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From:   Thomas Gleixner <tglx@linutronix.de>
To:     Huacai Chen <chenhuacai@gmail.com>
Cc:     Davidlohr Bueso <dave@stgolabs.net>,
        Huacai Chen <chenhuacai@loongson.cn>,
        Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Darren Hart <dvhart@infradead.org>,
        Thomas Bogendoerfer <tsbogend@alpha.franken.de>,
        "open list:MIPS" <linux-mips@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Xuefeng Li <lixuefeng@loongson.cn>,
        Jiaxun Yang <jiaxun.yang@flygoat.com>,
        Hongchen Zhang <zhanghongchen@loongson.cn>
Subject: Re: [PATCH] futex: Fix fault_in_user_writeable()
In-Reply-To: <CAAhV-H5TKLMi0GSQmh9RFK_k5eNRwx8AE8MjMKjJfbnyVYP-+w@mail.gmail.com>
References: <20210816065417.3987596-1-chenhuacai@loongson.cn>
 <20210816182750.26i535ilc6nef5k6@offworld> <874kbpp5zd.ffs@tglx>
 <CAAhV-H5TKLMi0GSQmh9RFK_k5eNRwx8AE8MjMKjJfbnyVYP-+w@mail.gmail.com>
Date:   Tue, 17 Aug 2021 09:07:32 +0200
Message-ID: <871r6spn0r.ffs@tglx>
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On Tue, Aug 17 2021 at 09:53, Huacai Chen wrote:
> On Tue, Aug 17, 2021 at 3:03 AM Thomas Gleixner <tglx@linutronix.de> wrote:
>> That's surely one way to fix that. If that does not work for whatever
>> reason, then we really don't want this find_vma() hack there, but rather
>> something like:
> I don't know why find_vma() is unacceptable here, there is also
> find_vma() in fixup_user_fault().

Wrong. find_extend_vma() != find_vma(). Aside of that fixup_user_fault()
does way more than that.

>>     if (IS_ENABLED(CONFIG_ARCH_USER_FAULT_VOODOO) && get_user(&tmp, uaddr))
>>         return -EFAULT;
>
> get_user() may be better than find_vma(), but can we drop
> CONFIG_ARCH_USER_FAULT_VOODOO here? On those "W implies R" archs,
> get_user() always success, this can simplify the logic.

For architectures which imply R fixup_user_fault() is way more
effinicient than taking the fault on get_user() and then invoking
fixup_user_fault() to ensure that the mapping is writeable.

No, we are not making stuff less efficient just because of MIPS.

Thanks,

        tglx