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Date:   Mon, 13 Jun 2022 16:42:57 +0200
From:   Michal Hocko <mhocko@suse.com>
To:     "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc:     Dave Hansen <dave.hansen@linux.intel.com>,
        Andy Lutomirski <luto@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>, x86@kernel.org,
        Kostya Serebryany <kcc@google.com>,
        Andrey Ryabinin <ryabinin.a.a@gmail.com>,
        Andrey Konovalov <andreyknvl@gmail.com>,
        Alexander Potapenko <glider@google.com>,
        Dmitry Vyukov <dvyukov@google.com>,
        "H . J . Lu" <hjl.tools@gmail.com>,
        Andi Kleen <ak@linux.intel.com>,
        Rick Edgecombe <rick.p.edgecombe@intel.com>,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCHv3 6/8] x86/mm: Provide ARCH_GET_UNTAG_MASK and
 ARCH_ENABLE_TAGGED_ADDR
Message-ID: <YqdM8ZIi0/35IhzF@dhcp22.suse.cz>
References: <20220610143527.22974-1-kirill.shutemov@linux.intel.com>
 <20220610143527.22974-7-kirill.shutemov@linux.intel.com>
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On Fri 10-06-22 17:35:25, Kirill A. Shutemov wrote:
[...]
> diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
> index 1962008fe743..93c8eba1a66d 100644
> --- a/arch/x86/kernel/process_64.c
> +++ b/arch/x86/kernel/process_64.c
> @@ -742,6 +742,32 @@ static long prctl_map_vdso(const struct vdso_image *image, unsigned long addr)
>  }
>  #endif
>  
> +static int prctl_enable_tagged_addr(unsigned long nr_bits)
> +{
> +	struct mm_struct *mm = current->mm;
> +
> +	/* Already enabled? */
> +	if (mm->context.lam_cr3_mask)
> +		return -EBUSY;
> +
> +	/* LAM has to be enabled before spawning threads */
> +	if (get_nr_threads(current) > 1)
> +		return -EBUSY;

This will not be sufficient in general. You can have mm shared with a
process without CLONE_THREAD. So you would also need to check also
MMF_MULTIPROCESS. But I do remember that general get_nr_threads is quite
tricky to use properly. Make sure to CC Oleg Nesterov for more details.

Also how does this work when the mm is shared with a kernel thread?

> +
> +	if (!nr_bits) {
> +		return -EINVAL;
> +	} else if (nr_bits <= 6) {
> +		mm->context.lam_cr3_mask = X86_CR3_LAM_U57;
> +		mm->context.untag_mask =  ~GENMASK(62, 57);
> +	} else {
> +		return -EINVAL;
> +	}
> +
> +	/* Update CR3 to get LAM active */
> +	switch_mm(current->mm, current->mm, current);
> +	return 0;
> +}

-- 
Michal Hocko
SUSE Labs