Another downside is
> we're taking a mutex on every KVM run, even if we were previously
> successful in starting the vhost task, but that's really not such a
> common operation that needs to be optimized to avoid this lock.

Yes, it is.  NAK to taking a VM-wide mutex on KVM_RUN.

I much prefer my (misguided in the original context[*]) approach of marking the
call_once() COMPLETED if and only if it succeeds.

diff --git a/include/linux/call_once.h b/include/linux/call_once.h
index 6261aa0b3fb0..9d47ed50139b 100644
--- a/include/linux/call_once.h
+++ b/include/linux/call_once.h
@@ -26,20 +26,28 @@ do {									\
 	__once_init((once), #once, &__key);				\
 } while (0)
 
-static inline void call_once(struct once *once, void (*cb)(struct once *))
+static inline int call_once(struct once *once, int (*cb)(struct once *))
 {
+        int r;
+
         /* Pairs with atomic_set_release() below.  */
         if (atomic_read_acquire(&once->state) == ONCE_COMPLETED)
-                return;
+                return 0;
 
         guard(mutex)(&once->lock);
         WARN_ON(atomic_read(&once->state) == ONCE_RUNNING);
         if (atomic_read(&once->state) != ONCE_NOT_STARTED)
-                return;
+                return -EINVAL;
 
         atomic_set(&once->state, ONCE_RUNNING);
-        cb(once);
+        r = cb(once);
+        if (r) {
+                atomic_set(&once->state, ONCE_NOT_STARTED);
+                return r;
+        }
+
         atomic_set_release(&once->state, ONCE_COMPLETED);
+        return 0;
 }
 
 #endif /* _LINUX_CALL_ONCE_H */

[*] https://lore.kernel.org/all/Z5e4w7IlEEk2cpH-@google.com