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Date: Tue, 21 Jun 2022 15:56:40 +0800
From: "Leizhen (ThunderTown)" <thunder.leizhen@...wei.com>
To: Baoquan He <bhe@...hat.com>,
Catalin Marinas <catalin.marinas@....com>,
Ard Biesheuvel <ardb@...nel.org>
CC: Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
<x86@...nel.org>, "H . Peter Anvin" <hpa@...or.com>,
Eric Biederman <ebiederm@...ssion.com>,
Rob Herring <robh+dt@...nel.org>,
Frank Rowand <frowand.list@...il.com>,
<devicetree@...r.kernel.org>, Dave Young <dyoung@...hat.com>,
Vivek Goyal <vgoyal@...hat.com>, <kexec@...ts.infradead.org>,
<linux-kernel@...r.kernel.org>, Will Deacon <will@...nel.org>,
<linux-arm-kernel@...ts.infradead.org>,
Jonathan Corbet <corbet@....net>, <linux-doc@...r.kernel.org>,
Randy Dunlap <rdunlap@...radead.org>,
Feng Zhou <zhoufeng.zf@...edance.com>,
Kefeng Wang <wangkefeng.wang@...wei.com>,
"Chen Zhou" <dingguo.cz@...group.com>,
John Donnelly <John.p.donnelly@...cle.com>,
Dave Kleikamp <dave.kleikamp@...cle.com>
Subject: Re: [PATCH 5/5] arm64: kdump: Don't defer the reservation of crash
high memory
On 2022/6/21 13:33, Baoquan He wrote:
> Hi,
>
> On 06/13/22 at 04:09pm, Zhen Lei wrote:
>> If the crashkernel has both high memory above DMA zones and low memory
>> in DMA zones, kexec always loads the content such as Image and dtb to the
>> high memory instead of the low memory. This means that only high memory
>> requires write protection based on page-level mapping. The allocation of
>> high memory does not depend on the DMA boundary. So we can reserve the
>> high memory first even if the crashkernel reservation is deferred.
>>
>> This means that the block mapping can still be performed on other kernel
>> linear address spaces, the TLB miss rate can be reduced and the system
>> performance will be improved.
>
> Ugh, this looks a little ugly, honestly.
>
> If that's for sure arm64 can't split large page mapping of linear
> region, this patch is one way to optimize linear mapping. Given kdump
> setting is necessary on arm64 server, the booting speed is truly
> impacted heavily.
There is also a performance impact when running.
>
> However, I would suggest letting it as is with below reasons:
>
> 1) The code will complicate the crashkernel reservatoin code which
> is already difficult to understand.
Yeah, I feel it, too.
> 2) It can only optimize the two cases, first is CONFIG_ZONE_DMA|DMA32
> disabled, the other is crashkernel=,high is specified. While both
> two cases are corner case, most of systems have CONFIG_ZONE_DMA|DMA32
> enabled, and most of systems have crashkernel=xM which is enough.
> Having them optimized won't bring benefit to most of systems.
The case of CONFIG_ZONE_DMA|DMA32 disabled have been resolved by
commit 031495635b46 ("arm64: Do not defer reserve_crashkernel() for platforms with no DMA memory zones").
Currently the performance problem to be optimized is that DMA is enabled.
> 3) Besides, the crashkernel=,high can be handled earlier because
> arm64 alwasys have memblock.bottom_up == false currently, thus we
> don't need worry arbout the lower limit of crashkernel,high
> reservation for now. If memblock.bottom_up is set true in the future,
> this patch doesn't work any more.
>
>
> ...
> crash_base = memblock_phys_alloc_range(crash_size, CRASH_ALIGN,
> crash_base, crash_max);
>
> So, in my opinion, we can leave the current NON_BLOCK|SECT mapping as
> is caused by crashkernel reserving, since no regression is brought.
> And meantime, turning to check if there's any way to make the contiguous
> linear mapping and later splitting work. The patch 4, 5 in this patchset
> doesn't make much sense to me, frankly speaking.
OK. As discussed earlier, I can rethink if there is a better way to patch 4-5,
and this time focus on patch 1-2. In this way, all the functions are complete,
and only optimization is left.
