On Sunday 05 Dec 2021 à 18:11:59 (+0000), Christophe Leroy wrote:

quoted

Is BAT5 needed here ?

Sure it is, because that's were kernel expects lowmem to be mapped. 
Allthough the kernel will unlikely access the 128M reserved for KASAN 
directly, the other 128M are still needed.

Yes that was my point

I'm wondering if for specific PAGE_OFFSET values, __mmu_mapin_ram()
ends using a BAT to map exactly the KASAN area, thus wasting it
because the kernel would never/rarely access it.

Or worse, it could consume the latest BAT available, and there would
be none left for the actual KASAN vm area

Maybe mmu_mapin_ram() could clamp top to KASAN phys start.

-- 
Maxime