| Message ID | 20210402005658.3024832-10-seanjc@google.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | KVM: Consolidate and optimize MMU notifiers | expand |
On 02/04/21 02:56, Sean Christopherson wrote: > Avoid taking mmu_lock for unrelated .invalidate_range_{start,end}() > notifications. Because mmu_notifier_count must be modified while holding > mmu_lock for write, and must always be paired across start->end to stay > balanced, lock elision must happen in both or none. To meet that > requirement, add a rwsem to prevent memslot updates across range_start() > and range_end(). > > Use a rwsem instead of a rwlock since most notifiers _allow_ blocking, > and the lock will be endl across the entire start() ... end() sequence. > If anything in the sequence sleeps, including the caller or a different > notifier, holding the spinlock would be disastrous. > > For notifiers that _disallow_ blocking, e.g. OOM reaping, simply go down > the slow path of unconditionally acquiring mmu_lock. The sane > alternative would be to try to acquire the lock and force the notifier > to retry on failure. But since OOM is currently the _only_ scenario > where blocking is disallowed attempting to optimize a guest that has been > marked for death is pointless. > > Unconditionally define and use mmu_notifier_slots_lock in the memslots > code, purely to avoid more #ifdefs. The overhead of acquiring the lock > is negligible when the lock is uncontested, which will always be the case > when the MMU notifiers are not used. > > Note, technically flag-only memslot updates could be allowed in parallel, > but stalling a memslot update for a relatively short amount of time is > not a scalability issue, and this is all more than complex enough. Proposal for the locking documentation: diff --git a/Documentation/virt/kvm/locking.rst b/Documentation/virt/kvm/locking.rst index b21a34c34a21..3e4ad7de36cb 100644 --- a/Documentation/virt/kvm/locking.rst +++ b/Documentation/virt/kvm/locking.rst @@ -16,6 +16,13 @@ The acquisition orders for mutexes are as follows: - kvm->slots_lock is taken outside kvm->irq_lock, though acquiring them together is quite rare. +- The kvm->mmu_notifier_slots_lock rwsem ensures that pairs of + invalidate_range_start() and invalidate_range_end() callbacks + use the same memslots array. kvm->slots_lock is taken outside the + write-side critical section of kvm->mmu_notifier_slots_lock, so + MMU notifiers must not take kvm->slots_lock. No other write-side + critical sections should be added. + On x86, vcpu->mutex is taken outside kvm->arch.hyperv.hv_lock. Everything else is a leaf: no other lock is taken inside the critical Paolo
On Fri, Apr 02, 2021, Paolo Bonzini wrote: > On 02/04/21 02:56, Sean Christopherson wrote: > > Avoid taking mmu_lock for unrelated .invalidate_range_{start,end}() > > notifications. Because mmu_notifier_count must be modified while holding > > mmu_lock for write, and must always be paired across start->end to stay > > balanced, lock elision must happen in both or none. To meet that > > requirement, add a rwsem to prevent memslot updates across range_start() > > and range_end(). > > > > Use a rwsem instead of a rwlock since most notifiers _allow_ blocking, > > and the lock will be endl across the entire start() ... end() sequence. > > If anything in the sequence sleeps, including the caller or a different > > notifier, holding the spinlock would be disastrous. > > > > For notifiers that _disallow_ blocking, e.g. OOM reaping, simply go down > > the slow path of unconditionally acquiring mmu_lock. The sane > > alternative would be to try to acquire the lock and force the notifier > > to retry on failure. But since OOM is currently the _only_ scenario > > where blocking is disallowed attempting to optimize a guest that has been > > marked for death is pointless. > > > > Unconditionally define and use