Re: [PATCH] x86/xen: Fix lazy MMU pause count wrap-around

["Jürgen Groß" <jgross@suse.com>]

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On 09.11.26 19:42, M. Vefa Bicakci wrote:
> On 2026-11-09 01:40, Juergen Gross wrote:
> 
>> On 02.11.26 16:31, M. Vefa Bicakci wrote:
>>> Commit bec6f41a6fe5 ("x86/xen: Get rid of last XEN_LAZY_MMU uses")
>>> unintentionally prevents the activation of lazy MMU mode for most tasks
>>> running on Xen PV VMs (including dom0).
>>>
>>> That commit made xen_end_context_switch() call
>>> __task_lazy_mmu_mode_resume() with the task being switched to. However,
>>> the latter function decrements the pause_count field of the task without
>>> checking whether that task's lazy MMU mode was paused or not. A task
>>> being switched to for the first time has never been switched from, so
>>> attempting to resume lazy MMU mode can wrap-around (underflow) the
>>> pause_count from 0 to 255 (U8_MAX).
>>>
>>> Furthermore, the non-zero pause_count is copied to child tasks at fork,
>>> so all descendants (including the idle tasks of the non-boot CPUs)
>>> inherit the non-zero pause_count. This prevents enabling lazy MMU mode
>>> for them, because lazy_mmu_mode_enable() does not do anything when
>>> pause_count is non-zero.
>>>
>>> Hence, call __task_lazy_mmu_mode_resume() from xen_end_context_switch()
>>> only if lazy MMU mode was paused.
>>>
>>> This issue was discovered with CONFIG_DEBUG_VM=y, which enables
>>> pause_count underflow and overflow warnings. In the examples quoted
>>> below, the first warning shows the underflow with PID 1. The second
>>> warning shows the overflow from 255 (U8_MAX) to 0, also for PID 1, as
>>> indicated by the same RDI register value.
>>>
>>> Example pause_count underflow warning:
>>>
>>>     WARNING: include/linux/pgtable.h:362 at xen_end_context_switch+0x4e/0x60, CPU#0: swapper/0/0
>>>     CPU: 0 UID: 0 PID: 0 Comm: swapper/0 Not tainted 7.2.4-1 #2 PREEMPT(full)
>>>     RIP: e030:xen_end_context_switch+0x4e/0x60
>>>     ...
>>>     RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffff888004930000
>>>
>>> Note: The PID is shown as 0 above, because current_task is updated in
>>> __switch_to() after arch_end_context_switch(). arch_end_context_switch()
>>> calls xen_end_context_switch() indirectly.
>>>
>>> Example pause_count overflow warning:
>>>
>>>     WARNING: include/linux/pgtable.h:319 at xen_start_context_switch+0x64/0x70, CPU#0: swapper/0/1
>>>     CPU: 0 UID: 0 PID: 1 Comm: swapper/0 Tainted: G        W  7.2.4-1 #2 PREEMPT(full)
>>>     ...
>>>     RIP: e030:xen_start_context_switch+0x64/0x70
>>>     ...
>>>     RDX: 0000000000000000 RSI: 0000000013f7894d RDI: ffff888004930000
>>>     ...
>>>     Call Trace:
>>>      <TASK>
>>>      __schedule+0x36a/0x1a90
>>>      schedule+0x27/0xa0
>>>      schedule_timeout+0xbd/0x100
>>>      __wait_for_common+0x97/0x1b0
>>>      wait_for_completion_killable+0x24/0x40
>>>      __kthread_create_on_node+0x13e/0x240
>>>      kthread_create_worker_on_node+0xee/0x1c0
>>>      workqueue_init+0x26/0x420
>>>      kernel_init_freeable+0x101/0x360
>>>      kernel_init+0x1a/0x130
>>>      ret_from_fork+0x2b0/0x340
>>>      ret_from_fork_asm+0x1a/0x30
>>>      </TASK>
>>>
>>> Fixes: bec6f41a6fe5 ("x86/xen: Get rid of last XEN_LAZY_MMU uses")
>>> Cc: stable@vger.kernel.org # 7.2.x
>>> Signed-off-by: M. Vefa Bicakci <m.v.b@runbox.com>
>>> ---
>>> === Testing notes ===
>>> This commit was tested with kernel v7.3-16755-gd135ab453a74 and v7.4-rc1
>>> with a domU PV VM, and versions from v7.2.4 to v7.2.14 with dom0 and
>>> domU PV VMs running on Xen v4.19.5. CONFIG_DEBUG_VM=y was set for all
>>> tests, and no warnings were observed.