>
> Thanks
> Baoquan
>
>>
>> Signed-off-by: Zhen Lei <thunder.leizhen@...wei.com>
>> ---
>> arch/arm64/mm/init.c | 71 ++++++++++++++++++++++++++++++++++++++++----
>> 1 file changed, 65 insertions(+), 6 deletions(-)
>>
>> diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
>> index fb24efbc46f5ef4..ae0bae2cafe6ab0 100644
>> --- a/arch/arm64/mm/init.c
>> +++ b/arch/arm64/mm/init.c
>> @@ -141,15 +141,44 @@ static void __init reserve_crashkernel(int dma_state)
>> unsigned long long crash_max = CRASH_ADDR_LOW_MAX;
>> char *cmdline = boot_command_line;
>> int dma_enabled = IS_ENABLED(CONFIG_ZONE_DMA) || IS_ENABLED(CONFIG_ZONE_DMA32);
>> - int ret;
>> + int ret, skip_res = 0, skip_low_res = 0;
>> bool fixed_base;
>>
>> if (!IS_ENABLED(CONFIG_KEXEC_CORE))
>> return;
>>
>> - if ((!dma_enabled && (dma_state != DMA_PHYS_LIMIT_UNKNOWN)) ||
>> - (dma_enabled && (dma_state != DMA_PHYS_LIMIT_KNOWN)))
>> - return;
>> + /*
>> + * In the following table:
>> + * X,high means crashkernel=X,high
>> + * unknown means dma_state = DMA_PHYS_LIMIT_UNKNOWN
>> + * known means dma_state = DMA_PHYS_LIMIT_KNOWN
>> + *
>> + * The first two columns indicate the status, and the last two
>> + * columns indicate the phase in which crash high or low memory
>> + * needs to be reserved.
>> + * ---------------------------------------------------
>> + * | DMA enabled | X,high used | unknown | known |
>> + * ---------------------------------------------------
>> + * | N N | low | NOP |
>> + * | Y N | NOP | low |
>> + * | N Y | high/low | NOP |
>> + * | Y Y | high | low |
>> + * ---------------------------------------------------
>> + *
>> + * But in this function, the crash high memory allocation of
>> + * crashkernel=Y,high and the crash low memory allocation of
>> + * crashkernel=X[@offset] for crashk_res are mixed at one place.
>> + * So the table above need to be adjusted as below:
>> + * ---------------------------------------------------
>> + * | DMA enabled | X,high used | unknown | known |
>> + * ---------------------------------------------------
>> + * | N N | res | NOP |
>> + * | Y N | NOP | res |
>> + * | N Y |res/low_res| NOP |
>> + * | Y Y | res | low_res |
>> + * ---------------------------------------------------
>> + *
>> + */
>>
>> /* crashkernel=X[@offset] */
>> ret = parse_crashkernel(cmdline, memblock_phys_mem_size(),
>> @@ -169,10 +198,33 @@ static void __init reserve_crashkernel(int dma_state)
>> else if (ret)
>> return;
>>
>> + /* See the third row of the second table above, NOP */
>> + if (!dma_enabled && (dma_state == DMA_PHYS_LIMIT_KNOWN))
>> + return;
>> +
>> + /* See the fourth row of the second table above */
>> + if (dma_enabled) {
>> + if (dma_state == DMA_PHYS_LIMIT_UNKNOWN)
>> + skip_low_res = 1;
>> + else
>> + skip_res = 1;
>> + }
>> +
>> crash_max = CRASH_ADDR_HIGH_MAX;
>> } else if (ret || !crash_size) {
>> /* The specified value is invalid */
>> return;
>> + } else {
>> + /* See the 1-2 rows of the second table above, NOP */
>> + if ((!dma_enabled && (dma_state == DMA_PHYS_LIMIT_KNOWN)) ||
>> + (dma_enabled && (dma_state == DMA_PHYS_LIMIT_UNKNOWN)))
>> + return;
>> + }
>> +
>> + if (skip_res) {
>> + crash_base = crashk_res.start;
>> + crash_size = crashk_res.end - crashk_res.start + 1;
>> + goto check_low;
>> }
>>
>> fixed_base = !!crash_base;
>> @@ -202,9 +254,18 @@ static void __init reserve_crashkernel(int dma_state)
>> return;
>> }
>>
>> + crashk_res.start = crash_base;
>> + crashk_res.end = crash_base + crash_size - 1;
>> +
>> +check_low:
>> + if (skip_low_res)
>> + return;
>> +
>> if ((crash_base >= CRASH_ADDR_LOW_MAX) &&
>> crash_low_size && reserve_crashkernel_low(crash_low_size)) {
>> memblock_phys_free(crash_base, crash_size);
>> + crashk_res.start = 0;
>> + crashk_res.end = 0;
>> return;
>> }
>>
>> @@ -219,8 +280,6 @@ static void __init reserve_crashkernel(int dma_state)
>> if (crashk_low_res.end)
>> kmemleak_ignore_phys(crashk_low_res.start);
>>
>> - crashk_res.start = crash_base;
>> - crashk_res.end = crash_base + crash_size - 1;
>> insert_resource(&iomem_resource, &crashk_res);
>> }
>>
>> --
>> 2.25.1
>>
>
> .
>
--
Regards,
Zhen Lei
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