mmu_notifier_slots_lock in the memslots > > code, purely to avoid more #ifdefs. The overhead of acquiring the lock > > is negligible when the lock is uncontested, which will always be the case > > when the MMU notifiers are not used. > > > > Note, technically flag-only memslot updates could be allowed in parallel, > > but stalling a memslot update for a relatively short amount of time is > > not a scalability issue, and this is all more than complex enough. > > Proposal for the locking documentation: Argh, sorry! Looks great, I owe you. > diff --git a/Documentation/virt/kvm/locking.rst b/Documentation/virt/kvm/locking.rst > index b21a34c34a21..3e4ad7de36cb 100644 > --- a/Documentation/virt/kvm/locking.rst > +++ b/Documentation/virt/kvm/locking.rst > @@ -16,6 +16,13 @@ The acquisition orders for mutexes are as follows: > - kvm->slots_lock is taken outside kvm->irq_lock, though acquiring > them together is quite rare. > +- The kvm->mmu_notifier_slots_lock rwsem ensures that pairs of > + invalidate_range_start() and invalidate_range_end() callbacks > + use the same memslots array. kvm->slots_lock is taken outside the > + write-side critical section of kvm->mmu_notifier_slots_lock, so > + MMU notifiers must not take kvm->slots_lock. No other write-side > + critical sections should be added. > + > On x86, vcpu->mutex is taken outside kvm->arch.hyperv.hv_lock. > Everything else is a leaf: no other lock is taken inside the critical > > Paolo >
On Fri, 2 Apr 2021 at 08:59, Sean Christopherson <seanjc@google.com> wrote: > > Avoid taking mmu_lock for unrelated .invalidate_range_{start,end}() > notifications. Because mmu_notifier_count must be modified while holding > mmu_lock for write, and must always be paired across start->end to stay > balanced, lock elision must happen in both or none. To meet that > requirement, add a rwsem to prevent memslot updates across range_start() > and range_end(). > > Use a rwsem instead of a rwlock since most notifiers _allow_ blocking, > and the lock will be endl across the entire start() ... end() sequence. > If anything in the sequence sleeps, including the caller or a different > notifier, holding the spinlock would be disastrous. > > For notifiers that _disallow_ blocking, e.g. OOM reaping, simply go down > the slow path of unconditionally acquiring mmu_lock. The sane > alternative would be to try to acquire the lock and force the notifier > to retry on failure. But since OOM is currently the _only_ scenario > where blocking is disallowed attempting to optimize a guest that has been > marked for death is pointless. > > Unconditionally define and use mmu_notifier_slots_lock in the memslots > code, purely to avoid more #ifdefs. The overhead of acquiring the lock > is negligible when the lock is uncontested, which will always be the case > when the MMU notifiers are not used. > > Note, technically flag-only memslot updates could be allowed in parallel, > but stalling a memslot update for a relatively short amount of time is > not a scalability issue, and this is all more than complex enough. > > Based heavily on code from Ben Gardon. > > Suggested-by: Ben Gardon <bgardon@google.com> > Signed-off-by: Sean Christopherson <seanjc@google.com> I saw this splatting: ====================================================== WARNING: possible circular locking dependency detected 5.12.0-rc3+ #6 Tainted: G OE ------------------------------------------------------ qemu-system-x86/3069 is trying to acquire lock: ffffffff9c775ca0 (mmu_notifier_invalidate_range_start){+.+.}-{0:0}, at: __mmu_notifier_invalidate_range_end+0x5/0x190 but task is already holding lock: ffffaff7410a9160 (&kvm->mmu_notifier_slots_lock){.+.+}-{3:3}, at: kvm_mmu_notifier_invalidate_range_start+0x36d/0x4f0 [kvm] which lock already depends on the new lock. the existing dependency chain (in reverse order) is: -> #1 (&kvm->mmu_notifier_slots_lock){.