Re: [PATCH v2 09/10] KVM: Don't take mmu_lock for range invalidation unless necessary

[Wanpeng Li]

On Fri, 2 Apr 2021 at 08:59, Sean Christopherson <seanjc@xxxxxxxxxx> 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@xxxxxxxxxx>
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>

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