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SubjectRe: [RFC 04/19] KVM: x86/mmu: Yield while processing disconnected_sps
On Wed, Nov 10, 2021 at 02:29:55PM -0800, Ben Gardon wrote:
> When preparing to free disconnected SPs, the list can accumulate many
> entries; enough that it is likely necessary to yeild while queuing RCU
> callbacks to free the SPs.
>
> Signed-off-by: Ben Gardon <bgardon@google.com>
> ---
> arch/x86/kvm/mmu/tdp_mmu.c | 18 +++++++++++++++---
> 1 file changed, 15 insertions(+), 3 deletions(-)
>
> diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> index a448f0f2d993..c2a9f7acf8ef 100644
> --- a/arch/x86/kvm/mmu/tdp_mmu.c
> +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> @@ -513,7 +513,8 @@ static void handle_changed_spte(struct kvm *kvm, int as_id, gfn_t gfn,
> * being removed from the paging structure and this function being called.
> */
> static void handle_disconnected_sps(struct kvm *kvm,
> - struct list_head *disconnected_sps)
> + struct list_head *disconnected_sps,
> + bool can_yield, bool shared)
> {
> struct kvm_mmu_page *sp;
> struct kvm_mmu_page *next;
> @@ -521,6 +522,16 @@ static void handle_disconnected_sps(struct kvm *kvm,
> list_for_each_entry_safe(sp, next, disconnected_sps, link) {
> list_del(&sp->link);
> call_rcu(&sp->rcu_head, tdp_mmu_free_sp_rcu_callback);
> +
> + if (can_yield &&
> + (need_resched() || rwlock_needbreak(&kvm->mmu_lock))) {
> + rcu_read_unlock();
> + if (shared)
> + cond_resched_rwlock_read(&kvm->mmu_lock);
> + else
> + cond_resched_rwlock_write(&kvm->mmu_lock);
> + rcu_read_lock();
> + }

What about something like this to cut down on the duplicate code?

diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
index c2a9f7acf8ef..2fd010f2421e 100644
--- a/arch/x86/kvm/mmu/tdp_mmu.c
+++ b/arch/x86/kvm/mmu/tdp_mmu.c
@@ -508,6 +508,26 @@ static void handle_changed_spte(struct kvm *kvm, int as_id, gfn_t gfn,
new_spte, level);
}

+static inline bool tdp_mmu_need_resched(struct kvm *kvm)
+{
+ return need_resched() || rwlock_needbreak(&kvm->mmu_lock);
+}
+
+static void tdp_mmu_cond_resched(struct kvm *kvm, bool shared, bool flush)
+{
+ rcu_read_unlock()
+
+ if (flush)
+ kvm_flush_remote_tlbs(kvm);
+
+ if (shared)
+ cond_resched_rwlock_read(&kvm->mmu_lock);
+ else
+ cond_resched_rwlock_write(&kvm->mmu_lock);
+
+ rcu_read_lock();
+}
+
/*
* The TLBs must be flushed between the pages linked from disconnected_sps
* being removed from the paging structure and this function being called.
@@ -523,15 +543,8 @@ static void handle_disconnected_sps(struct kvm *kvm,
list_del(&sp->link);
call_rcu(&sp->rcu_head, tdp_mmu_free_sp_rcu_callback);

- if (can_yield &&
- (need_resched() || rwlock_needbreak(&kvm->mmu_lock))) {
- rcu_read_unlock();
- if (shared)
- cond_resched_rwlock_read(&kvm->mmu_lock);
- else
- cond_resched_rwlock_write(&kvm->mmu_lock);
- rcu_read_lock();
- }
+ if (can_yield && tdp_mmu_need_resched(kvm))
+ tdp_mmu_cond_resched(kvm, shared, false);
}
}

@@ -724,18 +737,8 @@ static inline bool tdp_mmu_iter_cond_resched(struct kvm *kvm,
if (iter->next_last_level_gfn == iter->yielded_gfn)
return false;

- if (need_resched() || rwlock_needbreak(&kvm->mmu_lock)) {
- rcu_read_unlock();
-
- if (flush)
- kvm_flush_remote_tlbs(kvm);
-
- if (shared)
- cond_resched_rwlock_read(&kvm->mmu_lock);
- else
- cond_resched_rwlock_write(&kvm->mmu_lock);
-
- rcu_read_lock();
+ if (tdp_mmu_need_resched(kvm)) {
+ tdp_mmu_cond_resched(kvm, shared, flush);

WARN_ON(iter->gfn > iter->next_last_level_gfn);

> }
> }
> @@ -599,7 +610,7 @@ static inline bool tdp_mmu_zap_spte_atomic(struct kvm *kvm,
> */
> WRITE_ONCE(*rcu_dereference(iter->sptep), 0);
>
> - handle_disconnected_sps(kvm, &disconnected_sps);
> + handle_disconnected_sps(kvm, &disconnected_sps, false, true);
>
> return true;
> }
> @@ -817,7 +828,8 @@ static bool zap_gfn_range(struct kvm *kvm, struct kvm_mmu_page *root,
>
> if (!list_empty(&disconnected_sps)) {
> kvm_flush_remote_tlbs(kvm);
> - handle_disconnected_sps(kvm, &disconnected_sps);
> + handle_disconnected_sps(kvm, &disconnected_sps,
> + can_yield, shared);
> flush = false;
> }
>
> --
> 2.34.0.rc0.344.g81b53c2807-goog
>

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