Re: [RFC, PATCHv2 29/29] mm, x86: introduce RLIMIT_VADDR

["Kirill A. Shutemov" <kirill@shutemov.name>]

On Mon, Jan 02, 2017 at 10:08:28PM -0800, Andy Lutomirski wrote:
> On Mon, Jan 2, 2017 at 12:44 AM, Arnd Bergmann <arnd@arndb.de> wrote:
> > On Tuesday, December 27, 2016 4:54:13 AM CET Kirill A. Shutemov wrote:
> >> As with other resources you can set the limit lower than current usage.
> >> It would affect only future virtual address space allocations.
> 
> I still don't buy all these use cases:
> 
> >>
> >> Use-cases for new rlimit:
> >>
> >>   - Bumping the soft limit to RLIM_INFINITY, allows current process all
> >>     its children to use addresses above 47-bits.
> 
> OK, I get this, but only as a workaround for programs that make
> assumptions about the address space and don't use some mechanism (to
> be designed?) to work correctly in spite of a larger address space.

I guess you've misread the case. It's opt-in for large adrress space, not
other way around.

I believe 47-bit VA by default is right way to go to make the transition
without breaking userspace.

> >>   - Bumping the soft limit to RLIM_INFINITY after fork(2), but before
> >>     exec(2) allows the child to use addresses above 47-bits.
> 
> Ditto.
> 
> >>
> >>   - Lowering the hard limit to 47-bits would prevent current process all
> >>     its children to use addresses above 47-bits, unless a process has
> >>     CAP_SYS_RESOURCES.
> 
> I've tried and I can't imagine any reason to do this.

That's just if something went wrong and we want to stop an application
from use addresses above 47-bit.

> >>   - Ita??s also can be handy to lower hard or soft limit to arbitrary
> >>     address. User-mode emulation in QEMU may lower the limit to 32-bit
> >>     to emulate 32-bit machine on 64-bit host.
> 
> I don't understand.  QEMU user-mode emulation intercepts all syscalls.
> What QEMU would *actually* want is a way to say "allocate me some
> memory with the high N bits clear".  mmap-via-int80 on x86 should be
> fixed to do this, but a new syscall with an explicit parameter would
> work, as would a prctl changing the current limit.

Look at mess in mmap_find_vma(). QEmu has to guess where is free virtual
memory. That's unnessesary complex.

prctl would work for this too. new-mmap would *not*: there are more ways
to allocate vitual address space: shmat(), mremap(). Changing all of them
just for this is stupid.

> >>
> >> TODO:
> >>   - port to non-x86;
> >>
> >> Not-yet-signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> >> Cc: linux-api@vger.kernel.org
> >
> > This seems to nicely address the same problem on arm64, which has
> > run into the same issue due to the various page table formats
> > that can currently be chosen at compile time.
> 
> On further reflection, I think this has very little to do with paging
> formats except insofar as paging formats make us notice the problem.
> The issue is that user code wants to be able to assume an upper limit
> on an address, and it gets an upper limit right now that depends on
> architecture due to paging formats.  But someone really might want to
> write a *portable* 64-bit program that allocates memory with the high
> 16 bits clear.  So let's add such a mechanism directly.
> 
> As a thought experiment, what if x86_64 simply never allocated "high"
> (above 2^47-1) addresses unless a new mmap-with-explicit-limit syscall
> were used?  Old glibc would continue working.  Old VMs would work.
> New programs that want to use ginormous mappings would have to use the
> new syscall.  This would be totally stateless and would have no issues
> with CRIU.

Except, we need more than mmap as I mentioned.

And what about stack? I'm not sure that everybody would be happy with
stack in the middle of address space.

> If necessary, we could also have a prctl that changes a
> "personality-like" limit that is in effect when the old mmap was used.
> I say "personality-like" because it would reset under exactly the same
> conditions that personality resets itself.
> 
> Thoughts?
> 
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-- 
 Kirill A. Shutemov

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