On Tue, 30 Nov 2021, Vlastimil Babka wrote:

So either I missed something or we violate the rule that kmalloc() provides
blocks where ARCH_KMALLOC_MINALIGN is not just the alignment of their
beginning but also nothing else touches the N*ARCH_KMALLOC_MINALIGN area
containing the allocated object.

Indeed.... The DMA API documentation in the kernel states:

Documentation/DMA-API-HOWTO.rst

2) ARCH_KMALLOC_MINALIGN

+   Architectures must ensure that kmalloc'ed buffer is
+   DMA-safe. Drivers and subsystems depend on it. If an architecture
+   isn't fully DMA-coherent (i.e. hardware doesn't ensure that data in
+   the CPU cache is identical to data in main memory),
+   ARCH_KMALLOC_MINALIGN must be set so that the memory allocator
+   makes sure that kmalloc'ed buffer doesn't share a cache line with
+   the others. See arch/arm/include/asm/cache.h as an example.


Note that this is only the case for kmalloc. Not for a slab cache setup
separately from the kmalloc array. That is why ARCH_KMALLOC_MINALIGN
exists.