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Message-ID: <YqdM8ZIi0/35IhzF@dhcp22.suse.cz>
Date: Mon, 13 Jun 2022 16:42:57 +0200
From: Michal Hocko <mhocko@...e.com>
To: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc: Dave Hansen <dave.hansen@...ux.intel.com>,
Andy Lutomirski <luto@...nel.org>,
Peter Zijlstra <peterz@...radead.org>, x86@...nel.org,
Kostya Serebryany <kcc@...gle.com>,
Andrey Ryabinin <ryabinin.a.a@...il.com>,
Andrey Konovalov <andreyknvl@...il.com>,
Alexander Potapenko <glider@...gle.com>,
Dmitry Vyukov <dvyukov@...gle.com>,
"H . J . Lu" <hjl.tools@...il.com>,
Andi Kleen <ak@...ux.intel.com>,
Rick Edgecombe <rick.p.edgecombe@...el.com>,
linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCHv3 6/8] x86/mm: Provide ARCH_GET_UNTAG_MASK and
ARCH_ENABLE_TAGGED_ADDR
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
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