Re: [PATCH v4 29/30] x86/mm, mm/vmalloc: Defer flush_tlb_kernel_range() targeting NOHZ_FULL CPUs

Jann Horn
2025-01-17T15:52:57Z

On Fri, Jan 17, 2025 at 4:25 PM Valentin Schneider <vschneid@redhat.com> wrote:
> On 14/01/25 19:16, Jann Horn wrote:
> > On Tue, Jan 14, 2025 at 6:51 PM Valentin Schneider <vschneid@redhat.com> 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.
> >>
> >> Deferral vs early entry danger zone
> >> ===================================
> >>
> >> This requires a guarantee that nothing in the vmalloc range can be vunmap'd
> >> and then accessed in early entry code.
> >
> > In other words, it needs a guarantee that no vmalloc allocations that
> > have been created in the vmalloc region while the CPU was idle can
> > then be accessed during early entry, right?
>
> I'm not sure if that would be a problem (not an mm expert, please do
> correct me) - looking at vmap_pages_range(), flush_cache_vmap() isn't
> deferred anyway.

flush_cache_vmap() is about stuff like flushing data caches on
architectures with virtually indexed caches; that doesn't do TLB
maintenance. When you look for its definition on x86 or arm64, you'll
see that they use the generic implementation which is simply an empty
inline function.

> So after vmapping something, I wouldn't expect isolated CPUs to have
> invalid TLB entries for the newly vmapped page.
>
> However, upon vunmap'ing something, the TLB flush is deferred, and thus
> stale TLB entries can and will remain on isolated CPUs, up until they
> execute the deferred flush themselves (IOW for the entire duration of the
> "danger zone").
>
> Does that make sense?

The design idea wrt TLB flushes in the vmap code is that you don't do
TLB flushes when you unmap stuff or when you map stuff, because doing
TLB flushes across the entire system on every vmap/vunmap would be a
bit costly; instead you just do batched TLB flushes in between, in
__purge_vmap_area_lazy().

In other words, the basic idea is that you can keep calling vmap() and
vunmap() a bunch of times without ever doing TLB flushes until you run
out of virtual memory in the vmap region; then you do one big TLB
flush, and afterwards you can reuse the free virtual address space for
new allocations again.

So if you "defer" that batched TLB flush for CPUs that are not
currently running in the kernel, I think the consequence is that those
CPUs may end up with incoherent TLB state after a reallocation of the
virtual address space.

Actually, I think this would mean that your optimization is disallowed
at least on arm64 - I'm not sure about the exact wording, but arm64
has a "break before make" rule that forbids conflicting writable
address translations or something like that.

(I said "until you run out of virtual memory in the vmap region", but
that's not actually true - see the comment above lazy_max_pages() for
an explanation of the actual heuristic. You might be able to tune that
a bit if you'd be significantly happier with less frequent
interruptions, or something along those lines.)