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
