linux-kernel - Re: [PATCH 1/2 v7] x86/kdump: always reserve the low 1MiB when the crashkernel option is specified

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Date:   Tue, 29 Oct 2019 14:25:43 +0800
From:   Baoquan He <bhe@...hat.com>
To:     lijiang <lijiang@...hat.com>
Cc:     linux-kernel@...r.kernel.org, tglx@...utronix.de, mingo@...hat.com,
        bp@...en8.de, hpa@...or.com, x86@...nel.org, dyoung@...hat.com,
        jgross@...e.com, dhowells@...hat.com, Thomas.Lendacky@....com,
        ebiederm@...ssion.com, vgoyal@...hat.com, d.hatayama@...itsu.com,
        horms@...ge.net.au, kexec@...ts.infradead.org
Subject: Re: [PATCH 1/2 v7] x86/kdump: always reserve the low 1MiB when the
 crashkernel option is specified

On 10/29/19 at 02:06pm, lijiang wrote:
> >>  struct crash_memmap_data {
> >> @@ -68,6 +70,19 @@ static inline void cpu_crash_vmclear_loaded_vmcss(void)
> >>  	rcu_read_unlock();
> >>  }
> >>  
> >> +/*
> >> + * When the crashkernel option is specified, only use the low
> >> + * 1MiB for the real mode trampoline.
> >> + */
> >> +void __init kexec_reserve_low_1MiB(void)
> > 
> > Thanks for the effort, Lianbo. I believe everyone is confident with this
> > solution and fix.
> > 
> > I have a tiny concern, why the function name is
> > kexec_reserve_low_1MiB(), but not kexec_reserve_low_1M()?
> 
> Thanks for your comment, Baoquan.
> 
> It means that kernel will reserve 1M 'Byte' memory, the function name does not
> have special meaning.
> 
> Would you mind if i change it to the crash_reserve_low_1M()?

Yes, crash_xx looks better since it's only related to crash dumping. As
for 1M, not very sure, see if other people have comment about it. Anyway,
crash_reserve_low_1M() looks good to me. Thanks.

> 
> void __init crash_reserve_low_1M(void)
> 
> Thanks.
> Lianbo
> 
> > I searched in kernel code with below filter, didn't see MiB appearing in
> > a function name. I am not sure about it either, just ask.
> > 
> > git grep "_[1-9]*M " arch/ kernel/ mm include/ drivers/ net/ init fs crypto/ certs/ ipc lib
> > 
> > Thanks
> > Baoquan
> > 
> >> +{
> >> +	if (cmdline_find_option(boot_command_line, "crashkernel",
> >> +				NULL, 0) > 0) {
> >> +		memblock_reserve(0, 1<<20);
> >> +		pr_info("Reserving the low 1MiB of memory for crashkernel\n");
> >> +	}
> >> +}
> >> +
> >>  #if defined(CONFIG_SMP) && defined(CONFIG_X86_LOCAL_APIC)
> >>  
> >>  static void kdump_nmi_callback(int cpu, struct pt_regs *regs)
> >> diff --git a/arch/x86/realmode/init.c b/arch/x86/realmode/init.c
> >> index 7dce39c8c034..b8bbd0017ca8 100644
> >> --- a/arch/x86/realmode/init.c
> >> +++ b/arch/x86/realmode/init.c
> >> @@ -8,6 +8,7 @@
> >>  #include <asm/pgtable.h>
> >>  #include <asm/realmode.h>
> >>  #include <asm/tlbflush.h>
> >> +#include <asm/crash.h>
> >>  
> >>  struct real_mode_header *real_mode_header;
> >>  u32 *trampoline_cr4_features;
> >> @@ -34,6 +35,7 @@ void __init reserve_real_mode(void)
> >>  
> >>  	memblock_reserve(mem, size);
> >>  	set_real_mode_mem(mem);
> >> +	kexec_reserve_low_1MiB();
> >>  }
> >>  
> >>  static void __init setup_real_mode(void)
> >> -- 
> >> 2.17.1
> >>