On Sun, Apr 17, 2022 at 04:08:22PM +0800, Herbert Xu wrote: > On Fri, Apr 15, 2022 at 01:38:15PM +0200, Ard Biesheuvel wrote: > > If this is the case, things are already broken today. We never take > > ARCH_DMA_MINALIGN into account when adding the driver ctx size to the > > overall allocation size. > > No it's not broken because kmalloc guarantees alignment. For > example, if ARCH_DMA_MINALIGN is 128 bytes, then kmalloc will > always return a pointer that's 128-byte aligned. That guarantees > this object and the next object are on different cache-lines. > > If you reduce the kmalloc minimum alignment to 64 bytes, then > the two neighbouring objects can share cache-lines, even if > each object is bigger than 128 bytes (e.g., if they were 192 > bytes each). Not with my series, the non-sharing of cache lines is preserved. kmalloc() still returns objects aligned to a cache-line. ARCH_DMA_MINALIGN was chosen as the cover-all value for all SoCs supported but I want to reduce the kmalloc() alignment to a cache line size if a platform has a cache line smaller than ARCH_DMA_MINALIGN (most arm64 SoCs have a cache line of 64 bytes rather than 128). -- Catalin
