linux-kernel - Re: [RFC PATCH v2 20/20] x86/mm, mm/vmalloc: Defer flush_tlb_kernel_range() targeting NOHZ

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Date:   Tue, 25 Jul 2023 13:37:32 -0300
From:   Marcelo Tosatti <mtosatti@...hat.com>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Valentin Schneider <vschneid@...hat.com>,
        Nadav Amit <namit@...are.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
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Subject: 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:
> > 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.