linux-kernel - Re: [PATCH -next v4 1/2] riscv: kdump: Implement crashkernel=X,[high,low]

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Message-ID: <ZGjI0kDmtnxKY3NP@MiWiFi-R3L-srv>
Date:   Sat, 20 May 2023 21:19:14 +0800
From:   Baoquan He <bhe@...hat.com>
To:     "chenjiahao (C)" <chenjiahao16@...wei.com>
Cc:     linux-kernel@...r.kernel.org, linux-riscv@...ts.infradead.org,
        kexec@...ts.infradead.org, linux-doc@...r.kernel.org,
        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
Subject: Re: [PATCH -next v4 1/2] riscv: kdump: Implement
 crashkernel=X,[high,low]

On 05/11/23 at 04:47pm, chenjiahao (C) wrote:
......
> > > @@ -1163,8 +1185,12 @@ static void __init reserve_crashkernel(void)
> > >   {
> > >   	unsigned long long crash_base = 0;
> > >   	unsigned long long crash_size = 0;
> > > +	unsigned long long crash_low_size = 0;
> > >   	unsigned long search_start = memblock_start_of_DRAM();
> > >   	unsigned long search_end = memblock_end_of_DRAM();
> > > +	unsigned long search_low_max = (unsigned long)dma32_phys_limit;
> > > +	char *cmdline = boot_command_line;
> > > +	bool fixed_base = false;
> > >   	int ret = 0;
> > > @@ -1180,14 +1206,34 @@ static void __init reserve_crashkernel(void)
> > >   		return;
> > >   	}
> > > -	ret = parse_crashkernel(boot_command_line, memblock_phys_mem_size(),
> > > +	ret = parse_crashkernel(cmdline, memblock_phys_mem_size(),
> > >   				&crash_size, &crash_base);
> > > -	if (ret || !crash_size)
> > > +	if (ret == -ENOENT) {
> > > +		/* Fallback to crashkernel=X,[high,low] */
> > > +		ret = parse_crashkernel_high(cmdline, 0, &crash_size, &crash_base);
> > > +		if (ret || !crash_size)
> > > +			return;
> > > +
> > > +		/*
> > > +		 * crashkernel=Y,low is valid only when crashkernel=X,high
> > > +		 * is passed.
> > > +		 */
> > > +		ret = parse_crashkernel_low(cmdline, 0, &crash_low_size, &crash_base);
> > > +		if (ret == -ENOENT)
> > > +			crash_low_size = DEFAULT_CRASH_KERNEL_LOW_SIZE;
> > > +		else if (ret)
> > > +			return;
> > > +
> > > +		search_start = search_low_max;
> > > +	} else if (ret || !crash_size) {
> > > +		/* Invalid argument value specified */
> > >   		return;
> > > +	}
> > >   	crash_size = PAGE_ALIGN(crash_size);
> > >   	if (crash_base) {
> > > +		fixed_base = true;
> > >   		search_start = crash_base;
> > >   		search_end = crash_base + crash_size;
> > >   	}
> > > @@ -1201,16 +1247,31 @@ 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, search_low_max));
> > Here, it seems not right in case crashkernel=,high is specified. In that
> > case, search_start == search_low_max, then the min(search_end,
> > search_low_max) will get search_low_max too. Then you make the fallback
> > in below code block to try to get crashkernel reservation above 4G. This
> > doesn't comply with the crashkernel=,high grammer which has been
> > implemented in other architectures.
> > 
> > For crashkernel=,high, user explicitly require memory reservation above
> > 4G. Why does crashkernel=,high is needed? E.g on big end server with
> > huge memory, while the low memory under 4G is limited and precious.
> > Hence, user want to put the main crashkernel reservation above 4G to
> > contain kdump kernel/initrd and run user space program, while with few
> > low memory for pci device driver. E.g crashkernel=2G,high, it won't
> > impact much if there's huge memory above 4G and get crashkernel
> > reservation there. However, it impacts a lot if it reserves memory
> > below 4G.
> > 
> > I would strongly suggest that risc-v also reserve memory from above 4G
> > for crashkernel=,high, then fallback to below 4G. That's consistent with
> > crashkernel=,high grammer.
> 
> Sorry for late response.
> 
> I have got the point here. So with the original implication of "crashkernel=,high",
> there is even no need to try reserving low memory under 4G. I have arranged another
> version of patchset, in which I updated the allocation logic in that case.
> 
> For example, when "crashkernel=1G,high" is specified, the previous logic is like:
> alloc range: crash_size: 0x40000000 (1G), crash_base: 4G_limit,
>              crash_max: 4G_limit
> alloc range high: crash_size: 0x40000000 (1G), crash_base: 4G_limit,
>                   crash_max: memblock_range_end
> alloc range low: low_size: 0x8000000 (128MB,default), crash_base: 0x0,
>                  crash_max: 4G_limit
> 
> After revision, the logic is like:
> alloc range: crash_size: 0x40000000 (1G), crash_base: memblock_range_start,
>              crash_max: memblock_range_end
> alloc range low: low_size: 0x8000000 (128MB,default), crash_base: 0x0,
>                  crash_max: 4G_limit
> 
> Please let me know if there is any problem exist.

Sorry for late reply.

Hmm, it doesn't seem completely correct. I will comment in your v5
patch. Please see over there.