Re: [PATCH 8/8] x86/mm: Allow to have userspace mappings above 47-bits

[Dmitry Safonov <dsafonov@virtuozzo.com>]

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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