linux-kernel - Re: [PATCHv5, REBASED 9/9] x86/mm: Allow to have userspace mappings above 47-bits

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Message-ID: <20170518152736.GA18333@dhcp22.suse.cz>
Date:   Thu, 18 May 2017 17:27:36 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
Cc:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        x86@...nel.org, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, Andi Kleen <ak@...ux.intel.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Andy Lutomirski <luto@...capital.net>,
        Dan Williams <dan.j.williams@...el.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, linux-api@...r.kernel.org
Subject: Re: [PATCHv5, REBASED 9/9] x86/mm: Allow to have userspace mappings
 above 47-bits

On Thu 18-05-17 18:19:52, Kirill A. Shutemov wrote:
> On Thu, May 18, 2017 at 01:43:59PM +0200, Michal Hocko wrote:
> > On Mon 15-05-17 15:12:18, Kirill A. Shutemov wrote:
> > [...]
> > > @@ -195,6 +207,16 @@ arch_get_unmapped_area_topdown(struct file *filp, const unsigned long addr0,
> > >  	info.length = len;
> > >  	info.low_limit = PAGE_SIZE;
> > >  	info.high_limit = get_mmap_base(0);
> > > +
> > > +	/*
> > > +	 * If hint address is above DEFAULT_MAP_WINDOW, look for unmapped area
> > > +	 * in the full address space.
> > > +	 *
> > > +	 * !in_compat_syscall() check to avoid high addresses for x32.
> > > +	 */
> > > +	if (addr > DEFAULT_MAP_WINDOW && !in_compat_syscall())
> > > +		info.high_limit += TASK_SIZE_MAX - DEFAULT_MAP_WINDOW;
> > > +
> > >  	info.align_mask = 0;
> > >  	info.align_offset = pgoff << PAGE_SHIFT;
> > >  	if (filp) {
> > 
> > I have two questions/concerns here. The above assumes that any address above
> > 1<<47 will use the _whole_ address space. Is this what we want?
> 
> Yes, I believe so.
> 
> > What if somebody does mmap(1<<52, ...) because he wants to (ab)use 53+
> > bits for some other purpose? Shouldn't we cap the high_limit by the
> > given address?
> 
> This would screw existing semantics of hint address -- "map here if
> free, please".

Well, the given address is just _hint_. We are still allowed to map to a
different place. And it is not specified whether the resulting mapping
is above or below that address. So I do not think it would screw the
existing semantic. Or do I miss something?

-- 
Michal Hocko
SUSE Labs