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

Marcelo Tosatti
2023-07-25T16:38:52Z

On Mon, Jul 24, 2023 at 10:40:04AM -0700, Dave Hansen wrote:
> On 7/24/23 04:32, Valentin Schneider wrote:
> > AFAICT the only reasonable way to go about the deferral is to prove that no
> > such access happens before the deferred @operation is done. We got to prove
> > that for sync_core() deferral, cf. PATCH 18.
> >
> > I'd like to reason about it for deferring vunmap TLB flushes:
> >
> > What addresses in VMAP range, other than the stack, can early entry code
> > access? Yes, the ranges can be checked at runtime, but is there any chance
> > of figuring this out e.g. at build-time?
>
> Nadav was touching on a very important point: TLB flushes for addresses
> are relatively easy to defer. You just need to ensure that the CPU
> deferring the flush does an actual flush before it might architecturally
> consume the contents of the flushed entry.
>
> TLB flushes for freed page tables are another game entirely. The CPU is
> free to cache any part of the paging hierarchy it wants at any time.

Depend on CONFIG_PAGE_TABLE_ISOLATION=y, which flushes TLB (and page
table caches) on user->kernel and kernel->user context switches ?

So freeing a kernel pagetable page does not require interrupting a CPU
which is in userspace (therefore does not have visibility into kernel
pagetables).

> It's also free to set accessed and dirty bits at any time, even for
> instructions that may never execute architecturally.
>
> That basically means that if you have *ANY* freed page table page
> *ANYWHERE* in the page table hierarchy of any CPU at any time ... you're
> screwed.
>
> There's no reasoning about accesses or ordering. As soon as the CPU
> does *anything*, it's out to get you.
>
> You're going to need to do something a lot more radical to deal with
> free page table pages.