Re: [RFC PATCH v2 20/20] x86/mm, mm/vmalloc: Defer flush_tlb_kernel_range() targeting NOHZ_FULL CPUs

[Nadav Amit]

> On Jul 20, 2023, at 9:30 AM, Valentin Schneider <vschneid@xxxxxxxxxx> wrote:
> 
> vunmap()'s issued from housekeeping CPUs are a relatively common source of
> interference for isolated NOHZ_FULL CPUs, as they are hit by the
> flush_tlb_kernel_range() IPIs.
> 
> Given that CPUs executing in userspace do not access data in the vmalloc
> range, these IPIs could be deferred until their next kernel entry.

So I think there are a few assumptions here that it seems suitable to confirm
and acknowledge the major one in the commit log (assuming they hold).

There is an assumption that VMAP page-tables are not freed. I actually
never paid attention to that, but skimming the code it does seem so. To
clarify the issue: if page-tables were freed and their pages were reused,
there would be a problem that page-walk caches for instance would be used
and “junk” entries from the reused pages would be used. See [1].

I would also assume the memory-hot-unplug of some sorts is not an issue,
(i.e., you cannot have a stale TLB entry pointing to memory that was
unplugged).  

I also think that there might be speculative code execution using stale
TLB entries that would point to memory that has been reused and perhaps
controllable by the user. If somehow the CPU/OS is tricked to use the
stale executable TLB entries early enough on kernel entry that might be
an issue. I guess it is probably theoretical issue, but it would be helpful
to confirm.

In general, deferring TLB flushes can be done safely. This patch, I think,
takes it one step forward and allows the reuse of the memory before the TLB
flush is actually done. This is more dangerous.

[1] https://lore.kernel.org/lkml/tip-b956575bed91ecfb136a8300742ecbbf451471ab@xxxxxxxxxxxxxx/