Re: [PATCH] KVM: x86: Move kzalloc out of atomic context on PREEMPT_RT

[Sean Christopherson]

"x86/kvm:" is the preferred shortlog scope for the guest side of things, "KVM: x86"
is for the host, i.e. for arch/x86/kvm.

On Thu, May 19, 2022, Yajun Deng wrote:
> The memory allocator is fully preemptible and therefore cannot
> be invoked from truly atomic contexts.
> 
> See Documentation/locking/locktypes.rst (line: 470)
> 
> Add raw_spin_unlock() before memory allocation and raw_spin_lock()
> after it.
> 
> Signed-off-by: Yajun Deng <yajun.deng@xxxxxxxxx>
> ---
>  arch/x86/kernel/kvm.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
> index d0bb2b3fb305..8f8ec9bbd847 100644
> --- a/arch/x86/kernel/kvm.c
> +++ b/arch/x86/kernel/kvm.c
> @@ -205,7 +205,9 @@ void kvm_async_pf_task_wake(u32 token)
>  		 * async PF was not yet handled.
>  		 * Add dummy entry for the token.
>  		 */
> -		n = kzalloc(sizeof(*n), GFP_ATOMIC);
> +		raw_spin_unlock(&b->lock);
> +		n = kzalloc(sizeof(*n), GFP_KERNEL);
> +		raw_spin_lock(&b->lock);

This is flawed, if the async #PF is handled while the lock is dropped then this
will enqueue a second, duplicate entry and not call apf_task_wake_one() as it
should.  I.e. two entries will be leaked.

AFAICT, kfree() is safe to call under a raw spinlock, so this?  Compile tested
only...

--
From: Sean Christopherson <seanjc@xxxxxxxxxx>
Date: Thu, 19 May 2022 07:57:11 -0700
Subject: [PATCH] x86/kvm: Alloc dummy async #PF token outside of raw spinlock

Drop the raw spinlock in kvm_async_pf_task_wake() before allocating the
the dummy async #PF token, the allocator is preemptible on PREEMPT_RT
kernels and must not be called from truly atomic contexts.

Opportunistically document why it's ok to loop on allocation failure,
i.e. why the function won't get stuck in an infinite loop.

Reported-by: Yajun Deng <yajun.deng@xxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
---
 arch/x86/kernel/kvm.c | 38 ++++++++++++++++++++++++--------------
 1 file changed, 24 insertions(+), 14 deletions(-)

diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
index d0bb2b3fb305..5a4100896969 100644
--- a/arch/x86/kernel/kvm.c
+++ b/arch/x86/kernel/kvm.c
@@ -190,7 +190,7 @@ void kvm_async_pf_task_wake(u32 token)
 {
 	u32 key = hash_32(token, KVM_TASK_SLEEP_HASHBITS);
 	struct kvm_task_sleep_head *b = &async_pf_sleepers[key];
-	struct kvm_task_sleep_node *n;
+	struct kvm_task_sleep_node *n, *dummy = NULL;

 	if (token == ~0) {
 		apf_task_wake_all();
@@ -202,24 +202,34 @@ void kvm_async_pf_task_wake(u32 token)
 	n = _find_apf_task(b, token);
 	if (!n) {
 		/*
-		 * async PF was not yet handled.
-		 * Add dummy entry for the token.
+		 * Async #PF not yet handled, add a dummy entry for the token.
+		 * Allocating the token must be down outside of the raw lock
+		 * as the allocator is preemptible on PREEMPT_RT kernels.
 		 */
-		n = kzalloc(sizeof(*n), GFP_ATOMIC);
-		if (!n) {
-			/*
-			 * Allocation failed! Busy wait while other cpu
-			 * handles async PF.
-			 */
+		if (!dummy) {
 			raw_spin_unlock(&b->lock);
-			cpu_relax();
+			dummy = kzalloc(sizeof(*dummy), GFP_KERNEL);
+
+			/*
+			 * Continue looping on allocation failure, eventually
+			 * the async #PF will be handled and allocating a new
+			 * node will be unnecessary.
+			 */
+			if (!dummy)
+				cpu_relax();
+
+			/*
+			 * Recheck for async #PF completion before enqueueing
+			 * the dummy token to avoid duplicate list entries.
+			 */
 			goto again;
 		}
-		n->token = token;
-		n->cpu = smp_processor_id();
-		init_swait_queue_head(&n->wq);
-		hlist_add_head(&n->link, &b->list);
+		dummy->token = token;
+		dummy->cpu = smp_processor_id();
+		init_swait_queue_head(&dummy->wq);
+		hlist_add_head(&dummy->link, &b->list);
 	} else {
+		kfree(dummy);
 		apf_task_wake_one(n);
 	}
 	raw_spin_unlock(&b->lock);

base-commit: a3808d88461270c71d3fece5e51cc486ecdac7d0
--