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

Valentin Schneider
2025-01-17T16:53:40Z

On 17/01/25 16:52, Jann Horn wrote:
> 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.
>

Ah, gotcha, thank you for laying this out! In which case yes, any vmalloc
that occurred while an isolated CPU was NOHZ-FULL can be an issue if said
CPU accesses it during early entry;

> 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.
>

On the bright side of things, arm64 is not as bad as x86 when it comes to
IPI'ing isolated CPUs :-) I'll add that to my notes, thanks!

> (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.)