Re: [RFC PATCH v7 30/31] x86/mm, mm/vmalloc: Defer kernel TLB flush IPIs under CONFIG_COALESCE_TLBI=y

Dave Hansen
2025-11-21T17:50:16Z

On 11/21/25 09:37, Valentin Schneider wrote:
> On 19/11/25 10:31, Dave Hansen wrote:
>> On 11/14/25 07:14, Valentin Schneider wrote:
>>> +static bool flush_tlb_kernel_cond(int cpu, void *info)
>>> +{
>>> + return housekeeping_cpu(cpu, HK_TYPE_KERNEL_NOISE) ||
>>> + per_cpu(kernel_cr3_loaded, cpu);
>>> +}
>>
>> Is it OK that 'kernel_cr3_loaded' can be be stale? Since it's not part
>> of the instruction that actually sets CR3, there's a window between when
>> 'kernel_cr3_loaded' is set (or cleared) and CR3 is actually written.
>>
>> Is that OK?
>>
>> It seems like it could lead to both unnecessary IPIs being sent and for
>> IPIs to be missed.
>>
>
> So the pattern is
>
> SWITCH_TO_KERNEL_CR3
> FLUSH
> KERNEL_CR3_LOADED := 1
>
> KERNEL_CR3_LOADED := 0
> SWITCH_TO_USER_CR3
>
>
> The 0 -> 1 transition has a window between the unconditional flush and the
> write to 1 where a remote flush IPI may be omitted. Given that the write is
> immediately following the unconditional flush, that would really be just
> two flushes racing with each other,

Let me fix that for you. When you wrote "a remote flush IPI may be
omitted" you meant to write: "there's a bug." ;)

In the end, KERNEL_CR3_LOADED==0 means, "you don't need to send this CPU
flushing IPIs because it will flush the TLB itself before touching
memory that needs a flush".

SWITCH_TO_KERNEL_CR3
FLUSH
// On kernel CR3, *AND* not getting IPIs
KERNEL_CR3_LOADED := 1

> but I could punt the kernel_cr3_loaded
> write above the unconditional flush.

Yes, that would eliminate the window, as long as the memory ordering is
right. You not only need to have the KERNEL_CR3_LOADED:=1 CPU set that
variable, you need to ensure that it has seen the page table update.

> The 1 -> 0 transition is less problematic, worst case a remote flush races
> with the CPU returning to userspace and it'll get interrupted back to
> kernelspace.

It's also not just "returning to userspace". It could well be *in*
userspace by the point the IPI shows up. It's not the end of the world,
and the window isn't infinitely long. But there certainly is still a
possibility of getting spurious interrupts for the precious NOHZ_FULL
task while it's in userspace.