linux-kernel - Re: [PATCHv3 6/8] x86/mm: Provide ARCH_GET_UNTAG_MASK and ARCH_ENABLE_TAGGED

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Message-ID: <Yqr7mzvX1+hWM8NL@hirez.programming.kicks-ass.net>
Date:   Thu, 16 Jun 2022 11:44:59 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Rick P Edgecombe <rick.p.edgecombe@...el.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "H.J. Lu" <hjl.tools@...il.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "andreyknvl@...il.com" <andreyknvl@...il.com>,
        "kcc@...gle.com" <kcc@...gle.com>, Andi Kleen <ak@...ux.intel.com>,
        "dvyukov@...gle.com" <dvyukov@...gle.com>,
        the arch/x86 maintainers <x86@...nel.org>,
        "ryabinin.a.a@...il.com" <ryabinin.a.a@...il.com>,
        "glider@...gle.com" <glider@...gle.com>
Subject: Re: [PATCHv3 6/8] x86/mm: Provide ARCH_GET_UNTAG_MASK and
 ARCH_ENABLE_TAGGED_ADDR

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 ;-)