Date:   Thu, 23 Aug 2018 14:33:49 +1000
From:   Nicholas Piggin <npiggin@...il.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Andrew Lutomirski <luto@...nel.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        Will Deacon <will.deacon@....com>,
        Rik van Riel <riel@...riel.com>,
        Jann Horn <jannh@...gle.com>,
        Adin Scannell <ascannell@...gle.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>,
        David Miller <davem@...emloft.net>,
        Martin Schwidefsky <schwidefsky@...ibm.com>,
        Michael Ellerman <mpe@...erman.id.au>
Subject: Re: [PATCH 3/4] mm/tlb, x86/mm: Support invalidating TLB caches for
 RCU_TABLE_FREE

On Wed, 22 Aug 2018 20:59:46 -0700
Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> On Wed, Aug 22, 2018 at 8:45 PM Nicholas Piggin <npiggin@...il.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.

powerpc/radix does have a special instruction for it, that is why I
posted the patch :)

> It was because
> the regular code had lost the tlb flush _entirely_, because powerpc
> didn't want it.

I think that was long before I started looking at the code.
powerpc/hash hardware has no idea about the page tables so yeah they
don't need 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

I think it was quite well understood and fixed here, a145abf12c9 but
again that was before I really started looking at it.

The hooks I added recently are for a different reason, and it's
actaully the opposite problem -- to work around the hacky generic code
that x86 foisted on other archs.

> 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.

I don't really understand what the issue you have with powerpc here.
powerpc hash has the page table flushing accessors which are just
no-ops, it's the generic code that fails to call them properly. Surely
there was no powerpc patch that removed those calls from generic code?

powerpc/radix yes it does some arch specific things to do its page walk
cache flushing, but it is a better design than the hacks x86 has in
generic code, surely. I thought you basically agreed and thought x86 /
generic code could move to that kind of model.

Thanks,
Nick

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