From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Kirill A. Shutemov" Subject: Re: [PATCHv5, REBASED 9/9] x86/mm: Allow to have userspace mappings above 47-bits Date: Thu, 18 May 2017 18:19:52 +0300 Message-ID: <20170518151952.jzvz6aeelgx7ifmm@node.shutemov.name> References: <20170515121218.27610-1-kirill.shutemov@linux.intel.com> <20170515121218.27610-10-kirill.shutemov@linux.intel.com> <20170518114359.GB25471@dhcp22.suse.cz> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <20170518114359.GB25471-2MMpYkNvuYDjFM9bn6wA6Q@public.gmane.org> Sender: linux-api-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org To: Michal Hocko Cc: "Kirill A. Shutemov" , x86-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org, Thomas Gleixner , Ingo Molnar , "H. Peter Anvin" , Andi Kleen , Dave Hansen , Andy Lutomirski , Dan Williams , linux-mm-Bw31MaZKKs3YtjvyW6yDsg@public.gmane.org, linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-api-u79uwXL29TY76Z2rM5mHXA@public.gmane.org List-Id: linux-api@vger.kernel.org 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<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