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From:   Barry Song <21cnbao@gmail.com>
Date:   Fri, 8 Jul 2022 20:17:12 +1200
Message-ID: <CAGsJ_4wp03dV0JqRk5NiSSEwPxvMH7z3SE0FhX8Dc8DjDPuVyw@mail.gmail.com>
Subject: Re: [PATCH 2/4] mm: rmap: Allow platforms without mm_cpumask to defer
 TLB flush
To:     Nadav Amit <namit@vmware.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Arnd Bergmann <arnd@arndb.de>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Jonathan Corbet <corbet@lwn.net>,
        "darren@os.amperecomputing.com" <darren@os.amperecomputing.com>,
        "guojian@oppo.com" <guojian@oppo.com>,
        "huzhanyuan@oppo.com" <huzhanyuan@oppo.com>,
        "linux-arm-kernel@lists.infradead.org" 
        <linux-arm-kernel@lists.infradead.org>,
        "linux-doc@vger.kernel.org" <linux-doc@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>,
        "lipeifeng@oppo.com" <lipeifeng@oppo.com>,
        "mgorman@suse.de" <mgorman@suse.de>,
        "realmz6@gmail.com" <realmz6@gmail.com>,
        "v-songbaohua@oppo.com" <v-songbaohua@oppo.com>,
        "will@kernel.org" <will@kernel.org>,
        "x86@kernel.org" <x86@kernel.org>,
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On Fri, Jul 8, 2022 at 8:08 PM Nadav Amit <namit@vmware.com> wrote:
>
> On Jul 7, 2022, at 11:59 PM, Barry Song <21cnbao@gmail.com> wrote:
>
> >> The cpumask_empty() is indeed just another memory access, which is mos=
t
> >> likely ok. But wouldn=E2=80=99t adding something like CONFIG_ARCH_HAS_=
MM_CPUMASK
> >> make the code simpler and (slightly, certainly slightly) more performa=
nt?
> >
> > Yep. good suggestion, Nadav. So the code will be as below, right?
>
> Hmmm=E2=80=A6 Although it is likely to work (because only x86 and arm wou=
ld use this
> batch flushing), I think that for consistency ARCH_HAS_MM_CPUMASK should =
be
> correct for all architectures.
>
> Is it really only x86 that has mm_cpumask()?

i am quite sure there are some other platforms having mm_cpumask().
for example, arm(not arm64).
but i am not exactly sure of the situation of each individual arch. thus,
i don't risk changing their source code.
but arm64 is the second platform looking for tlbbatch, and
ARCH_HAS_MM_CPUMASK only affects tlbbatch. so i would
expect those platforms to fill their ARCH_HAS_MM_CPUMASK
while they start to bringup their tlbbatch? for this moment,
we only need to make certain we don't break x86?
does it make sense?

Thanks
Barry


>