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

[Michal Hocko <mhocko@kernel.org>]

On Thu 18-05-17 19:13:30, Michal Hocko wrote:
> On Thu 18-05-17 19:22:55, Kirill A. Shutemov wrote:
> > On Thu, May 18, 2017 at 05:59:14PM +0200, Michal Hocko wrote:
> [...]
> > > 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;
> 
> unless I am missing something fundamental here this is not how it works.
> We just map a different range if the requested one is not free (in
> absence of MAP_FIXED). And we do that in top->down direction so this is
> already how it works. And you _do_ rely on the same thing when allowing
> larger than 47b except you start from the top of the supported address
> space. So how exactly is your new behavior any different and more clear?

OK, I take that back because I am clearly wrong. We simply always start
from top. Sorry about my confusion.

Feel free to add
Acked-by: Michal Hocko <mhocko@suse.com>
-- 
Michal Hocko
SUSE Labs

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