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From: Nicholas Piggin <npiggin@gmail.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>,
	Andrew Lutomirski <luto@kernel.org>,
	"the arch/x86 maintainers" <x86@kernel.org>,
	Borislav Petkov <bp@alien8.de>, Will Deacon <will.deacon@arm.com>,
	Rik van Riel <riel@surriel.com>, Jann Horn <jannh@google.com>,
	Adin Scannell <ascannell@google.com>,
	Dave Hansen <dave.hansen@intel.com>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	linux-mm <linux-mm@kvack.org>, David Miller <davem@davemloft.net>,
	Martin Schwidefsky <schwidefsky@de.ibm.com>,
	Michael Ellerman <mpe@ellerman.id.au>
Subject: Re: [PATCH 3/4] mm/tlb, x86/mm: Support invalidating TLB caches for RCU_TABLE_FREE
Date: Thu, 23 Aug 2018 14:33:49 +1000	[thread overview]
Message-ID: <20180823143349.65cb0da0@roar.ozlabs.ibm.com> (raw)
In-Reply-To: <CA+55aFxneZTFxxxAjLZmj92VUJg6z7hERxJ2cHoth-GC0RuELw@mail.gmail.com>

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

> 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.

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

  reply	other threads:[~2018-08-23  4:34 UTC|newest]

Thread overview: 96+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-08-22 15:30 [PATCH 0/4] x86: TLB invalidate fixes Peter Zijlstra
2018-08-22 15:30 ` [PATCH 1/4] x86/mm/tlb: Revert the recent lazy TLB patches Peter Zijlstra
2018-08-22 21:37   ` Linus Torvalds
2018-08-22 22:11     ` Rik van Riel
2018-08-22 15:30 ` [PATCH 2/4] mm/tlb: Remove tlb_remove_table() non-concurrent condition Peter Zijlstra
2018-08-23  3:31   ` Nicholas Piggin
2018-08-23  3:35     ` Linus Torvalds
2018-08-23  3:44       ` Linus Torvalds
2018-08-23  4:16       ` Nicholas Piggin
2018-08-23  4:54         ` Linus Torvalds
2018-08-23  5:15           ` Nicholas Piggin
2018-08-24  8:42           ` Peter Zijlstra
2018-08-23 13:40   ` Will Deacon
2018-08-22 15:30 ` [PATCH 3/4] mm/tlb, x86/mm: Support invalidating TLB caches for RCU_TABLE_FREE Peter Zijlstra
2018-08-22 15:55   ` Peter Zijlstra
2018-08-23  3:45     ` Nicholas Piggin
2018-08-23  3:59       ` Linus Torvalds
2018-08-23  4:33         ` Nicholas Piggin [this message]
2018-08-23  5:03           ` Linus Torvalds
2018-08-23  5:58             ` Nicholas Piggin
2018-08-23  4:54         ` Benjamin Herrenschmidt
2018-08-23  5:11           ` Linus Torvalds
2018-08-23  5:20             ` Linus Torvalds
2018-08-23  6:48               ` Martin Schwidefsky
2018-08-23  5:21             ` Benjamin Herrenschmidt
2018-08-23  6:15               ` Nicholas Piggin
2018-08-23 13:39             ` Will Deacon
2018-08-24  8:47               ` Peter Zijlstra
2018-08-24 11:32                 ` Peter Zijlstra
2018-08-24 11:39                   ` Peter Zijlstra
2018-08-27  5:00                     ` Nicholas Piggin
2018-08-27  7:47                       ` Peter Zijlstra
2018-08-27  8:04                         ` Nicholas Piggin
2018-08-27  8:09                           ` Benjamin Herrenschmidt
2018-08-27  8:20                             ` Peter Zijlstra
2018-08-27  8:54                               ` Nicholas Piggin
2018-08-27  9:02                             ` Nicholas Piggin
2018-08-27 22:13                               ` Benjamin Herrenschmidt
2018-08-27 13:36                           ` Rik van Riel
2018-08-27 14:29                             ` Nicholas Piggin
2018-08-27  8:57                         ` removig ia64, was: " Christoph Hellwig
2018-08-27 11:28                           ` Peter Zijlstra
2018-08-27 11:45                           ` Jason Duerstock
2018-08-27 11:00                         ` Peter Zijlstra
2018-08-30  0:13                           ` Vineet Gupta
2018-08-30 10:23                             ` Peter Zijlstra
2018-08-24 17:26                 ` Nadav Amit
2018-08-24 18:04                   ` Peter Zijlstra
2018-08-24 18:35                     ` TLB flushes on fixmap changes Nadav Amit
2018-08-24 19:31                       ` Linus Torvalds
2018-08-24 20:24                         ` Nadav Amit
2018-08-25  0:58                           ` Linus Torvalds
2018-08-25  2:29                             ` nadav.amit
2018-08-25  4:23                               ` Andy Lutomirski
2018-08-26  2:23                                 ` Masami Hiramatsu
2018-08-26  4:21                                   ` Andy Lutomirski
2018-08-26  4:43                                     ` Kees Cook
2018-08-26  5:53                                       ` Nadav Amit
2018-08-26 14:20                                       ` Andy Lutomirski
2018-08-26 16:47                                         ` Kees Cook
2018-08-26 17:25                                           ` Andy Lutomirski
2018-08-26 20:15                                             ` Thomas Gleixner
2018-08-26 22:03                                               ` Kees Cook
2018-08-26 22:15                                                 ` Matthew Wilcox
2018-08-26 22:29                                                 ` Jann Horn
2018-08-26  9:09                                     ` Peter Zijlstra
2018-08-27  3:03                                       ` Masami Hiramatsu
2018-08-27  3:26                                         ` Nadav Amit
2018-08-27  8:05                                           ` Masami Hiramatsu
2018-08-27 17:34                                             ` Nadav Amit
2018-08-27 18:45                                               ` Andy Lutomirski
2018-08-27 18:54                                                 ` Nadav Amit
2018-08-27 18:58                                                   ` Andy Lutomirski
2018-08-27 19:10                                                     ` Nadav Amit
2018-08-27 19:43                                                       ` Nadav Amit
2018-08-27 19:58                                                         ` Andy Lutomirski
2018-08-27 20:16                                                           ` Nadav Amit
2018-08-27 21:55                                                             ` Nadav Amit
2018-08-27 22:32                                                               ` Andy Lutomirski
2018-08-27 22:54                                                                 ` Nadav Amit
2018-08-27 23:01                                                                   ` Andy Lutomirski
2018-08-28  8:49                                                                     ` Masami Hiramatsu
2018-08-28 17:33                                                                       ` Nadav Amit
2018-08-27  8:13                                         ` Peter Zijlstra
2018-08-27  9:39                                           ` Masami Hiramatsu
2018-08-27  9:55                                           ` Jann Horn
2018-08-26 22:48                                     ` Jann Horn
2018-08-24  8:35           ` [PATCH 3/4] mm/tlb, x86/mm: Support invalidating TLB caches for RCU_TABLE_FREE Peter Zijlstra
2018-08-24 13:13             ` Peter Zijlstra
2018-08-24 13:14               ` Peter Zijlstra
2018-08-24 15:49               ` Will Deacon
2018-08-23 23:31     ` Will Deacon
2018-08-22 21:34   ` Linus Torvalds
2018-08-23  8:46   ` Nicholas Piggin
2018-08-22 15:30 ` [PATCH 4/4] x86/mm: Only use tlb_remove_table() for paravirt Peter Zijlstra
2018-08-22 22:12   ` Eduardo Valentin

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