Re: [PATCH v4] arm64: mm: fix linear mem mapping access performance degradation

[Will Deacon]

On Sat, Jul 02, 2022 at 11:57:53PM +0800, Guanghui Feng wrote:
> The arm64 can build 2M/1G block/sectiion mapping. When using DMA/DMA32 zone
> (enable crashkernel, disable rodata full, disable kfence), the mem_map will
> use non block/section mapping(for crashkernel requires to shrink the region
> in page granularity). But it will degrade performance when doing larging
> continuous mem access in kernel(memcpy/memmove, etc).

Hmm. It seems a bit silly to me that we take special care to unmap the
crashkernel from the linear map even when can_set_direct_map() is false, as
we won't be protecting the main kernel at all!

Why don't we just leave the crashkernel mapped if !can_set_direct_map()
and then this problem just goes away?

Will