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Re: [PATCH v2 14/20] mm: Provide speculative fault infrastructure

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On Tue, Aug 29, 2017 at 07:14:37AM +1000, Benjamin Herrenschmidt wrote:
> On Mon, 2017-08-28 at 11:37 +0200, Peter Zijlstra wrote:
> > > Doing all this job and just give up because we cannot allocate page tables
> > > looks very wasteful to me.
> > > 
> > > Have you considered to look how we can hand over from speculative to
> > > non-speculative path without starting from scratch (when possible)?
> > 
> > So we _can_ in fact allocate and install page-tables, but we have to be
> > very careful about it. The interesting case is where we race with
> > free_pgtables() and install a page that was just taken out.
> > 
> > But since we already have the VMA I think we can do something like:
> 
> That makes me extremely nervous... there could be all sort of
> assumptions esp. in arch code about the fact that we never populate the
> tree without the mm sem.

That _would_ be somewhat dodgy, because that means it needs to rely on
taking mmap_sem for _writing_ to undo things and arch/powerpc/ doesn't
have many down_write.*mmap_sem:

$ git grep "down_write.*mmap_sem" arch/powerpc/
arch/powerpc/kernel/vdso.c:     if (down_write_killable(&mm->mmap_sem))
arch/powerpc/kvm/book3s_64_vio.c:       down_write(&current->mm->mmap_sem);
arch/powerpc/mm/mmu_context_iommu.c:    down_write(&mm->mmap_sem);
arch/powerpc/mm/subpage-prot.c: down_write(&mm->mmap_sem);
arch/powerpc/mm/subpage-prot.c: down_write(&mm->mmap_sem);
arch/powerpc/mm/subpage-prot.c:         down_write(&mm->mmap_sem);

Then again, I suppose it could be relying on the implicit down_write
from things like munmap() and the like..

And things _ought_ to be ordered by the various PTLs
(mm->page_table_lock and pmd->lock) which of course doesn't mean
something accidentally snuck through.

> We'd have to audit archs closely. Things like the page walk cache
> flushing on power etc...

If you point me where to look, I'll have a poke around. I'm not
quite sure what you mean with pagewalk cache flushing. Your hash thing
flushes everything inside the PTL IIRC and the radix code appears fairly
'normal'.

> I don't mind the "retry" .. .we've brought stuff in the L1 cache
> already which I would expect to be the bulk of the overhead, and the
> allocation case isn't that common. Do we have numbers to show how
> destrimental this is today ?

No numbers, afaik. And like I said, I didn't consider this an actual
problem when I did these patches. But since Kirill asked ;-)

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Re: [PATCH v2 14/20] mm: Provide speculative fault infrastructure "Kirill A. Shutemov" <kirill@shutemov.name> - 2017-08-27 02:20 +0200
  Re: [PATCH v2 14/20] mm: Provide speculative fault infrastructure Peter Zijlstra <peterz@infradead.org> - 2017-08-28 11:40 +0200
    Re: [PATCH v2 14/20] mm: Provide speculative fault infrastructure Benjamin Herrenschmidt <benh@kernel.crashing.org> - 2017-08-28 23:30 +0200
      Re: [PATCH v2 14/20] mm: Provide speculative fault infrastructure Andi Kleen <ak@linux.intel.com> - 2017-08-29 00:40 +0200
        Re: [PATCH v2 14/20] mm: Provide speculative fault infrastructure Peter Zijlstra <peterz@infradead.org> - 2017-08-29 10:20 +0200
      Re: [PATCH v2 14/20] mm: Provide speculative fault infrastructure Peter Zijlstra <peterz@infradead.org> - 2017-08-29 10:40 +0200
        Re: [PATCH v2 14/20] mm: Provide speculative fault infrastructure Peter Zijlstra <peterz@infradead.org> - 2017-08-29 13:30 +0200
          Re: [PATCH v2 14/20] mm: Provide speculative fault infrastructure Benjamin Herrenschmidt <benh@kernel.crashing.org> - 2017-08-30 00:00 +0200
            Re: [PATCH v2 14/20] mm: Provide speculative fault infrastructure Peter Zijlstra <peterz@infradead.org> - 2017-08-30 08:20 +0200
  Re: [PATCH v2 14/20] mm: Provide speculative fault infrastructure Laurent Dufour <ldufour@linux.vnet.ibm.com> - 2017-08-29 10:00 +0200
    Re: [PATCH v2 14/20] mm: Provide speculative fault infrastructure Peter Zijlstra <peterz@infradead.org> - 2017-08-29 14:10 +0200
      Re: [PATCH v2 14/20] mm: Provide speculative fault infrastructure Laurent Dufour <ldufour@linux.vnet.ibm.com> - 2017-08-29 15:20 +0200
        Re: [PATCH v2 14/20] mm: Provide speculative fault infrastructure Peter Zijlstra <peterz@infradead.org> - 2017-08-29 15:50 +0200
          Re: [PATCH v2 14/20] mm: Provide speculative fault infrastructure Anshuman Khandual <khandual@linux.vnet.ibm.com> - 2017-08-30 07:10 +0200
            Re: [PATCH v2 14/20] mm: Provide speculative fault infrastructure Peter Zijlstra <peterz@infradead.org> - 2017-08-30 08:00 +0200
              Re: [PATCH v2 14/20] mm: Provide speculative fault infrastructure Laurent Dufour <ldufour@linux.vnet.ibm.com> - 2017-08-30 11:40 +0200
                Re: [PATCH v2 14/20] mm: Provide speculative fault infrastructure Anshuman Khandual <khandual@linux.vnet.ibm.com> - 2017-08-31 09:00 +0200
                Re: [PATCH v2 14/20] mm: Provide speculative fault infrastructure Peter Zijlstra <peterz@infradead.org> - 2017-08-31 09:40 +0200
            Re: [PATCH v2 14/20] mm: Provide speculative fault infrastructure Laurent Dufour <ldufour@linux.vnet.ibm.com> - 2017-08-30 12:00 +0200
      Re: [PATCH v2 14/20] mm: Provide speculative fault infrastructure Anshuman Khandual <khandual@linux.vnet.ibm.com> - 2017-08-30 05:50 +0200
  Re: [PATCH v2 14/20] mm: Provide speculative fault infrastructure Anshuman Khandual <khandual@linux.vnet.ibm.com> - 2017-08-30 07:30 +0200
  Re: [PATCH v2 14/20] mm: Provide speculative fault infrastructure Laurent Dufour <ldufour@linux.vnet.ibm.com> - 2017-08-30 11:00 +0200

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