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From: Jann Horn <jannh@google.com>
To: "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>
Cc: linux-arch <linux-arch@vger.kernel.org>,
	linux-man <linux-man@vger.kernel.org>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Linux-MM <linux-mm@kvack.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Will Deacon <will.deacon@arm.com>,
	linuxppc-dev@lists.ozlabs.org,
	Andy Lutomirski <luto@amacapital.net>,
	Dave Hansen <dave.hansen@intel.com>,
	Paul Mackerras <paulus@samba.org>,
	linux-arm-kernel@lists.infradead.org,
	Andrew Morton <akpm@linux-foundation.org>,
	Linux API <linux-api@vger.kernel.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	"Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>
Subject: Re: [PATCH] mmap.2: describe the 5level paging hack
Date: Mon, 25 Feb 2019 16:02:48 +0100
Message-ID: <CAG48ez2h6eavM9YWXYa69OLY7mFFKn1KN=roH2dZJCi7PSSmuQ@mail.gmail.com> (raw)
In-Reply-To: <f89de711-d73b-96be-75b6-0e9054022708@gmail.com>

On Mon, Feb 25, 2019 at 3:55 PM Michael Kerrisk (man-pages)
<mtk.manpages@gmail.com> wrote:
> On 2/11/19 5:36 PM, Jann Horn wrote:
> > The manpage is missing information about the compatibility hack for
> > 5-level paging that went in in 4.14, around commit ee00f4a32a76 ("x86/mm:
> > Allow userspace have mappings above 47-bit"). Add some information about
> > that.
> >
> > While I don't think any hardware supporting this is shipping yet (?), I
> > think it's useful to try to write a manpage for this API, partly to
> > figure out how usable that API actually is, and partly because when this
> > hardware does ship, it'd be nice if distro manpages had information about
> > how to use it.
> >
> > Signed-off-by: Jann Horn <jannh@google.com>
> > ---
> > This patch goes on top of the patch "[PATCH] mmap.2: fix description of
> > treatment of the hint" that I just sent, but I'm not sending them in a
> > series because I want the first one to go in, and I think this one might
> > be a bit more controversial.
> >
> > It would be nice if the architecture maintainers and mm folks could have
> > a look at this and check that what I wrote is right - I only looked at
> > the source for this, I haven't tried it.
> >
> >  man2/mmap.2 | 15 +++++++++++++++
> >  1 file changed, 15 insertions(+)
> >
> > diff --git a/man2/mmap.2 b/man2/mmap.2
> > index 8556bbfeb..977782fa8 100644
> > --- a/man2/mmap.2
> > +++ b/man2/mmap.2
> > @@ -67,6 +67,8 @@ is NULL,
> >  then the kernel chooses the (page-aligned) address
> >  at which to create the mapping;
> >  this is the most portable method of creating a new mapping.
> > +On Linux, in this case, the kernel may limit the maximum address that can be
> > +used for allocations to a legacy limit for compatibility reasons.
> >  If
> >  .I addr
> >  is not NULL,
> > @@ -77,6 +79,19 @@ or equal to the value specified by
> >  and attempt to create the mapping there.
> >  If another mapping already exists there, the kernel picks a new
> >  address, independent of the hint.
> > +However, if a hint above the architecture's legacy address limit is provided
> > +(on x86-64: above 0x7ffffffff000, on arm64: above 0x1000000000000, on ppc64 with
> > +book3s: above 0x7fffffffffff or 0x3fffffffffff, depending on page size), the
> > +kernel is permitted to allocate mappings beyond the architecture's legacy
> > +address limit. The availability of such addresses is hardware-dependent.
> > +Therefore, if you want to be able to use the full virtual address space of
> > +hardware that supports addresses beyond the legacy range, you need to specify an
> > +address above that limit; however, for security reasons, you should avoid
> > +specifying a fixed valid address outside the compatibility range,
> > +since that would reduce the value of userspace address space layout
> > +randomization. Therefore, it is recommended to specify an address
> > +.I beyond
> > +the end of the userspace address space.
> >  .\" Before Linux 2.6.24, the address was rounded up to the next page
> >  .\" boundary; since 2.6.24, it is rounded down!
> >  The address of the new mapping is returned as the result of the call.
> >
>
> Hi Jann,
>
> A few comments came in on this patch. Is there anything from
> those comments that should be rolled into the text?

Hi!

Yeah, I think all the feedback on the patch were good points, and I'll
have to integrate that into my patch.

      reply index

Thread overview: 6+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-02-11 16:36 Jann Horn
2019-02-12  9:41 ` Kirill A. Shutemov
2019-02-13 12:48 ` Will Deacon
2019-02-15  9:13 ` Michael Ellerman
2019-02-25 14:55 ` Michael Kerrisk (man-pages)
2019-02-25 15:02   ` Jann Horn [this message]

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