Re: [PATCH v2 24/30] KVM: x86/mmu: Allow yielding when zapping GFNs for defunct TDP MMU root
From: David Matlack
Date: Tue Jan 04 2022 - 20:25:47 EST
On Thu, Dec 23, 2021 at 10:23:12PM +0000, Sean Christopherson wrote:
> Allow yielding when zapping SPTEs after the last reference to a valid
> root is put. Because KVM must drop all SPTEs in response to relevant
> mmu_notifier events, mark defunct roots invalid and reset their refcount
> prior to zapping the root. Keeping the refcount elevated while the zap
> is in-progress ensures the root is reachable via mmu_notifier until the
> zap completes and the last reference to the invalid, defunct root is put.
>
> Allowing kvm_tdp_mmu_put_root() to yield fixes soft lockup issues if the
> root in being put has a massive paging structure, e.g. zapping a root
> that is backed entirely by 4kb pages for a guest with 32tb of memory can
> take hundreds of seconds to complete.
>
> watchdog: BUG: soft lockup - CPU#49 stuck for 485s! [max_guest_memor:52368]
> RIP: 0010:kvm_set_pfn_dirty+0x30/0x50 [kvm]
> __handle_changed_spte+0x1b2/0x2f0 [kvm]
> handle_removed_tdp_mmu_page+0x1a7/0x2b8 [kvm]
> __handle_changed_spte+0x1f4/0x2f0 [kvm]
> handle_removed_tdp_mmu_page+0x1a7/0x2b8 [kvm]
> __handle_changed_spte+0x1f4/0x2f0 [kvm]
> tdp_mmu_zap_root+0x307/0x4d0 [kvm]
> kvm_tdp_mmu_put_root+0x7c/0xc0 [kvm]
> kvm_mmu_free_roots+0x22d/0x350 [kvm]
> kvm_mmu_reset_context+0x20/0x60 [kvm]
> kvm_arch_vcpu_ioctl_set_sregs+0x5a/0xc0 [kvm]
> kvm_vcpu_ioctl+0x5bd/0x710 [kvm]
> __se_sys_ioctl+0x77/0xc0
> __x64_sys_ioctl+0x1d/0x20
> do_syscall_64+0x44/0xa0
> entry_SYSCALL_64_after_hwframe+0x44/0xae
>
> KVM currently doesn't put a root from a non-preemptible context, so other
> than the mmu_notifier wrinkle, yielding when putting a root is safe.
>
> Yield-unfriendly iteration uses for_each_tdp_mmu_root(), which doesn't
> take a reference to each root (it requires mmu_lock be held for the
> entire duration of the walk).
>
> tdp_mmu_next_root() is used only by the yield-friendly iterator.
>
> kvm_tdp_mmu_zap_invalidated_roots() is explicitly yield friendly.
>
> kvm_mmu_free_roots() => mmu_free_root_page() is a much bigger fan-out,
> but is still yield-friendly in all call sites, as all callers can be
> traced back to some combination of vcpu_run(), kvm_destroy_vm(), and/or
> kvm_create_vm().
