Re: [PATCH 05/31] x86/mm: Reduce tlb flushes from ptep_set_access_flags()

[Linus Torvalds <torvalds@linux-foundation.org>]

On Fri, Oct 26, 2012 at 5:34 AM, Michel Lespinasse <walken@google.com> wrote:
> On Thu, Oct 25, 2012 at 9:23 PM, Linus Torvalds <torvalds@linux-foundation.org> wrote:
>>
>> Yes. It's not architected as far as I know, though. But I agree, it's
>> possible - even likely - we could avoid TLB flushing entirely on x86.
>
> Actually, it is architected on x86. This was first described in the
> intel appnote 317080 "TLBs, Paging-Structure Caches, and Their
> Invalidation", last paragraph of section 5.1. Nowadays, the same
> contents are buried somewhere in Volume 3 of the architecture manual
> (in my copy: 4.10.4.1 Operations that Invalidate TLBs and
> Paging-Structure Caches)

Good. I should have known it must be architected, because we've gone
back-and-forth on this in the kernel historically. We used to have
some TLB invalidates in the faulting path because I wasn't sure
whether they were needed or not, but we clearly don't have them any
more (and I suspect coverage was always spotty).

And Intel (and AMD) have been very good at documenting as architected
these kinds of details that people end up relying on even if they
weren't necessarily originally explicitly documented.

>> I *suspect* that whole TLB flush just magically became an SMP one
>> without anybody ever really thinking about it.
>
> I would be very worried about assuming every non-x86 arch has similar
> TLB semantics. However, if their fault handlers always invalidate TLB
> for pages that get spurious faults, then skipping the remote
> invalidation would be fine. (I believe this is what
> tlb_fix_spurious_fault() is for ?)

Yes. Of course, there may be some case where we unintentionally don't
necessarily flush a faulting address (on some architecture that needs
it), and then removing the cross-cpu invalidate could expose that
pre-existing bug-let, and cause an infinite loop of page faults due to
a TLB entry that never gets invalidated even if the page tables are
actually up-to-date.

So changing the mm/pgtable-generic.c function sounds like the right
thing to do, but would be a bit more scary.

Changing the x86 version sounds safe, *especially* since you point out
that the "fault-causes-tlb-invalidate" is architected behavior.

So I'd almost be willing to drop the invalidate in just one single
commit, because it really should be safe. The only thing it does is
guarantee that the accessed bit gets updated, and the accessed bit
just isn't that important. If we never flush the TLB on another CPU
that continues to use a TLB entry where the accessed bit is set (even
if it's cleared in the in-memory page tables), the worst that can
happen is that the accessed bit doesn't ever get set even if that CPU
constantly uses the page.

And nobody will *ever* care. The A bit is purely a heuristic for the
page LRU thing, we don't care about irrelevant special cases that
won't even affect correctness (much less performance - if that thing
is really hot and stays in the TLB, if we evict it, it will
immediately get reloaded anyway).

And doing a TLB invalidate even locally is worthless: sure, setting
the dirty bit and not invalidating the TLB can cause a local micro-tlb
fault (not a software-visible one, just microarchitectural pipeline
restart with TLB reload) on the next write access (because the TLB
would still contain D=0), so *eve*if* the CPU didn't
invalidate-on-fault, there's no reason we should invalidate in
software on x86.

Again, this can be different on non-x86 architectures with software
dirty bits, where a stale TLB entry that never gets flushed could
cause infinite TLB faults that never make progress, but that's really
a TLB _walker_ issue, not a generic VM issue.

          Linus

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