[PATCH v2 1/2] mm: slab: Introduce __GFP_PACKED for smaller kmalloc() alignments

[Greg Kroah-Hartman]

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.

thanks,

greg k-h