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

["M. Vefa Bicakci" <m.v.b@runbox.com>]

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?

Thank you,

Vefa