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Date:   Thu, 16 Jun 2022 11:44:59 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Andy Lutomirski <luto@kernel.org>
Cc:     Rick P Edgecombe <rick.p.edgecombe@intel.com>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
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        Dave Hansen <dave.hansen@linux.intel.com>,
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        the arch/x86 maintainers <x86@kernel.org>,
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Subject: Re: [PATCHv3 6/8] x86/mm: Provide ARCH_GET_UNTAG_MASK and
 ARCH_ENABLE_TAGGED_ADDR
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On Sun, Jun 12, 2022 at 02:03:43PM -0700, Andy Lutomirski wrote:

> >> > > > +       /* LAM has to be enabled before spawning threads */
> >> > > > +       if (get_nr_threads(current) > 1)
> >> > > > +               return -EBUSY;

> >> > > Does this work for vfork()? I guess the idea is that locking is

vfork() isn't the problem, the problem is that Linux allows CLONE_VM
without CLONE_THREAD. Now, mostly nobody does that these days, but it is
possible.

> get_nr_threads() is the wrong thing.  Either look at mm->mm_users or
> find a way to get rid of this restriction entirely.

mm->mm_users should indeed be sufficient here.

> IMO it would not be insane to have a way to iterate over all tasks
> using an mm.  But doing this for io_uring, etc might be interesting.

That has come up so often over the past 15+ years I've no idea how come
we've still not managed to actually do that ;-)