+.+}-{3:3}: down_read+0x48/0x250 kvm_mmu_notifier_invalidate_range_start+0x36d/0x4f0 [kvm] __mmu_notifier_invalidate_range_start+0xe8/0x260 wp_page_copy+0x82b/0xa30 do_wp_page+0xde/0x420 __handle_mm_fault+0x935/0x1230 handle_mm_fault+0x179/0x420 do_user_addr_fault+0x1b3/0x690 exc_page_fault+0x82/0x2b0 asm_exc_page_fault+0x1e/0x30 -> #0 (mmu_notifier_invalidate_range_start){+.+.}-{0:0}: __lock_acquire+0x110f/0x1980 lock_acquire+0x1bc/0x400 __mmu_notifier_invalidate_range_end+0x47/0x190 wp_page_copy+0x796/0xa30 do_wp_page+0xde/0x420 __handle_mm_fault+0x935/0x1230 handle_mm_fault+0x179/0x420 do_user_addr_fault+0x1b3/0x690 exc_page_fault+0x82/0x2b0 asm_exc_page_fault+0x1e/0x30 other info that might help us debug this: Possible unsafe locking scenario: CPU0 CPU1 ---- ---- lock(&kvm->mmu_notifier_slots_lock); lock(mmu_notifier_invalidate_range_start); lock(&kvm->mmu_notifier_slots_lock); lock(mmu_notifier_invalidate_range_start); *** DEADLOCK *** 2 locks held by qemu-system-x86/3069: #0: ffff9e4269f8a9e0 (&mm->mmap_lock#2){++++}-{3:3}, at: do_user_addr_fault+0x10e/0x690 #1: ffffaff7410a9160 (&kvm->mmu_notifier_slots_lock){.+.+}-{3:3}, at: kvm_mmu_notifier_invalidate_range_start+0x36d/0x4f0 [kvm] stack backtrace: CPU: 0 PID: 3069 Comm: qemu-system-x86 Tainted: G OE 5.12.0-rc3+ #6 Hardware name: LENOVO ThinkCentre M8500t-N000/SHARKBAY, BIOS FBKTC1AUS 02/16/2016 Call Trace: dump_stack+0x87/0xb7 print_circular_bug.isra.39+0x1b4/0x210 check_noncircular+0x103/0x150 __lock_acquire+0x110f/0x1980 ? __lock_acquire+0x110f/0x1980 lock_acquire+0x1bc/0x400 ? __mmu_notifier_invalidate_range_end+0x5/0x190 ? find_held_lock+0x40/0xb0 __mmu_notifier_invalidate_range_end+0x47/0x190 ? __mmu_notifier_invalidate_range_end+0x5/0x190 wp_page_copy+0x796/0xa30 do_wp_page+0xde/0x420 __handle_mm_fault+0x935/0x1230 handle_mm_fault+0x179/0x420 do_user_addr_fault+0x1b3/0x690 ? rcu_read_lock_sched_held+0x4f/0x80 exc_page_fault+0x82/0x2b0 ? asm_exc_page_fault+0x8/0x30 asm_exc_page_fault+0x1e/0x30 RIP: 0033:0x55f5bef2560f
diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h index 40ac2d40bb5a..bc3dd2838bb8 100644 --- a/include/linux/kvm_host.h +++ b/include/linux/kvm_host.h @@ -472,6 +472,7 @@ struct kvm { #endif /* KVM_HAVE_MMU_RWLOCK */ struct mutex slots_lock; + struct rw_semaphore mmu_notifier_slots_lock; struct mm_struct *mm; /* userspace tied to this vm */ struct kvm_memslots __rcu *memslots[KVM_ADDRESS_SPACE_NUM]; struct kvm_vcpu *vcpus[KVM_MAX_VCPUS]; @@ -660,8 +661,9 @@ static inline struct kvm_memslots *__kvm_memslots(struct kvm *kvm, int as_id) { as_id = array_index_nospec(as_id, KVM_ADDRESS_SPACE_NUM); return srcu_dereference_check(kvm->memslots[as_id], &kvm->srcu, - lockdep_is_held(&kvm->slots_lock) || - !refcount_read(&kvm->users_count)); + lockdep_is_held(&kvm->slots_lock) || + lockdep_is_held(&kvm->mmu_notifier_slots_lock) || + !refcount_read(&kvm->users_count)); } static inline struct kvm_memslots *kvm_memslots(struct kvm *kvm) diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c index f6697ad741ed..af28f39817a5 100644 --- a/virt/kvm/kvm_main.c +++ b/virt/kvm/kvm_main.c @@ -462,6 +462,7 @@ struct kvm_hva_range { pte_t pte; hva_handler_t handler; on_lock_fn_t on_lock; + bool must_lock; bool flush_on_ret; bool may_block; }; @@ -479,6 +480,25 @@ static void kvm_null_fn(void) } #define IS_KVM_NULL_FN(fn) ((fn) == (void *)kvm_null_fn) + +/* Acquire mmu_lock if necessary. Returns %true if @handler is "null" */ +static __always_inline bool kvm_mmu_lock_and_check_handler(struct kvm *kvm, + const struct kvm_hva_range *range, + bool *locked) +{ + if (*locked) + return false; + + *locked = true; + + KVM_MMU_LOCK(kvm); + + if (!IS_KVM_NULL_FN(range->on_lock)) + range->on_lock(kvm, range->start, range->end); + + return IS_KVM_NULL_FN(range->handler); +} + static __always_inline int __kvm_handle_hva_range(struct kvm *kvm, const