Question on the five-level page table support patches

[John Paul Adrian Glaubitz]

Hi Kirill!

I recently read the LWN article on your and your colleagues work to
add five-level page table support for x86 to the Linux kernel [1]
and I got your email address from the last patch of the series.

Since this extends the address space beyond 48-bits, as you may know,
it will cause potential headaches with Javascript engines which use
tagged pointers. On SPARC, the virtual address space already extends
to 52 bits and we are running into these very issues with Javascript
engines on SPARC.

Now, a possible way to mitigate this problem would be to pass the
"hint" parameter to mmap() in order to tell the kernel not to allocate
memory beyond the 48 bits address space. Unfortunately, on Linux this
will only work when the area pointed to by "hint" is unallocated which
means one cannot simply use a hardcoded "hint" to mitigate this problem.

However, since this trick still works on NetBSD and used to work on
Linux [3], I was wondering whether there are plans to bring back
this behavior to mmap() in Linux.

Currently, people are using ugly work-arounds [4] to address this
problem which involve a manual iteration over memory blocks and
basically implementing another allocator in the user space
application.

Thanks,
Adrian

> [1] https://lwn.net/Articles/717293/
> [2] https://lwn.net/Articles/717300/
> [3] https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=824449#22
> [4] https://hg.mozilla.org/mozilla-central/rev/dfaafbaaa291

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 .''`.  John Paul Adrian Glaubitz
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