Messages in this thread | | | Subject | Re: [PATCH 8/8] x86/mm: Allow to have userspace mappings above 47-bits | From | Dmitry Safonov <> | Date | Fri, 7 Apr 2017 13:06:35 +0300 |
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On 04/07/2017 02:21 AM, Kirill A. Shutemov wrote: > 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.
Ok
> >> 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?
I'll comment to v2 to keep all in one place.
-- Dmitry
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