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

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

On Fri, Sep 06, 2024 at 07:17:44AM GMT, Arnd Bergmann wrote:
> On Thu, Sep 5, 2024, at 21:15, Charlie Jenkins wrote:
> > Create a personality flag ADDR_LIMIT_47BIT to support applications
> > that wish to transition from running in environments that support at
> > most 47-bit VAs to environments that support larger VAs. This
> > personality can be set to cause all allocations to be below the 47-bit
> > boundary. Using MAP_FIXED with mmap() will bypass this restriction.
> >
> > Signed-off-by: Charlie Jenkins <charlie@rivosinc.com>
>
> I think having an architecture-independent mechanism to limit the size
> of the 64-bit address space is useful in general, and we've discussed
> the same thing for arm64 in the past, though we have not actually
> reached an agreement on the ABI previously.

The thread on the original proposals attests to this being rather a fraught
topic, and I think the weight of opinion was more so in favour of opt-in
rather than opt-out.

>
> > @@ -22,6 +22,7 @@ enum {
> >  	WHOLE_SECONDS =		0x2000000,
> >  	STICKY_TIMEOUTS	=	0x4000000,
> >  	ADDR_LIMIT_3GB = 	0x8000000,
> > +	ADDR_LIMIT_47BIT = 	0x10000000,
> > };
>
> I'm a bit worried about having this done specifically in the
> personality flag bits, as they are rather limited. We obviously
> don't want to add many more such flags when there could be
> a way to just set the default limit.

Since I'm the one who suggested it, I feel I should offer some kind of
vague defence here :)

We shouldn't let perfect be the enemy of the good. This is a relatively
straightforward means of achieving the aim (assuming your concern about
arch_get_mmap_end() below isn't a blocker) which has the least impact on
existing code.

Of course we can end up in absurdities where we start doing
ADDR_LIMIT_xxBIT... but again - it's simple, shouldn't represent an
egregious maintenance burden and is entirely opt-in so has things going for
it.

>
> It's also unclear to me how we want this flag to interact with
> the existing logic in arch_get_mmap_end(), which attempts to
> limit the default mapping to a 47-bit address space already.

How does ADDR_LIMIT_3GB presently interact with that?

>
> For some reason, it appears that the arch_get_mmap_end()
> logic on RISC-V defaults to the maximum address
> space for the 'addr==0' case which is inconsistentn with
> the other architectures, so we should probably fix that
> part first, possibly moving more of that logic into a
> shared implementation.
>
>       Arnd