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

[Catalin Marinas <catalin.marinas@arm.com>]

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