>>>
>>> === Debugging notes ===
>>> For debugging, I instrumented xen_start_context_switch(),
>>> xen_end_context_switch() and copy_process(), and I confirmed that the
>>> inherited pause_count value is decremented for each level of the process
>>> tree, starting from 255.
>>>
>>> === Credits ===
>>> The patch was hand-written, but the commit message was polished with a
>>> few rounds of LLM review.
>>> ---
>>>    arch/x86/xen/enlighten_pv.c | 10 +++++++++-
>>>    include/linux/sched.h       | 19 +++++++++++++++++++
>>>    2 files changed, 28 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/arch/x86/xen/enlighten_pv.c b/arch/x86/xen/enlighten_pv.c
>>> index c5117a7730d6..68f8a977999b 100644
>>> --- a/arch/x86/xen/enlighten_pv.c
>>> +++ b/arch/x86/xen/enlighten_pv.c
>>> @@ -431,7 +431,15 @@ static void xen_end_context_switch(struct task_struct *next)
>>>          xen_mc_flush();
>>>        this_cpu_write(xen_cpu_lazy_mode, false);
>>> -    __task_lazy_mmu_mode_resume(next);
>>> +
>>> +    /*
>>> +     * A newly created task can be switched to without ever having been
>>> +     * switched from, so it will not have a prior lazy MMU pause to undo
>>> +     * from xen_start_context_switch(). Hence, resume lazy MMU only if it
>>> +     * was paused.
>>> +     */
>>> +    if (__task_lazy_mmu_mode_paused(next))
>>> +        __task_lazy_mmu_mode_resume(next);
>>>    }
>>>      static unsigned long xen_store_tr(void)
>>> diff --git a/include/linux/sched.h b/include/linux/sched.h
>>> index 1c93da75226e..3f9341f1f827 100644
>>> --- a/include/linux/sched.h
>>> +++ b/include/linux/sched.h
>>> @@ -1791,6 +1791,25 @@ static inline bool is_lazy_mmu_mode_active(void)
>>>          return __task_lazy_mmu_mode_active(current);
>>>    }
>>> +
>>> +/**
>>> + * __task_lazy_mmu_mode_paused() - Return a task's lazy MMU mode pause state
>>> + * @tsk: The task to check.
>>> + *
>>> + * Test whether @tsk has its lazy MMU mode state set to paused
>>> + * (i.e., pause_count > 0).
>>> + *
>>> + * This function is intended for architectures that implement the lazy MMU
>>> + * mode; it must not be called from generic code.
>>> + *
>>> + * Return: Whether task @tsk has paused lazy MMU mode.
>>> + */
>>> +static inline bool __task_lazy_mmu_mode_paused(struct task_struct *tsk)
>>> +{
>>> +    struct lazy_mmu_state *state = &tsk->lazy_mmu_state;
>>> +
>>> +    return state->pause_count > 0;
>>> +}
>>>    #endif
>>>      extern struct pid __rcu *cad_pid;
>> Thanks for finding this issue.
> 
> Thanks for taking the time to review and provide feedback!
> 
>> While your patch is fixing the problem, I'd prefer fixing the root cause:
>> When setting up a new struct task_struct, the initial lazy_mmu_state state
>> should be initialized accordingly. This would require another lazy_mmu_state
>> hook for initializing it, probably via a simple static_call(). Xen could use
>> that for setting the pause count to 1.
>>
>> At the same time __task_lazy_mmu_mode_resume() should probably WARN() if
>> the pause count is 0 already and NOT decrement the pause count.
>>
>> Thoughts?
> 
> I agree that your proposed approach is indeed more elegant.
> 
> To confirm my understanding: Setting every new task's pause count to 1
> will ensure that the first context switch into that task will not underflow
> the pause count. Also, making __task_lazy_mmu_mode_resume() return early
> with a warning if the pause_count is already zero would guard against
> encountering the same issue again.
> 
> Do you intend to take this on, or would you like me to take a stab at this?
I'll take it, thanks for the offering.


Juergen

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