On Wed, Oct 26, 2022 at 08:39:04AM +0200, Greg Kroah-Hartman wrote: > On Tue, Oct 25, 2022 at 09:52:46PM +0100, Catalin Marinas wrote: > > By default kmalloc() returns objects aligned to ARCH_KMALLOC_MINALIGN. > > This can be somewhat large on architectures defining ARCH_DMA_MINALIGN > > (e.g. 128 on arm64) and significant memory is wasted through small > > kmalloc() allocations. > > > > Reduce the minimum alignment for kmalloc() to the default > > KMALLOC_MIN_SIZE (8 for slub, 32 for slab) but align the > > requested size to the bigger ARCH_KMALLOC_MINALIGN unless a newly added > > __GFP_PACKED flag is passed. With this gfp flag, the alignment is > > reduced to KMALLOC_PACKED_ALIGN, at least sizeof(unsigned long long). > > Can memory allocated with __GFP_PACKED be sent to DMA controllers? > > If not, you should say that somewhere here or I'm going to get a bunch > of patches trying to add this flag to tiny USB urb allocations (where we > allocate 8 or 16 bytes) that is then going to fail on some hardware. Good point, I'll add a comment. We can also add a check to the DMA API when debugging is enabled, something like WARN_ON_ONCE(ksize(ptr) < cache_line_size()) for non-coherent devices. -- Catalin
