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

["Kirill A. Shutemov" <kirill@shutemov.name>]

On Thu, May 18, 2017 at 05:59:14PM +0200, Michal Hocko wrote:
> On Thu 18-05-17 17:50:03, Michal Hocko wrote:
> > On Thu 18-05-17 18:41:35, Kirill A. Shutemov wrote:
> > > On Thu, May 18, 2017 at 05:27:36PM +0200, Michal Hocko wrote:
> > > > 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?
> > > 
> > > You are right, that this behaviour is not fixed by any standard or written
> > > down in documentation, but it's de-facto policy of Linux mmap(2) the
> > > beginning.
> > > 
> > > And we need to be very careful when messing with this.
> > 
> > I am sorry but I still do not understand. You already touch this
> > semantic. mmap(-1UL,...) will already returns basically arbitrary
> > address. All I am asking for is that mmap doesn't return higher address
> > than the given one whent address > 1<<47. We do not have any such users
> > currently so it won't be a change in behavior while it would allow
> > different sized address spaces naturally.
> 
> I basically mean something like the following
> ---
> diff --git a/arch/x86/kernel/sys_x86_64.c b/arch/x86/kernel/sys_x86_64.c
> index 74d1587b181d..d6f66ff02d0a 100644
> --- a/arch/x86/kernel/sys_x86_64.c
> +++ b/arch/x86/kernel/sys_x86_64.c
> @@ -195,7 +195,7 @@ arch_get_unmapped_area_topdown(struct file *filp, const unsigned long addr0,
>  		goto bottomup;
>  
>  	/* requesting a specific address */
> -	if (addr) {
> +	if (addr && addr <= DEFAULT_MAP_WINDOW) {
>  		addr = PAGE_ALIGN(addr);
>  		vma = find_vma(mm, addr);
>  		if (TASK_SIZE - len >= addr &&
> @@ -215,7 +215,7 @@ arch_get_unmapped_area_topdown(struct file *filp, const unsigned long addr0,
>  	 * !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.high_limit += min(TASK_SIZE_MAX, address) - DEFAULT_MAP_WINDOW;
>  
>  	info.align_mask = 0;
>  	info.align_offset = pgoff << PAGE_SHIFT;

You try to stretch the interface too far. With the patch you propose we
have totally different behaviour wrt hint address if it below and above
47-bits:

 * <= 47-bits: allocate VM [addr; addr + len - 1], if free;
 * > 47-bits: allocate VM anywhere under addr;

Sorry, no. That's ugly.

If you feel that we need to guarantee that bits above certain limit are
unused, introduce new interface. We have enough logic encoded in hint
address already.

-- 
 Kirill A. Shutemov

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