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. -- Michal Hocko SUSE Labs -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx"> email@xxxxxxxxx </a>