On 12/7/18 7:16 AM, Nicolas Boichat wrote: > IOMMUs using ARMv7 short-descriptor format require page tables > to be allocated within the first 4GB of RAM, even on 64-bit systems. > On arm64, this is done by passing GFP_DMA32 flag to memory allocation > functions. > > For IOMMU L2 tables that only take 1KB, it would be a waste to allocate > a full page using get_free_pages, so we considered 3 approaches: > 1. This patch, adding support for GFP_DMA32 slab caches. > 2. genalloc, which requires pre-allocating the maximum number of L2 > page tables (4096, so 4MB of memory). > 3. page_frag, which is not very memory-efficient as it is unable > to reuse freed fragments until the whole page is freed. > > This change makes it possible to create a custom cache in DMA32 zone > using kmem_cache_create, then allocate memory using kmem_cache_alloc. > > We do not create a DMA32 kmalloc cache array, as there are currently > no users of kmalloc(..., GFP_DMA32). These calls will continue to > trigger a warning, as we keep GFP_DMA32 in GFP_SLAB_BUG_MASK. > > This implies that calls to kmem_cache_*alloc on a SLAB_CACHE_DMA32 > kmem_cache must _not_ use GFP_DMA32 (it is anyway redundant and > unnecessary). > > Signed-off-by: Nicolas Boichat <drinkcat@xxxxxxxxxxxx> Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
