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

["David Hildenbrand (Arm)" <david@kernel.org>]

On 2/5/26 19:36, Dave Hansen wrote:
> On 2/5/26 09:06, David Hildenbrand (Arm) wrote:
>>> But can't we RCU-free the page table?  Why do we need to wait for the
>>> RCU readers to finish?
>>
>> For unsharing hugetlb PMD tables the problem is not the freeing but the
>> reuse of the PMD table for other purposes in the last remaining user.
>> It's complicated.
> 
> Letting the previously-shared table get released to everything else in
> the system sounds like a fixable problem. tlb_flush_unshared_tables()
> talks about this, and it makes sense that once locks get dropped that
> something else could get mapped in and start using the PMD.

Yeah, I tried to document that carefully.

> 
> The RCU way of fixing that would be to allocate new page table, replace
> the old one, and RCU-free the old one. Read, Copy, Update. :)
> 
> It does temporarily eat up an extra page, and cost an extra copy. But
> neither of those seems expensive compared to IPI'ing the world.

I played with many such ideas, including never reusing a page table 
again once it was once shared. All turned out rather horrible.

RCU-way: replacing a shared page table involves updating all processes 
that share the page table :/ . I think another issue I stumbled into 
while trying to implement was around failing to allocate memory (but 
being required to make progress). It all turned to quite some complexity 
and inefficiency, so I had to give up on that. :)

> 
>> For page table freeing, we only do it if we fail to allocate memory --
>> if we cannot use RCU IIRC.
> 
> But that case is fine to be slow and use synchronize_rcu(). If you're
> failing to allocate a single page, you're in a way slow path anyway.
That's true. We could likely do that already and avoid the IPI broadcast 
there that was once reported to be a problem for RT applications.

-- 
Cheers,

David