Re: [PATCH 2/2] arm64: Allocate crashkernel always in ZONE_DMA

[Will Deacon]

On Thu, Jul 02, 2020 at 03:44:20AM +0530, Bhupesh Sharma wrote:
> commit bff3b04460a8 ("arm64: mm: reserve CMA and crashkernel in
> ZONE_DMA32") allocates crashkernel for arm64 in the ZONE_DMA32.
> 
> However as reported by Prabhakar, this breaks kdump kernel booting in
> ThunderX2 like arm64 systems. I have noticed this on another ampere
> arm64 machine. The OOM log in the kdump kernel looks like this:
> 
>   [    0.240552] DMA: preallocated 128 KiB GFP_KERNEL pool for atomic allocations
>   [    0.247713] swapper/0: page allocation failure: order:1, mode:0xcc1(GFP_KERNEL|GFP_DMA), nodemask=(null),cpuset=/,mems_allowed=0
>   <..snip..>
>   [    0.274706] Call trace:
>   [    0.277170]  dump_backtrace+0x0/0x208
>   [    0.280863]  show_stack+0x1c/0x28
>   [    0.284207]  dump_stack+0xc4/0x10c
>   [    0.287638]  warn_alloc+0x104/0x170
>   [    0.291156]  __alloc_pages_slowpath.constprop.106+0xb08/0xb48
>   [    0.296958]  __alloc_pages_nodemask+0x2ac/0x2f8
>   [    0.301530]  alloc_page_interleave+0x20/0x90
>   [    0.305839]  alloc_pages_current+0xdc/0xf8
>   [    0.309972]  atomic_pool_expand+0x60/0x210
>   [    0.314108]  __dma_atomic_pool_init+0x50/0xa4
>   [    0.318504]  dma_atomic_pool_init+0xac/0x158
>   [    0.322813]  do_one_initcall+0x50/0x218
>   [    0.326684]  kernel_init_freeable+0x22c/0x2d0
>   [    0.331083]  kernel_init+0x18/0x110
>   [    0.334600]  ret_from_fork+0x10/0x18
> 
> This patch limits the crashkernel allocation to the first 1GB of
> the RAM accessible (ZONE_DMA), as otherwise we might run into OOM
> issues when crashkernel is executed, as it might have been originally
> allocated from either a ZONE_DMA32 memory or mixture of memory chunks
> belonging to both ZONE_DMA and ZONE_DMA32.

How does this interact with this ongoing series:

https://lore.kernel.org/r/20200628083458.40066-1-chenzhou10@xxxxxxxxxx

(patch 4, in particular)

> Fixes: bff3b04460a8 ("arm64: mm: reserve CMA and crashkernel in ZONE_DMA32")
> Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxxxx>
> Cc: Vladimir Davydov <vdavydov.dev@xxxxxxxxx>
> Cc: James Morse <james.morse@xxxxxxx>
> Cc: Mark Rutland <mark.rutland@xxxxxxx>
> Cc: Will Deacon <will@xxxxxxxxxx>
> Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
> Cc: cgroups@xxxxxxxxxxxxxxx
> Cc: linux-mm@xxxxxxxxx
> Cc: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> Cc: kexec@xxxxxxxxxxxxxxxxxxx
> Reported-by: Prabhakar Kushwaha <pkushwaha@xxxxxxxxxxx>
> Signed-off-by: Bhupesh Sharma <bhsharma@xxxxxxxxxx>
> ---
>  arch/arm64/mm/init.c | 16 ++++++++++++++--
>  1 file changed, 14 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
> index 1e93cfc7c47a..02ae4d623802 100644
> --- a/arch/arm64/mm/init.c
> +++ b/arch/arm64/mm/init.c
> @@ -91,8 +91,15 @@ static void __init reserve_crashkernel(void)
>  	crash_size = PAGE_ALIGN(crash_size);
>  
>  	if (crash_base == 0) {
> -		/* Current arm64 boot protocol requires 2MB alignment */
> -		crash_base = memblock_find_in_range(0, arm64_dma32_phys_limit,
> +		/* Current arm64 boot protocol requires 2MB alignment.
> +		 * Also limit the crashkernel allocation to the first
> +		 * 1GB of the RAM accessible (ZONE_DMA), as otherwise we
> +		 * might run into OOM issues when crashkernel is executed,
> +		 * as it might have been originally allocated from
> +		 * either a ZONE_DMA32 memory or mixture of memory
> +		 * chunks belonging to both ZONE_DMA and ZONE_DMA32.
> +		 */

This comment needs help. Why does putting the crashkernel in ZONE_DMA
prevent "OOM issues"?

Will