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Subject: TLB flushes on fixmap changes 
From:   Nadav Amit <nadav.amit@gmail.com>
In-Reply-To: <20180824180438.GS24124@hirez.programming.kicks-ass.net>
Date:   Fri, 24 Aug 2018 11:35:57 -0700
Cc:     Will Deacon <will.deacon@arm.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Benjamin Herrenschmidt <benh@au1.ibm.com>,
        Nick Piggin <npiggin@gmail.com>,
        Andrew Lutomirski <luto@kernel.org>,
        the arch/x86 maintainers <x86@kernel.org>,
        Borislav Petkov <bp@alien8.de>,
        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>
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To:     Peter Zijlstra <peterz@infradead.org>
Sender: linux-kernel-owner@vger.kernel.org
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at 11:04 AM, Peter Zijlstra <peterz@infradead.org> wrote:

> On Fri, Aug 24, 2018 at 10:26:50AM -0700, Nadav Amit wrote:
>> at 1:47 AM, Peter Zijlstra <peterz@infradead.org> wrote:
>>=20
>>> On Thu, Aug 23, 2018 at 02:39:59PM +0100, Will Deacon wrote:
>>>> The only problem with this approach is that we've lost track of the =
granule
>>>> size by the point we get to the tlb_flush(), so we can't adjust the =
stride of
>>>> the TLB invalidations for huge mappings, which actually works =
nicely in the
>>>> synchronous case (e.g. we perform a single invalidation for a 2MB =
mapping,
>>>> rather than iterating over it at a 4k granule).
>>>>=20
>>>> One thing we could do is switch to synchronous mode if we detect a =
change in
>>>> granule (i.e. treat it like a batch failure).
>>>=20
>>> We could use tlb_start_vma() to track that, I think. Shouldn't be =
too
>>> hard.
>>=20
>> Somewhat unrelated, but I use this opportunity that TLB got your =
attention
>> for something that bothers me for some time. clear_fixmap(), which is =
used
>> in various places (e.g., text_poke()), ends up in doing only a local =
TLB
>> flush (in __set_pte_vaddr()).
>>=20
>> Is that sufficient?
>=20
> Urgh.. weren't the fixmaps per cpu? Bah, I remember looking at this
> during PTI, but I seem to have forgotten everything again.

[ Changed the title. Sorry for hijacking the thread. ]

Since:

native_set_fixmap()->set_pte_vaddr()->pgd_offset_k()

And pgd_offset_k() uses init_mm, they do not seem to be per-CPU.

In addition, the __flush_tlb_one_kernel() in text_poke() seems redundant
(since set_fixmap() should do it as well).

If you also think the current behavior is inappropriate, I can take a =
stab
at fixing it by adding a shootdown. But, if text_poke() is called when
interrupts are disabled, the fix would be annoying.