>
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> ---
> arch/x86/kvm/mmu/mmu_internal.h | 7 +-
> arch/x86/kvm/mmu/tdp_mmu.c | 145 +++++++++++++++++++++++---------
> 2 files changed, 109 insertions(+), 43 deletions(-)
>
> diff --git a/arch/x86/kvm/mmu/mmu_internal.h b/arch/x86/kvm/mmu/mmu_internal.h
> index be063b6c91b7..8ce3d58fdf7f 100644
> --- a/arch/x86/kvm/mmu/mmu_internal.h
> +++ b/arch/x86/kvm/mmu/mmu_internal.h
> @@ -65,7 +65,12 @@ struct kvm_mmu_page {
> struct kvm_rmap_head parent_ptes; /* rmap pointers to parent sptes */
> tdp_ptep_t ptep;
> };
> - DECLARE_BITMAP(unsync_child_bitmap, 512);
> + union {
> + DECLARE_BITMAP(unsync_child_bitmap, 512);
> + struct {
> + bool tdp_mmu_defunct_root;
> + };
> + };
>
> struct list_head lpage_disallowed_link;
> #ifdef CONFIG_X86_32
> diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> index 72bcec2cd23c..aec97e037a8d 100644
> --- a/arch/x86/kvm/mmu/tdp_mmu.c
> +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> @@ -91,21 +91,67 @@ void kvm_tdp_mmu_put_root(struct kvm *kvm, struct kvm_mmu_page *root,
>
> WARN_ON(!root->tdp_mmu_page);
>
> - spin_lock(&kvm->arch.tdp_mmu_pages_lock);
> - list_del_rcu(&root->link);
> - spin_unlock(&kvm->arch.tdp_mmu_pages_lock);
> + /*
> + * Ensure root->role.invalid is read after the refcount reaches zero to
> + * avoid zapping the root multiple times, e.g. if a different task
> + * acquires a reference (after the root was marked invalid+defunct) and
> + * puts the last reference, all while holding mmu_lock for read. Pairs
> + * with the smp_mb__before_atomic() below.
> + */
> + smp_mb__after_atomic();
> +
> + /*
> + * Free the root if it's already invalid. Invalid roots must be zapped
> + * before their last reference is put, i.e. there's no work to be done,
> + * and all roots must be invalidated (see below) before they're freed.
> + * Re-zapping defunct roots, which are always invalid, would put KVM
> + * into an infinite loop (again, see below).
> + */
> + if (root->role.invalid) {
> + spin_lock(&kvm->arch.tdp_mmu_pages_lock);
> + list_del_rcu(&root->link);
> + spin_unlock(&kvm->arch.tdp_mmu_pages_lock);
> +
> + call_rcu(&root->rcu_head, tdp_mmu_free_sp_rcu_callback);
> + return;
> + }
> +
> + /*
> + * Invalidate the root to prevent it from being reused by a vCPU, and
> + * mark it defunct so that kvm_tdp_mmu_zap_invalidated_roots() doesn't
> + * try to put a reference it didn't acquire.
> + */
> + root->role.invalid = true;
> + root->tdp_mmu_defunct_root = true;
Ah ok so tdp_mmu_defunct_root indicates the root became invalid due to
losing all its references while it was valid. This is in contrast to
kvm_tdp_mmu_invalidate_all_roots() which marks roots invalid while they
still have valid references.
But I wonder if tdp_mmu_defunct_root is necessary? It's only used to
skip a put in zap_invalidated_roots. Could we instead unconditionally
grab a reference in invalidate_all_roots and then unconditionally drop
it?
e.g. Apply this on top:
diff --git a/arch/x86/kvm/mmu/mmu_internal.h b/arch/x86/kvm/mmu/mmu_internal.h
index 8ce3d58fdf7f..be063b6c91b7 100644
--- a/arch/x86/kvm/mmu/mmu_internal.h
+++ b/arch/x86/kvm/mmu/mmu_internal.h
@@ -65,12 +65,7 @@ struct kvm_mmu_page {
struct kvm_rmap_head parent_ptes; /* rmap pointers to parent sptes */
tdp_ptep_t ptep;
};
- union {
- DECLARE_BITMAP(unsync_child_bitmap, 512);
- struct {
- bool tdp_mmu_defunct_root;
- };
- };
+ DECLARE_BITMAP(unsync_child_bitmap, 512);
struct list_head lpage_disallowed_link;
#ifdef CONFIG_X86_32
diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
index aec97e037a8d..6bc5556b4cb7 100644
--- a/arch/x86/kvm/mmu/tdp_mmu.c
+++ b/arch/x86/kvm/mmu/tdp_mmu.c
@@ -122,7 +122,6 @@ void kvm_tdp_mmu_put_root(struct kvm *kvm, struct kvm_mmu_page *root,
* try to put a reference it didn't acquire.
*/
root->role.invalid = true;
- root->tdp_mmu_defunct_root = true;
/*
* Ensure tdp_mmu_defunct_root is visible if a concurrent reader acquires
@@ -140,7 +139,7 @@ void kvm_tdp_mmu_put_root(struct kvm *kvm, struct kvm_mmu_page *root,
* faults, and the only flow that must not consume an invalid root is
* allocating a new root for a vCPU, which also takes mmu_lock for write.
