[PATCH v3, RFC] iommu/io-pgtable-arm-v7s: Use page_frag to request DMA32 memory

[Christoph Hellwig]

On Tue, Dec 04, 2018 at 04:23:00PM +0800, Nicolas Boichat wrote:
> IOMMUs using ARMv7 short-descriptor format require page tables
> (level 1 and 2) to be allocated within the first 4GB of RAM, even
> on 64-bit systems.
> 
> For level 1/2 tables, ensure GFP_DMA32 is used if CONFIG_ZONE_DMA32
> is defined (e.g. on arm64 platforms).
> 
> For level 2 tables (1 KB), we use page_frag to allocate these pages,
> as we cannot directly use kmalloc (no slab cache for GFP_DMA32) or
> kmem_cache (mm/ code treats GFP_DMA32 as an invalid flag).
> 
> One downside is that we only free the allocated page if all the
> 4 fragments (4 IOMMU L2 tables) are freed, but given that we
> usually only allocate limited number of IOMMU L2 tables, this
> should not have too much impact on memory usage: In the absolute
> worst case (4096 L2 page tables, each on their own 4K page),
> we would use 16 MB of memory for 4 MB of L2 tables.

I think this needs to be documemented in the code.  That is move
the explanation about into a comment in the code.