On Sat, 1 Feb 2020, Kees Cook wrote:
>
> I can't find where the address limit for dma-kmalloc is implemented.
include/linux/mmzones.h
enum zone_type {
/*
* ZONE_DMA and ZONE_DMA32 are used when there are peripherals not able
* to DMA to all of the addressable memory (ZONE_NORMAL).
* On architectures where this area covers the whole 32 bit address
* space ZONE_DMA32 is used. ZONE_DMA is left for the ones with smaller
* DMA addressing constraints. This distinction is important as a 32bit
* DMA mask is assumed when ZONE_DMA32 is defined. Some 64-bit
* platforms may need both zones as they support peripherals with
* different DMA addressing limitations.
*
* Some examples:
*
* - i386 and x86_64 have a fixed 16M ZONE_DMA and ZONE_DMA32 for the
* rest of the lower 4G.
*
* - arm only uses ZONE_DMA, the size, up to 4G, may vary depending on
* the specific device.
*
* - arm64 has a fixed 1G ZONE_DMA and ZONE_DMA32 for the rest of the
* lower 4G.
*
* - powerpc only uses ZONE_DMA, the size, up to 2G, may vary
* depending on the specific device.
*
* - s390 uses ZONE_DMA fixed to the lower 2G.
*
* - ia64 and riscv only use ZONE_DMA32.
*
* - parisc uses neither.
*/
#ifdef CONFIG_ZONE_DMA
ZONE_DMA,
#endif
#ifdef CONFIG_ZONE_DMA32
ZONE_DMA32,
#endif
/*
* Normal addressable memory is in ZONE_NORMAL. DMA operations can
be
* performed on pages in ZONE_NORMAL if the DMA devices support
* transfers to all addressable memory.
*/
ZONE_NORMAL,
#ifdef CONFIG_HIGHMEM
/*
* A memory area that is only addressable by the kernel through
* mapping portions into its own address space. This is for example
* used by i386 to allow the kernel to address the memory beyond
* 900MB. The kernel will set up special mappings (page
* table entries on i386) for each page that the kernel needs to
* access.
*/
ZONE_HIGHMEM,
#endif
ZONE_MOVABLE,
#ifdef CONFIG_ZONE_DEVICE
ZONE_DEVICE,
#endif
__MAX_NR_ZONES
};
