Re: [PATCH v5 1/3] mm: Add support for kmem caches in DMA32 zone

[Vlastimil Babka <vbabka@suse.cz>]

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@chromium.org>

Acked-by: Vlastimil Babka <vbabka@suse.cz>