linux-kernel - Re: [PATCH -next v2 1/2] riscv: kdump: Implement crashkernel=X,[high,low]

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Message-ID: <f1fcf149-4d40-f10e-39d4-6580f025629c@huawei.com>
Date:   Fri, 31 Mar 2023 19:36:42 +0800
From:   "chenjiahao (C)" <chenjiahao16@...wei.com>
To:     Baoquan He <bhe@...hat.com>
CC:     <paul.walmsley@...ive.com>, <palmer@...belt.com>,
        <conor.dooley@...rochip.com>, <guoren@...nel.org>,
        <heiko@...ech.de>, <bjorn@...osinc.com>, <alex@...ti.fr>,
        <akpm@...ux-foundation.org>, <atishp@...osinc.com>,
        <thunder.leizhen@...wei.com>, <horms@...nel.org>,
        <linux-kernel@...r.kernel.org>, <linux-riscv@...ts.infradead.org>,
        <kexec@...ts.infradead.org>, <linux-doc@...r.kernel.org>
Subject: Re: [PATCH -next v2 1/2] riscv: kdump: Implement
 crashkernel=X,[high,low]


On 2023/3/31 7:32, Baoquan He wrote:
> On 03/30/23 at 09:40pm, chenjiahao (C) wrote:
> ......
>> Agreed, I will clean this up later in next version.
>>>> +		if (ret || !crash_size)
>>>> +			return;
>>>> +
>>>> +		/*
>>>> +		 * crashkernel=Y,low is valid only when crashkernel=X,high
>>>> +		 * is passed and high memory is reserved successful.
>>>> +		 */
>>>> +		ret = parse_crashkernel_low(boot_command_line, 0, &crash_low_size, &crash_base);
>>>> +		if (ret == -ENOENT)
>>>> +			crash_low_size = DEFAULT_CRASH_KERNEL_LOW_SIZE;
>>>> +		else if (ret)
>>>> +			return;
>>>> +
>>>> +		search_start = dma32_phys_limit;
>>>> +	} else if (ret || !crash_size) {
>>>> +		/* Invalid argument value specified */
>>>>    		return;
>>>> +	}
>>>>    	crash_size = PAGE_ALIGN(crash_size);
>>>> @@ -1201,16 +1246,26 @@ static void __init reserve_crashkernel(void)
>>>>    	 */
>>>>    	crash_base = memblock_phys_alloc_range(crash_size, PMD_SIZE,
>>>>    					       search_start,
>>>> -					       min(search_end, (unsigned long) SZ_4G));
>>>> +					       min(search_end, (unsigned long)dma32_phys_limit));
>>>>    	if (crash_base == 0) {
>>> The above conditional check isn't right. If crashkernel=size@...set
>>> specified, the reservation failure won't trigger retry. This seems to be
>>> originally introduced by old commit, while this need be fixed firstly.
>> Just a little curious about the rule to cope with this specific case. If
>> "crashkernel=size@...set" was passed
>>
>> but reserve failed, should try again to allocate in high memory, regardless
>> the specified size@...set,
>>
>> or just throw a warning and return? Since I noticed the current logic here
>> on Arm64 is to check if !fixed_base first
> Yeah, we need mark the "crashkernel=size@...set" case and avoid to
> retry. Because you won't succeed if  memblock has already failed to
> reserve an unavailable memory region, retry is meaningless. This has
> been done in x86, arm64.

Make sense, thanks.

Actually, in my previous tests, the result in this case is the same
as expectation, i.e. when allocating "crashkernel=size@...set" failed
on low memory, it would retry but return on failure. Since the
search_end is assigned with offset + size, which is lower than DMA32
limit, the second allocation is definitely invalid.

But for sure, to make the code easy to read and eradicate other
possible corner cases, I will check if !fixed_base first on retry.

>