struct kvm_hva_range *range) { @@ -495,16 +515,9 @@ static __always_inline int __kvm_handle_hva_range(struct kvm *kvm, idx = srcu_read_lock(&kvm->srcu); - /* The on_lock() path does not yet support lock elision. */ - if (!IS_KVM_NULL_FN(range->on_lock)) { - locked = true; - KVM_MMU_LOCK(kvm); - - range->on_lock(kvm, range->start, range->end); - - if (IS_KVM_NULL_FN(range->handler)) - goto out_unlock; - } + if (range->must_lock && + kvm_mmu_lock_and_check_handler(kvm, range, &locked)) + goto out_unlock; for (i = 0; i < KVM_ADDRESS_SPACE_NUM; i++) { slots = __kvm_memslots(kvm, i); @@ -534,10 +547,9 @@ static __always_inline int __kvm_handle_hva_range(struct kvm *kvm, gfn_range.end = hva_to_gfn_memslot(hva_end + PAGE_SIZE - 1, slot); gfn_range.slot = slot; - if (!locked) { - locked = true; - KVM_MMU_LOCK(kvm); - } + if (kvm_mmu_lock_and_check_handler(kvm, range, &locked)) + goto out_unlock; + ret |= range->handler(kvm, &gfn_range); } } @@ -568,6 +580,7 @@ static __always_inline int kvm_handle_hva_range(struct mmu_notifier *mn, .pte = pte, .handler = handler, .on_lock = (void *)kvm_null_fn, + .must_lock = false, .flush_on_ret = true, .may_block = false, }; @@ -587,6 +600,7 @@ static __always_inline int kvm_handle_hva_range_no_flush(struct mmu_notifier *mn .pte = __pte(0), .handler = handler, .on_lock = (void *)kvm_null_fn, + .must_lock = false, .flush_on_ret = false, .may_block = false, }; @@ -603,11 +617,15 @@ static void kvm_mmu_notifier_change_pte(struct mmu_notifier *mn, trace_kvm_set_spte_hva(address); /* - * .change_pte() must be bookended by .invalidate_range_{start,end}(), - * and so always runs with an elevated notifier count. This obviates - * the need to bump the sequence count. + * .change_pte() must be bookended by .invalidate_range_{start,end}(). + * If mmu_notifier_count is zero, then start() didn't find a relevant + * memslot and wasn't forced down the slow path; rechecking here is + * unnecessary. This can only occur if memslot updates are blocked. */ - WARN_ON_ONCE(!kvm->mmu_notifier_count); + if (!kvm->mmu_notifier_count) { + lockdep_assert_held(&kvm->mmu_notifier_slots_lock); + return; + } kvm_handle_hva_range(mn, address, address + 1, pte, kvm_set_spte_gfn); } @@ -644,6 +662,7 @@ static void kvm_inc_notifier_count(struct kvm *kvm, unsigned long start, static int kvm_mmu_notifier_invalidate_range_start(struct mmu_notifier *mn, const struct mmu_notifier_range *range) { + bool blockable = mmu_notifier_range_blockable(range); struct kvm *kvm = mmu_notifier_to_kvm(mn); const struct kvm_hva_range hva_range = { .start = range->start, @@ -651,12 +670,29 @@ static int kvm_mmu_notifier_invalidate_range_start(struct mmu_notifier *mn, .pte = __pte(0), .handler = kvm_unmap_gfn_range, .on_lock = kvm_inc_notifier_count, + .must_lock = !blockable, .flush_on_ret = true, - .may_block = mmu_notifier_range_blockable(range), + .may_block = blockable, }; trace_kvm_unmap_hva_range(range->start, range->end); + /* + * Prevent memslot modification between range_start() and range_end() + * so that conditionally locking provides the same result in both + * functions. Without that guarantee, the mmu_notifier_count + * adjustments will be imbalanced. + * + * Skip the memslot-lookup lock elision (set @must_lock above) to avoid + * having to take the semaphore on non-blockable calls, e.g. OOM kill. + * The complexity required to handle conditional locking for this case + * is not worth the marginal benefits, the VM is likely doomed anyways. + * + * Pairs with the unlock in range_end(). + */ + if (blockable) + down_read(&kvm->mmu_notifier_slots_lock); + __kvm_handle_hva_range(kvm, &hva_range); return 0; @@ -683,6 +719,7 @@ static void kvm_dec_notifier_count(struct kvm *kvm, unsigned long start, static void kvm_mmu_notifier_invalidate_range_end(struct mmu_notifier *mn, const struct mmu_notifier_range *range) { + bool blockable = mmu_notifier_range_blockable(range); struct kvm *kvm = mmu_notifier_to_kvm(mn); const struct kvm_hva_range hva_range = { .start = range->start, @@ -690,12 +727,17 @@ static void kvm_mmu_notifier_invalidate_range_end(struct mmu_notifier *mn, .pte = __pte(0), .handler = (void *)kvm_null_fn, .on_lock = kvm_dec_notifier_count, + .must_lock = !blockable, .flush_on_ret = true, - .may_block = mmu_notifier_range_blockable(range), + .may_block = blockable, }; __kvm_handle_hva_range(kvm, &hva_range); + /* Pairs with the lock in range_start(). */ + if (blockable) + up_read(&kvm->mmu_notifier_slots_lock); + BUG_ON(kvm->mmu_notifier_count < 0); } @@ -908,6 +950,7 @@ static struct kvm *kvm_create_vm(unsigned long type) mutex_init(&kvm->lock); mutex_init(&kvm->irq_lock); mutex_init(&kvm->slots_lock); + init_rwsem(&kvm->mmu_notifier_slots_lock); INIT_LIST_HEAD(&kvm->devices); BUILD_BUG_ON(KVM_MEM_SLOTS_NUM > SHRT_MAX); @@ -1028,6 +1071,16 @@ static void kvm_destroy_vm(struct kvm *kvm) kvm_coalesced_mmio_free(kvm); #if defined(CONFIG_MMU_NOTIFIER) && defined(KVM_ARCH_WANT_MMU_NOTIFIER) mmu_notifier_unregister(&kvm->mmu_notifier, kvm->mm); + /* + * Reset the lock used to prevent memslot updates between MMU notifier + * invalidate_range_start() and invalidate_range_end(). At this point, + * no more MMU notifiers will run and pending calls to ...start() have + * completed. But, the lock could still be held if KVM's notifier was + * removed between ...start() and ...end(). No threads can be waiting + * on the lock as the last reference on KVM has been dropped. If the + * lock is still held, freeing memslots will deadlock. + */ + init_rwsem(&kvm->mmu_notifier_slots_lock); #else kvm_arch_flush_shadow_all(kvm); #endif @@ -1279,7 +1332,10 @@ static struct kvm_memslots *install_new_memslots(struct kvm *kvm, WARN_ON(gen & KVM_MEMSLOT_GEN_UPDATE_IN_PROGRESS); slots->generation = gen | KVM_MEMSLOT_GEN_UPDATE_IN_PROGRESS; + down_write(&kvm->mmu_notifier_slots_lock); rcu_assign_pointer(kvm->memslots[as_id], slots); + up_write(&kvm->mmu_notifier_slots_lock); + synchronize_srcu_expedited(&kvm->srcu); /*
Avoid taking mmu_lock for unrelated .invalidate_range_{start,end}() notifications. Because mmu_notifier_count must be modified while holding mmu_lock for write, and must always be paired across start->end to stay balanced, lock elision must happen in both or none. To meet that requirement, add a rwsem to prevent memslot updates across range_start() and range_end(). Use a rwsem instead of a rwlock since most notifiers _allow_ blocking, and the lock will be endl across the entire start() ... end() sequence. If anything in the sequence sleeps, including the caller or a different notifier, holding the spinlock would be disastrous. For notifiers that _disallow_ blocking, e.g. OOM reaping, simply go down the slow path of unconditionally acquiring mmu_lock. The sane alternative would be to try to acquire the lock and force the notifier to retry on failure. But since OOM is currently the _only_ scenario where blocking is disallowed attempting to optimize a guest that has been marked for death is pointless. Unconditionally define and use mmu_notifier_slots_lock in the memslots code, purely to avoid more #ifdefs. The overhead of acquiring the lock is negligible when the lock is uncontested, which will always be the case when the MMU notifiers are not used. Note, technically flag-only memslot updates could be allowed in parallel, but stalling a memslot update for a relatively short amount of time is not a scalability issue, and this is all more than complex enough. Based heavily on code from Ben Gardon. Suggested-by: Ben Gardon <bgardon@google.com> Signed-off-by: Sean Christopherson <seanjc@google.com> --- include/linux/kvm_host.h | 6 ++- virt/kvm/kvm_main.c | 96 +++++++++++++++++++++++++++++++--------- 2 files changed, 80 insertions(+), 22 deletions(-)