Re: [PATCH v4] slob: add size header to all allocations

Christoph Lameter
2021-11-30T15:26:32Z

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.