Re: [PATCH 3/4] mm/tlb, x86/mm: Support invalidating TLB caches for RCU_TABLE_FREE

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

On Wed, Aug 22, 2018 at 8:45 PM Nicholas Piggin <npiggin@gmail.com> wrote:
>
> powerpc/radix has no such issue, it already does this tracking.

Yeah, I now realize that this was why you wanted to add that hacky
thing to the generic code, so that you can add the tlb_flush_pgtable()
call.

I thought it was because powerpc had some special flush instruction
for it, and the regular tlb flush didn't do it. But no. It was because
the regular code had lost the tlb flush _entirely_, because powerpc
didn't want it.

> We were discussing this a couple of months ago, I wasn't aware of ARM's
> issue but I suggested x86 could go the same way as powerpc.

The problem is that x86 _used_ to do this all correctly long long ago.

And then we switched over to the "generic" table flushing (which
harkens back to the powerpc code).

Which actually turned out to be not generic at all, and did not flush
the internal pages like x86 used to (back when x86 just used
tlb_remove_page for everything).

So as a result, x86 had unintentionally lost the TLB flush we used to
have, because tlb_remove_table() had lost the tlb flushing because of
a powerpc quirk.

You then added it back as a hacky per-architecture hook (apparently
having realized that you never did it at all), which didn't fix the
unintentional lack of flushing on x86.

So now we're going to do it right.  No more "oh, powerpc didn't need
to flush because the hash tables weren't in the tlb at all" thing in
the generic code that then others need to work around.

              Linus