Re: [PATCHv3 6/8] x86/mm: Provide ARCH_GET_UNTAG_MASK and ARCH_ENABLE_TAGGED_ADDR

From: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
To: "Edgecombe, Rick P" <rick.p.edgecombe@intel.com>
Cc: "peterz@infradead.org" <peterz@infradead.org>,
	"Lutomirski, Andy" <luto@kernel.org>,
	"dave.hansen@linux.intel.com" <dave.hansen@linux.intel.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"hjl.tools@gmail.com" <hjl.tools@gmail.com>,
	"linux-mm@kvack.org" <linux-mm@kvack.org>,
	"kcc@google.com" <kcc@google.com>,
	"andreyknvl@gmail.com" <andreyknvl@gmail.com>,
	"ak@linux.intel.com" <ak@linux.intel.com>,
	"dvyukov@google.com" <dvyukov@google.com>,
	"x86@kernel.org" <x86@kernel.org>,
	"ryabinin.a.a@gmail.com" <ryabinin.a.a@gmail.com>,
	"glider@google.com" <glider@google.com>
Subject: Re: [PATCHv3 6/8] x86/mm: Provide ARCH_GET_UNTAG_MASK and ARCH_ENABLE_TAGGED_ADDR
Date: Fri, 10 Jun 2022 21:06:35 +0300	[thread overview]
Message-ID: <20220610180635.l44opq2votd3gxpl@black.fi.intel.com> (raw)
In-Reply-To: <c3b4f3ccf8ee547a588bf8a971064e4d62b6a44c.camel@intel.com>

On Fri, Jun 10, 2022 at 04:16:01PM +0000, Edgecombe, Rick P wrote:
> On Fri, 2022-06-10 at 17:35 +0300, Kirill A. Shutemov wrote:
> > +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;
> 
> Does this work for vfork()? I guess the idea is that locking is not
> needed below because there is only one thread with the MM, but with
> vfork() another task could operate on the MM, call fork(), etc. I'm not
> sure...

I'm not sure I follow. vfork() blocks parent process until child exit or
execve(). I don't see how it is a problem.

-- 
 Kirill A. Shutemov