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

On Mon, Jul 24, 2023 at 10:40:04AM -0700, Dave Hansen wrote:

On 7/24/23 04:32, Valentin Schneider wrote:

quoted

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.