Re: [PATCH v6 3/4] arm64: use both ZONE_DMA and ZONE_DMA32

From: Nicolas Saenz Julienne
Date: Tue Oct 22 2019 - 07:23:41 EST


On Mon, 2019-10-21 at 16:36 -0400, Qian Cai wrote:
> I managed to get more information here,
>
> [ 0.000000] cma: dma_contiguous_reserve(limit c0000000)
> [ 0.000000] cma: dma_contiguous_reserve: reserving 64 MiB for global area
> [ 0.000000] cma: cma_declare_contiguous(size 0x0000000004000000, base
> 0x0000000000000000, limit 0x00000000c0000000 alignment 0x0000000000000000)
> [ 0.000000] cma: Failed to reserve 512 MiB
>
> Full dmesg:
>
> https://cailca.github.io/files/dmesg.txt

OK I got it, reproduced it too.

Here are the relevant logs:

[ 0.000000] DMA [mem 0x00000000802f0000-0x00000000bfffffff]
[ 0.000000] DMA32 [mem 0x00000000c0000000-0x00000000ffffffff]
[ 0.000000] Normal [mem 0x0000000100000000-0x00000097fcffffff]

As you can see ZONE_DMA spans from 0x00000000802f0000-0x00000000bfffffff which
is slightly smaller than 1GB.

[ 0.000000] crashkernel reserved: 0x000000009fe00000 - 0x00000000bfe00000 (512 MB)

Here crashkernel reserved 512M in ZONE_DMA.

[ 0.000000] cma: Failed to reserve 512 MiB

CMA tried to allocate 512M in ZONE_DMA which fails as there is no enough space.
Makes sense.

A fix could be moving crashkernel reservations after CMA and then if unable to
fit in ZONE_DMA try ZONE_DMA32 before bailing out. Maybe it's a little over the
top, yet although most devices will be fine with ZONE_DMA32, the RPi4 needs
crashkernel to be reserved in ZONE_DMA.

My knowledge of Kdump is limited, so I'd love to see what Catalin has to say.
Here's a tested patch of what I'm proposing:

diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
index 120c26af916b..49f3c3a34ae2 100644
--- a/arch/arm64/mm/init.c
+++ b/arch/arm64/mm/init.c
@@ -76,6 +76,7 @@ phys_addr_t arm64_dma32_phys_limit __ro_after_init;
static void __init reserve_crashkernel(void)
{
unsigned long long crash_base, crash_size;
+ phys_addr_t limit = arm64_dma_phys_limit;
int ret;

ret = parse_crashkernel(boot_command_line, memblock_phys_mem_size(),
@@ -86,11 +87,14 @@ static void __init reserve_crashkernel(void)

crash_size = PAGE_ALIGN(crash_size);

+again:
if (crash_base == 0) {
/* Current arm64 boot protocol requires 2MB alignment */
- crash_base = memblock_find_in_range(0, ARCH_LOW_ADDRESS_LIMIT,
- crash_size, SZ_2M);
- if (crash_base == 0) {
+ crash_base = memblock_find_in_range(0, limit, crash_size,
SZ_2M);
+ if (!crash_base && limit == arm64_dma_phys_limit) {
+ limit = arm64_dma32_phys_limit;
+ goto again;
+ } else if (!crash_base && limit == arm64_dma32_phys_limit) {
pr_warn("cannot allocate crashkernel (size:0x%llx)\n",
crash_size);
return;
@@ -448,13 +452,13 @@ void __init arm64_memblock_init(void)
else
arm64_dma32_phys_limit = PHYS_MASK + 1;

- reserve_crashkernel();
-
reserve_elfcorehdr();

high_memory = __va(memblock_end_of_DRAM() - 1) + 1;

dma_contiguous_reserve(arm64_dma_phys_limit ? : arm64_dma32_phys_limit);
+
+ reserve_crashkernel();
}

void __init bootmem_init(void)


Regards,
Nicolas

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