On Tue, May 31, 2022 at 04:16:19PM +1000, Michael Ellerman wrote:

I was thinking of doing it in C, similar to the way arm64 handles it.

Something like below. It means we always double the stack size when
KASAN is enabled. I think it's preferable, as it will always work
regardless of whether the user has an old .config (or is bisectting)?

Is there any reason to even offer the Kconfig?  It is super cryptic and
just picking the right value directly in the header would seem much
more sensible:

#if defined(CONFIG_PPC_256K_PAGES)
#define MIN_THREAD_SHIFT		15
#elif defined(CONFIG_PPC64)
#define MIN_THREAD_SHIFT		14
#else
#define MIN_THREAD_SHIFT		13
#endif

#ifdef CONFIG_KASAN
#define THREAD_SHIFT		(MIN_THREAD_SHIFT + 1)
#else
#define THREAD_SHIFT		MIN_THREAD_SHIFT
#endif

#if defined(CONFIG_VMAP_STACK) && THREAD_SHIFT < PAGE_SHIFT
#undef THREAD_SHIFT
#define THREAD_SHIFT		PAGE_SHIFT
#endif