linux-kernel - Re: [PATCH v6 3/4] arm64: use both ZONE_DMA and ZONE

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Date:   Wed, 23 Oct 2019 09:11:59 +0200
From:   Matthias Brugger <mbrugger@...e.com>
To:     Nicolas Saenz Julienne <nsaenzjulienne@...e.de>,
        Qian Cai <cai@....pw>, catalin.marinas@....com
Cc:     f.fainelli@...il.com, marc.zyngier@....com,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-mm@...ck.org, Rob Herring <robh+dt@...nel.org>,
        linux-rpi-kernel@...ts.infradead.org, m.szyprowski@...sung.com,
        Robin Murphy <Robin.Murphy@....com>, phill@...pberrypi.org,
        will@...nel.org, Christoph Hellwig <hch@....de>,
        linux-arm-kernel@...ts.infradead.org, wahrenst@....net
Subject: Re: [PATCH v6 3/4] arm64: use both ZONE_DMA and ZONE_DMA32



On 22/10/2019 13:23, Nicolas Saenz Julienne wrote:
> 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;

I'd try to avoid the goto.
Apart from that we should write some information message that the crashkernel
got reserved in arm64_dma_phys_limit. Otherwise RPi4 might break silently and
this will give the user at least a hint what happened.

Regards,
Matthias

> +               } 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
>