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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