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  1. 06 Feb, 2024 3 commits
    • Sean Christopherson's avatar
      KVM: Get reference to VM's address space in the async #PF worker · 8284765f
      Sean Christopherson authored 
      Get a reference to the target VM's address space in async_pf_execute()
instead of gifting a reference from kvm_setup_async_pf().  Keeping the
address space alive just to service an async #PF is counter-productive,
i.e. if the process is exiting and all vCPUs are dead, then NOT doing
get_user_pages_remote() and freeing the address space asap is desirable.

Handling the mm reference entirely within async_pf_execute() also
simplifies the async #PF flows as a whole, e.g. it's not immediately
obvious when the worker task vs. the vCPU task is responsible for putting
the gifted mm reference.
Reviewed-by: default avatarVitaly Kuznetsov <vkuznets@redhat.com>
Reviewed-by: default avatarXu Yilun <yilun.xu@intel.com>
Link: https://lore.kernel.org/r/20240110011533.503302-4-seanjc@google.comSigned-off-by: default avatarSean Christopherson <seanjc@google.com>
      8284765f
    • Sean Christopherson's avatar
      KVM: Put mm immediately after async #PF worker completes remote gup() · 422eeb54
      Sean Christopherson authored 
      Put the async #PF worker's reference to the VM's address space as soon as
the worker is done with the mm.  This will allow deferring getting a
reference to the worker itself without having to track whether or not
getting a reference succeeded.

Note, if the vCPU is still alive, there is no danger of the worker getting
stuck with tearing down the host page tables, as userspace also holds a
reference (obviously), i.e. there is no risk of delaying the page-present
notification due to triggering the slow path in mmput().
Reviewed-by: default avatarVitaly Kuznetsov <vkuznets@redhat.com>
Reviewed-by: default avatarXu Yilun <yilun.xu@intel.com>
Link: https://lore.kernel.org/r/20240110011533.503302-3-seanjc@google.comSigned-off-by: default avatarSean Christopherson <seanjc@google.com>
      422eeb54
    • Sean Christopherson's avatar
      KVM: Always flush async #PF workqueue when vCPU is being destroyed · 3d75b8aa
      Sean Christopherson authored 
      Always flush the per-vCPU async #PF workqueue when a vCPU is clearing its
completion queue, e.g. when a VM and all its vCPUs is being destroyed.
KVM must ensure that none of its workqueue callbacks is running when the
last reference to the KVM _module_ is put.  Gifting a reference to the
associated VM prevents the workqueue callback from dereferencing freed
vCPU/VM memory, but does not prevent the KVM module from being unloaded
before the callback completes.

Drop the misguided VM refcount gifting, as calling kvm_put_kvm() from
async_pf_execute() if kvm_put_kvm() flushes the async #PF workqueue will
result in deadlock.  async_pf_execute() can't return until kvm_put_kvm()
finishes, and kvm_put_kvm() can't return until async_pf_execute() finishes:

 WARNING: CPU: 8 PID: 251 at virt/kvm/kvm_main.c:1435 kvm_put_kvm+0x2d/0x320 [kvm]
 Modules linked in: vhost_net vhost vhost_iotlb tap kvm_intel kvm irqbypass
 CPU: 8 PID: 251 Comm: kworker/8:1 Tainted: G        W          6.6.0-rc1-e7af8d17224a-x86/gmem-vm #119
 Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0 02/06/2015
 Workqueue: events async_pf_execute [kvm]
 RIP: 0010:kvm_put_kvm+0x2d/0x320 [kvm]
 Call Trace:
  <TASK>
  async_pf_execute+0x198/0x260 [kvm]
  process_one_work+0x145/0x2d0
  worker_thread+0x27e/0x3a0
  kthread+0xba/0xe0
  ret_from_fork+0x2d/0x50
  ret_from_fork_asm+0x11/0x20
  </TASK>
 ---[ end trace 0000000000000000 ]---
 INFO: task kworker/8:1:251 blocked for more than 120 seconds.
       Tainted: G        W          6.6.0-rc1-e7af8d17224a-x86/gmem-vm #119
 "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
 task:kworker/8:1     state:D stack:0     pid:251   ppid:2      flags:0x00004000
 Workqueue: events async_pf_execute [kvm]
 Call Trace:
  <TASK>
  __schedule+0x33f/0xa40
  schedule+0x53/0xc0
  schedule_timeout+0x12a/0x140
  __wait_for_common+0x8d/0x1d0
  __flush_work.isra.0+0x19f/0x2c0
  kvm_clear_async_pf_completion_queue+0x129/0x190 [kvm]
  kvm_arch_destroy_vm+0x78/0x1b0 [kvm]
  kvm_put_kvm+0x1c1/0x320 [kvm]
  async_pf_execute+0x198/0x260 [kvm]
  process_one_work+0x145/0x2d0
  worker_thread+0x27e/0x3a0
  kthread+0xba/0xe0
  ret_from_fork+0x2d/0x50
  ret_from_fork_asm+0x11/0x20
  </TASK>

If kvm_clear_async_pf_completion_queue() actually flushes the workqueue,
then there's no need to gift async_pf_execute() a reference because all
invocations of async_pf_execute() will be forced to complete before the
vCPU and its VM are destroyed/freed.  And that in turn fixes the module
unloading bug as __fput() won't do module_put() on the last vCPU reference
until the vCPU has been freed, e.g. if closing the vCPU file also puts the
last reference to the KVM module.

Note that kvm_check_async_pf_completion() may also take the work item off
the completion queue and so also needs to flush the work queue, as the
work will not be seen by kvm_clear_async_pf_completion_queue().  Waiting
on the workqueue could theoretically delay a vCPU due to waiting for the
work to complete, but that's a very, very small chance, and likely a very
small delay.  kvm_arch_async_page_present_queued() unconditionally makes a
new request, i.e. will effectively delay entering the guest, so the
remaining work is really just:

        trace_kvm_async_pf_completed(addr, cr2_or_gpa);

        __kvm_vcpu_wake_up(vcpu);

        mmput(mm);

and mmput() can't drop the last reference to the page tables if the vCPU is
still alive, i.e. the vCPU won't get stuck tearing down page tables.

Add a helper to do the flushing, specifically to deal with "wakeup all"
work items, as they aren't actually work items, i.e. are never placed in a
workqueue.  Trying to flush a bogus workqueue entry rightly makes
__flush_work() complain (kudos to whoever added that sanity check).

Note, commit 5f6de5cb ("KVM: Prevent module exit until all VMs are
freed") *tried* to fix the module refcounting issue by having VMs grab a
reference to the module, but that only made the bug slightly harder to hit
as it gave async_pf_execute() a bit more time to complete before the KVM
module could be unloaded.

Fixes: af585b92 ("KVM: Halt vcpu if page it tries to access is swapped out")
Cc: stable@vger.kernel.org
Cc: David Matlack <dmatlack@google.com>
Reviewed-by: default avatarXu Yilun <yilun.xu@intel.com>
Reviewed-by: default avatarVitaly Kuznetsov <vkuznets@redhat.com>
Link: https://lore.kernel.org/r/20240110011533.503302-2-seanjc@google.comSigned-off-by: default avatarSean Christopherson <seanjc@google.com>
      3d75b8aa
  3. 29 Jan, 2024 1 commit
  5. 28 Jan, 2024 7 commits
  7. 27 Jan, 2024 9 commits
  9. 26 Jan, 2024 20 commits
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