*/
- refcount_set(&root->tdp_mmu_root_count, 1);
+ refcount_add(1, &root->tdp_mmu_root_count);
/*
* Zap the root, then put the refcount "acquired" above. Recursively
@@ -984,16 +983,10 @@ void kvm_tdp_mmu_zap_invalidated_roots(struct kvm *kvm)
/*
* Put the reference acquired in kvm_tdp_mmu_invalidate_roots()
- * unless the root is defunct worker data, in which case no
- * reference was taken. Roots become defunct only when a valid
- * root has its last reference put, thus holding a reference
- * means the root can't become defunct between invalidating the
- * root and re-checking the data here.
*
* Note, the iterator holds its own reference.
*/
- if (!root->tdp_mmu_defunct_root)
- kvm_tdp_mmu_put_root(kvm, root, true);
+ kvm_tdp_mmu_put_root(kvm, root, true);
}
}
@@ -1015,10 +1008,6 @@ void kvm_tdp_mmu_zap_invalidated_roots(struct kvm *kvm)
* dropped, so even though all invalid roots are zapped, a root may not go away
* for quite some time, e.g. if a vCPU blocks across multiple memslot updates.
*
- * Don't take a reference if the root is defunct, vCPUs cannot hold references
- * to defunct roots and so will never get stuck with zapping the root. Note,
- * defunct roots still need to be zapped by kvm_tdp_mmu_zap_invalidated_roots().
- *
* Because mmu_lock is held for write, it should be impossible to observe a
* root with zero refcount, i.e. the list of roots cannot be stale.
*
@@ -1032,9 +1021,8 @@ void kvm_tdp_mmu_invalidate_all_roots(struct kvm *kvm)
lockdep_assert_held_write(&kvm->mmu_lock);
list_for_each_entry(root, &kvm->arch.tdp_mmu_roots, link) {
- if (!root->tdp_mmu_defunct_root &&
- !WARN_ON_ONCE(!kvm_tdp_mmu_get_root(root)))
- root->role.invalid = true;
+ refcount_add(1, &root->tdp_mmu_root_count);
+ root->role.invalid = true;
}
}
> +
> + /*
> + * Ensure tdp_mmu_defunct_root is visible if a concurrent reader acquires
> + * a reference after the root's refcount is reset. Pairs with the
> + * smp_mb__after_atomic() above and in kvm_tdp_mmu_zap_invalidated_roots().
> + */
> + smp_mb__before_atomic();
> +
> + /*
> + * Note, if mmu_lock is held for read this can race with other readers,
> + * e.g. they may acquire a reference without seeing the root as invalid,
> + * and the refcount may be reset after the root is skipped. Both races
> + * are benign, as flows that must visit all roots, e.g. need to zap
> + * SPTEs for correctness, must take mmu_lock for write to block page
> + * faults, and the only flow that must not consume an invalid root is
> + * allocating a new root for a vCPU, which also takes mmu_lock for write.
> + */
> + refcount_set(&root->tdp_mmu_root_count, 1);
>
> /*
> - * A TLB flush is not necessary as KVM performs a local TLB flush when
> - * allocating a new root (see kvm_mmu_load()), and when migrating vCPU
> - * to a different pCPU. Note, the local TLB flush on reuse also
> - * invalidates any paging-structure-cache entries, i.e. TLB entries for
> - * intermediate paging structures, that may be zapped, as such entries
> - * are associated with the ASID on both VMX and SVM.
> + * Zap the root, then put the refcount "acquired" above. Recursively
> + * call kvm_tdp_mmu_put_root() to test the above logic for avoiding an
> + * infinite loop by freeing invalid roots. By design, the root is
> + * reachable while it's being zapped, thus a different task can put its
> + * last reference, i.e. flowing through kvm_tdp_mmu_put_root() for a
> + * defunct root is unavoidable.
