[PATCH] arm64: compat: Keep alignment address arithmetic 32-bit

[Karl Mehltretter]

On Wed, Aug 19, 2026 at 01:19:20PM +0100, Arnd Bergmann wrote:
> 
> I don't see yet why 64K_PAGES makes a difference. I had
> expected TASK_SIZE to be 4GiB regardless of the page size, and
> the top page (but not the bottom page) to be reachable from normal
> userspace.

There is a special case in arch/arm64/include/asm/processor.h:

#if defined(CONFIG_ARM64_64K_PAGES) && defined(CONFIG_KUSER_HELPERS)
#define TASK_SIZE_32		UL(0x100000000)
#else
#define TASK_SIZE_32		(UL(0x100000000) - PAGE_SIZE)
#endif

Only ARM64_64K_PAGES with KUSER_HELPERS gives compat tasks the
full 4 GiB. The 64K kuser mapping then occupies the last
page.

I'll send a v2 soon.

Karl