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

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Date:   Tue, 22 Oct 2019 13:23:32 +0200
From:   Nicolas Saenz Julienne <nsaenzjulienne@...e.de>
To:     Qian Cai <cai@....pw>, catalin.marinas@....com
Cc:     f.fainelli@...il.com, mbrugger@...e.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 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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