> */
> tdp_mmu_zap_root(kvm, root, shared);
> -
> - call_rcu(&root->rcu_head, tdp_mmu_free_sp_rcu_callback);
> + kvm_tdp_mmu_put_root(kvm, root, shared);
> }
>
> enum tdp_mmu_roots_iter_type {
> @@ -758,12 +804,23 @@ static inline gfn_t tdp_mmu_max_gfn_host(void)
> static void tdp_mmu_zap_root(struct kvm *kvm, struct kvm_mmu_page *root,
> bool shared)
> {
> - bool root_is_unreachable = !refcount_read(&root->tdp_mmu_root_count);
> struct tdp_iter iter;
>
> gfn_t end = tdp_mmu_max_gfn_host();
> gfn_t start = 0;
>
> + /*
> + * The root must have an elevated refcount so that it's reachable via
> + * mmu_notifier callbacks, which allows this path to yield and drop
> + * mmu_lock. When handling an unmap/release mmu_notifier command, KVM
> + * must drop all references to relevant pages prior to completing the
> + * callback. Dropping mmu_lock with an unreachable root would result
> + * in zapping SPTEs after a relevant mmu_notifier callback completes
> + * and lead to use-after-free as zapping a SPTE triggers "writeback" of
> + * dirty accessed bits to the SPTE's associated struct page.
> + */
> + WARN_ON_ONCE(!refcount_read(&root->tdp_mmu_root_count));
> +
> kvm_lockdep_assert_mmu_lock_held(kvm, shared);
>
> rcu_read_lock();
> @@ -775,19 +832,7 @@ static void tdp_mmu_zap_root(struct kvm *kvm, struct kvm_mmu_page *root,
> for_each_tdp_pte_min_level(iter, root->spt, root->role.level,
> root->role.level, start, end) {
> retry:
> - /*
> - * Yielding isn't allowed when zapping an unreachable root as
> - * the root won't be processed by mmu_notifier callbacks. When
> - * handling an unmap/release mmu_notifier command, KVM must
> - * drop all references to relevant pages prior to completing
> - * the callback. Dropping mmu_lock can result in zapping SPTEs
> - * for an unreachable root after a relevant callback completes,
> - * which leads to use-after-free as zapping a SPTE triggers
> - * "writeback" of dirty/accessed bits to the SPTE's associated
> - * struct page.
> - */
> - if (!root_is_unreachable &&
> - tdp_mmu_iter_cond_resched(kvm, &iter, false, shared))
> + if (tdp_mmu_iter_cond_resched(kvm, &iter, false, shared))
> continue;
>
> if (!is_shadow_present_pte(iter.old_spte))
> @@ -796,22 +841,9 @@ static void tdp_mmu_zap_root(struct kvm *kvm, struct kvm_mmu_page *root,
> if (!shared) {
> tdp_mmu_set_spte(kvm, &iter, 0);
> } else if (!tdp_mmu_set_spte_atomic(kvm, &iter, 0)) {
> - /*
> - * cmpxchg() shouldn't fail if the root is unreachable.
> - * to be unreachable. Re-read the SPTE and retry so as
> - * not to leak the page and its children.
> - */
> - WARN_ONCE(root_is_unreachable,
> - "Contended TDP MMU SPTE in unreachable root.");
> iter.old_spte = kvm_tdp_mmu_read_spte(iter.sptep);
> goto retry;
> }
> - /*
> - * WARN if the root is invalid and is unreachable, all SPTEs
> - * should've been zapped by kvm_tdp_mmu_zap_invalidated_roots(),
> - * and inserting new SPTEs under an invalid root is a KVM bug.
> - */
> - WARN_ON_ONCE(root_is_unreachable && root->role.invalid);
> }
>
> rcu_read_unlock();
> @@ -899,6 +931,9 @@ void kvm_tdp_mmu_zap_all(struct kvm *kvm)
> int i;
>
> /*
> + * Zap all roots, including defunct roots, as all SPTEs must be dropped
> + * before returning to the caller.
