On Mon, 27 Aug 2018 09:47:01 +0200
Peter Zijlstra [off-list ref] wrote:

On Mon, Aug 27, 2018 at 03:00:08PM +1000, Nicholas Piggin wrote:

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On Fri, 24 Aug 2018 13:39:53 +0200
Peter Zijlstra [off-list ref] wrote:  

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On Fri, Aug 24, 2018 at 01:32:14PM +0200, Peter Zijlstra wrote:  

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Hurm.. look at commit:

  e77b0852b551 ("mm/mmu_gather: track page size with mmu gather and force flush if page size change")    

Ah, good, it seems that already got cleaned up a lot. But it all moved
into the power code.. blergh.  

I lost track of what the problem is here?  

Aside from the commit above being absolute crap (which did get fixed up,
luckily) I would really like to get rid of all arch specific mmu_gather.

We can have opt-in bits to the generic code, but the endless back and
forth between common and arch code is an utter pain in the arse.

And there's only like 4 architectures that still have a custom
mmu_gather:

  - sh
  - arm
  - ia64
  - s390

sh is trivial, arm seems doable, with a bit of luck we can do 'rm -rf
arch/ia64' leaving us with s390.

Well I don't see a big problem in having an arch_mmu_gather field
or small bits. powerpc would actually like that rather than trying
to add things it wants into generic code (and it wants more than
just a few flags bits, ideally).

After that everyone uses the common code and we can clean up.

quoted

For powerpc, tlb_start_vma is not the right API to use for this because
it wants to deal with different page sizes within a vma.  

Yes.. I see that. tlb_remove_check_page_size_change() really is a rather
ugly thing, it can cause loads of TLB flushes. Do you really _have_ to
do that? The way ARM and x86 work is that using INVLPG in a 4K stride is
still correct for huge pages, inefficient maybe, but so is flushing
every other page because 'sparse' transparant-huge-pages.

It could do that. It requires a tlbie that matches the page size,
so it means 3 sizes. I think possibly even that would be better
than current code, but we could do better if we had a few specific
fields in there.

Thanks,
Nick