Re: [PATCH v4 0/3] targeted TLB sync IPIs for lockless page table

[Lance Yang <lance.yang@linux.dev>]

On 2026/2/5 23:41, Dave Hansen wrote:
> On 2/5/26 07:31, Lance Yang wrote:
>> On 2026/2/5 23:09, Dave Hansen wrote:
>>> On 2/5/26 07:01, Lance Yang wrote:
>>>> So for now, neither approach looks good: tracking on the read side adss
>>>> cost to GUP-fast, and syncing on the write side e.g. synchronize_rcu()
>>>> is too slow on large systems.
>>>
>>> Which of the writers truly *need* synchronize_rcu()?
>>>
>>> What are they doing with the memory that they can't move forward unless
>>> it's quiescent *now*?
>>
>> Without IPIs or synchronize_rcu(), IIUC, we have no way to know if there
>> are ongoing concurrent lockless page-table walks — the walkers just disable
>> IRQs and walk.
> 
> Yeah, but one aim of RCU is ensuring that readers see valid data but not
> necessarily the most up to date data.
> 
> Are there cases where ongoing concurrent lockless page-table walks need
> to see the writes and they can't tolerate seeing valid but slightly
> stale data?

The issue is we're about to free the page table (e.g. 
pmdp_collapse_flush()).

We have to ensure no walker is still doing a lockless page-table walk
when the page directories are freed, otherwise we get use-after-free.

> Don't forget that we also have pesky concurrent lockless page-table
> walkers called CPUs. They're extra pesky in that they don't even stop
> for IPIs. ;)

I assume those walkers that don't disable IRQs only read the PMD and
don't walk into the table; otherwise the current sync wouldn't work
for them.

Thanks,
Lance