> + *
> * A TLB flush is unnecessary, KVM zaps everything if and only the VM
> * is being destroyed or the userspace VMM has exited. In both cases,
> * KVM_RUN is unreachable, i.e. no vCPUs will ever service the request.
> @@ -924,6 +959,12 @@ void kvm_tdp_mmu_zap_invalidated_roots(struct kvm *kvm)
>
> for_each_invalid_tdp_mmu_root_yield_safe(kvm, root) {
> /*
> + * Zap the root, even if it's defunct, as all SPTEs must be
> + * dropped before returning to the caller, e.g. if the root was
> + * invalidated by a memslot update, then SPTEs associated with
> + * a deleted/moved memslot are unreachable via the mmu_notifier
> + * once the memslot update completes.
> + *
> * A TLB flush is unnecessary, invalidated roots are guaranteed
> * to be unreachable by the guest (see kvm_tdp_mmu_put_root()
> * for more details), and unlike the legacy MMU, no vCPU kick
> @@ -935,10 +976,24 @@ void kvm_tdp_mmu_zap_invalidated_roots(struct kvm *kvm)
> tdp_mmu_zap_root(kvm, root, true);
>
> /*
> - * Put the reference acquired in kvm_tdp_mmu_invalidate_roots().
> + * Leverages kvm_tdp_mmu_get_root() in the iterator, pairs with
> + * the smp_mb__before_atomic() in kvm_tdp_mmu_put_root() to
> + * ensure a defunct root is seen as such.
> + */
> + smp_mb__after_atomic();
> +
> + /*
> + * Put the reference acquired in kvm_tdp_mmu_invalidate_roots()
Correction: kvm_tdp_mmu_invalidate_all_roots
(Looks like the typo comes from a previous patch though.)
> + * unless the root is defunct worker data, in which case no
> + * reference was taken. Roots become defunct only when a valid
> + * root has its last reference put, thus holding a reference
> + * means the root can't become defunct between invalidating the
> + * root and re-checking the data here.
> + *
> * Note, the iterator holds its own reference.
> */
> - kvm_tdp_mmu_put_root(kvm, root, true);
> + if (!root->tdp_mmu_defunct_root)
> + kvm_tdp_mmu_put_root(kvm, root, true);
> }
> }
>
> @@ -953,13 +1008,17 @@ void kvm_tdp_mmu_zap_invalidated_roots(struct kvm *kvm)
> * a vCPU drops the last reference to a root prior to the root being zapped, it
> * will get stuck with tearing down the entire paging structure.
> *
> - * Get a reference even if the root is already invalid,
> + * Get a reference even if the root is already invalid, unless it's defunct, as
> * kvm_tdp_mmu_zap_invalidated_roots() assumes it was gifted a reference to all
> * invalid roots, e.g. there's no epoch to identify roots that were invalidated
> * by a previous call. Roots stay on the list until the last reference is
> * dropped, so even though all invalid roots are zapped, a root may not go away
> * for quite some time, e.g. if a vCPU blocks across multiple memslot updates.
> *
> + * Don't take a reference if the root is defunct, vCPUs cannot hold references
> + * to defunct roots and so will never get stuck with zapping the root. Note,
> + * defunct roots still need to be zapped by kvm_tdp_mmu_zap_invalidated_roots().
> + *
> * Because mmu_lock is held for write, it should be impossible to observe a
> * root with zero refcount, i.e. the list of roots cannot be stale.
> *
> @@ -971,8 +1030,10 @@ void kvm_tdp_mmu_invalidate_all_roots(struct kvm *kvm)
> struct kvm_mmu_page *root;
>
> lockdep_assert_held_write(&kvm->mmu_lock);
> +
> list_for_each_entry(root, &kvm->arch.tdp_mmu_roots, link) {
> - if (!WARN_ON_ONCE(!kvm_tdp_mmu_get_root(kvm, root)))
> + if (!root->tdp_mmu_defunct_root &&
> + !WARN_ON_ONCE(!kvm_tdp_mmu_get_root(root)))
> root->role.invalid = true;
> }
> }
> --
> 2.34.1.448.ga2b2bfdf31-goog
>