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

[Guo Ren <guoren@kernel.org>]

On Fri, Sep 6, 2024 at 3:18 PM Arnd Bergmann <arnd@arndb.de> 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.
>
> > @@ -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.
>
> 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.

To optimize RISC-V progress, I recommend:

Step 1: Approve the patch.
Step 2: Update Go and OpenJDK's RISC-V backend to utilize it.
Step 3: Wait approximately several iterations for Go & OpenJDK
Step 4: Remove the 47-bit constraint in arch_get_mmap_end()


>
> 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
>


-- 
Best Regards
 Guo Ren