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

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Date:   Thu, 18 May 2017 18:19:52 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Michal Hocko <mhocko@...nel.org>
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, 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".

> Another thing would be that 
> 	/* requesting a specific address */
> 	if (addr) {
> 		addr = PAGE_ALIGN(addr);
> 		vma = find_vma(mm, addr);
> 		if (TASK_SIZE - len >= addr &&
> 				(!vma || addr + len <= vma->vm_start))
> 			return addr;
> 	}
> 
> would fail for mmap(-1UL, ...) which is good because we do want to
> fallback to vm_unmapped_area and have randomized address which is
> ensured by your info.high_limit += ... but that wouldn't work for
> mmap(1<<N, ...) where N>47. So the first such mapping won't be
> randomized while others will be. This is quite unexpected I would say.
> So it should be documented at least or maybe we want to skip the above
> shortcut for addr > DEFAULT_MAP_WINDOW altogether.

Again, you're missing existing semantics of hint address. You may have a
reason to set hint address above 47-bit, besides getting access to full
address space.

-- 
 Kirill A. Shutemov