On Thu, Apr 06, 2017 at 10:15:47PM +0300, Dmitry Safonov wrote: > On 04/06/2017 09:43 PM, Dmitry Safonov wrote: > > Hi Kirill, > > > > On 04/06/2017 05:01 PM, Kirill A. Shutemov wrote: > > > On x86, 5-level paging enables 56-bit userspace virtual address space. > > > Not all user space is ready to handle wide addresses. It's known that > > > at least some JIT compilers use higher bits in pointers to encode their > > > information. It collides with valid pointers with 5-level paging and > > > leads to crashes. > > > > > > To mitigate this, we are not going to allocate virtual address space > > > above 47-bit by default. > > > > > > But userspace can ask for allocation from full address space by > > > specifying hint address (with or without MAP_FIXED) above 47-bits. > > > > > > If hint address set above 47-bit, but MAP_FIXED is not specified, we try > > > to look for unmapped area by specified address. If it's already > > > occupied, we look for unmapped area in *full* address space, rather than > > > from 47-bit window. > > > > Do you wish after the first over-47-bit mapping the following mmap() > > calls return also over-47-bits if there is free space? > > It so, you could simplify all this code by changing only mm->mmap_base > > on the first over-47-bit mmap() call. > > This will do simple trick. No. I want every allocation to explicitely opt-in large address space. It's additional fail-safe: if a library can't handle large addresses it has better chance to survive if its own allocation will stay within 47-bits. > I just tried to define it like this: > -#define DEFAULT_MAP_WINDOW ((1UL << 47) - PAGE_SIZE) > +#define DEFAULT_MAP_WINDOW (test_thread_flag(TIF_ADDR32) ? \ > + IA32_PAGE_OFFSET : ((1UL << 47) - > PAGE_SIZE)) > > And it looks working better. Okay, thanks. I'll send v2. > > > + if (addr > DEFAULT_MAP_WINDOW && !in_compat_syscall()) > > > + info.high_limit += TASK_SIZE - DEFAULT_MAP_WINDOW; > > > > Hmm, TASK_SIZE depends now on TIF_ADDR32, which is set during exec(). > > That means for ia32/x32 ELF which has TASK_SIZE < 4Gb as TIF_ADDR32 > > is set, which can do 64-bit syscalls - the subtraction will be > > a negative.. With your proposed change to DEFAULT_MAP_WINDOW difinition it should be okay, right? -- Kirill A. Shutemov
