On Mon, Oct 12, 2020 at 08:47:15AM +0200, Christoph Hellwig wrote:

On Fri, Oct 09, 2020 at 06:10:52PM +0100, Catalin Marinas wrote:

quoted

kdump wants DMA-able memory and,

DMAable by whom?  The only way to guranteed DMAable memory is to use
the DMA memory allocator(s) and pass a specific device to them.  Everyting
else is just fundamentally broken.  Note that even when device is not
DMAable we can still use swiotlb to access it.

What I meant is that the new kexec'ed kernel needs some memory in the
ZONE_DMA range, currently set to the bottom 30-bit even for platforms
that can cope with the whole 32-bit range (anything other than RPi4).
The memory range available to the kdump kernels is limited to what
reserve_crashkernel() allocated, which may not fit in the lower 1GB.

There are two ongoing threads (complementary):

1. Change the arm64 reserve_crashkernel() similar to x86 which allocates
   memory above 4G with a small block in the ZONE_DMA range.

2. Allow zone_dma_bits to be 32 for arm64 platforms other than RPi4.

The second point also fixes some regressions with CMA reservations that
could no longer fit in the lower 1GB.

-- 
Catalin

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