On 11/9/18 12:57 PM, Nicolas Boichat wrote:

On Fri, Nov 9, 2018 at 6:43 PM Vlastimil Babka [off-list ref] wrote:

quoted

Also I'm probably missing the point of this all. In patch 3 you use
__get_dma32_pages() thus __get_free_pages(__GFP_DMA32), which uses
alloc_pages, thus the page allocator directly, and there's no slab
caches involved.

__get_dma32_pages fixes level 1 page allocations in the patch 3.

This change fixes level 2 page allocations
(kmem_cache_zalloc(data->l2_tables, gfp | GFP_DMA)), by transparently
remapping GFP_DMA to an underlying ZONE_DMA32.

The alternative would be to create a new SLAB_CACHE_DMA32 when
CONFIG_ZONE_DMA32 is defined, but then I'm concerned that the callers
would need to choose between the 2 (GFP_DMA or GFP_DMA32...), and also
need to use some ifdefs (but maybe that's not a valid concern?).

quoted

It makes little sense to involve slab for page table
allocations anyway, as those tend to be aligned to a page size (or
high-order page size). So what am I missing?

Level 2 tables are ARM_V7S_TABLE_SIZE(2) => 1kb, so we'd waste 3kb if
we allocated a full page.

Oh, I see.

Well, I think indeed the most transparent would be to support
SLAB_CACHE_DMA32. The callers of kmem_cache_zalloc() would then need not
add anything special to gfp, as that's stored internally upon
kmem_cache_create(). Of course SLAB_BUG_MASK would no longer have to
treat __GFP_DMA32 as unexpected. It would be unexpected when passed to
kmalloc() which doesn't have special dma32 caches, but for a cache
explicitly created to allocate from ZONE_DMA32, I don't see why not. I'm
somewhat surprised that there wouldn't be a need for this earlier, so
maybe I'm still missing something.

Thanks,