Re: [PATCH RFC v3 1/2] mm: Add personality flag to limit address to 47 bits

On Thu, Sep 12, 2024 at 02:15:59PM -0700, Charlie Jenkins wrote:
> On Thu, Sep 12, 2024 at 11:53:49AM +0100, Catalin Marinas wrote:
> > On Wed, Sep 11, 2024 at 11:18:12PM -0700, Charlie Jenkins wrote:
> > > Opting-in to the higher address space is reasonable. However, it is not
> > > my preference, because the purpose of this flag is to ensure that
> > > allocations do not exceed 47-bits, so it is a clearer ABI to have the
> > > applications that want this guarantee to be the ones setting the flag,
> > > rather than the applications that want the higher bits setting the flag.
> > 
> > Yes, this would be ideal. Unfortunately those applications don't know
> > they need to set a flag in order to work.
> 
> It's not a regression, the applications never worked (on platforms that
> do not have this default). The 47-bit default would allow applications
> that didn't work to start working at the cost of a non-ideal ABI. That
> doesn't seem like a reasonable tradeoff to me.  If applications want to
> run on new hardware that has different requirements, shouldn't they be
> required to update rather than expect the kernel will solve their
> problems for them?

That's a valid point but it depends on the application and how much you
want to spend updating user-space. OpenJDK is fine, if you need a JIT
you'll have to add support for that architecture anyway. But others are
arch-agnostic, you just recompile to your target. It's not an ABI
problem, more of an API one.

The x86 case (and powerpc/arm64) was different, the 47-bit worked for a
long time before expanding it. So it made a lot of sense to keep the
same default.

Anyway, the prctl() can go both ways, either expanding or limiting the
default address space. So I'd be fine with such interface.

-- 
Catalin