linux-kernel - Re: [PATCH 1/2] vmcore: allow alternate dump capturing methods to export vmcore without is_kdump

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Message-ID: <ZPP/UeP1zUbGPzrt@MiWiFi-R3L-srv>
Date:   Sun, 3 Sep 2023 11:36:49 +0800
From:   Baoquan He <bhe@...hat.com>
To:     Hari Bathini <hbathini@...ux.ibm.com>
Cc:     lkml <linux-kernel@...r.kernel.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        Kexec-ml <kexec@...ts.infradead.org>,
        Dave Young <dyoung@...hat.com>,
        Mahesh J Salgaonkar <mahesh@...ux.ibm.com>,
        Sourabh Jain <sourabhjain@...ux.ibm.com>,
        Michael Ellerman <mpe@...erman.id.au>
Subject: Re: [PATCH 1/2] vmcore: allow alternate dump capturing methods to
 export vmcore without is_kdump_kernel()

Hi Hari,

On 09/02/23 at 12:34am, Hari Bathini wrote:
> Currently, is_kdump_kernel() returns true when elfcorehdr_addr is set.
> While elfcorehdr_addr is set for kexec based kernel dump mechanism,
> alternate dump capturing methods like fadump [1] also set it to export
> the vmcore. is_kdump_kernel() is used to restrict resources in crash
> dump capture kernel but such restrictions may not be desirable for
> fadump. Allow is_kdump_kernel() to be defined differently for such
> scenarios. With this, is_kdump_kernel() could be false while vmcore
> is usable. So, introduce is_crashdump_kernel() to return true when
> elfcorehdr_addr is set and use it for vmcore related checks.

I got what is done in these two patches, but didn't get why they need be
done. vmcore_unusable()/is_vmcore_usable() are only unitilized in ia64.
Why do you care if it's is_crashdump_kernel() or is_kdump_kernel()?
If you want to override the generic is_kdump_kernel() with powerpc's own
is_kdump_kernel(), your below change is enough to allow you to do that.
I can't see why is_crashdump_kernel() is needed. Could you explain that
specifically?

#ifndef is_kdump_kernel
static inline bool is_kdump_kernel(void)
{
        return elfcorehdr_addr != ELFCORE_ADDR_MAX;
}
#endif

Thanks
Baoquan

> [1] https://docs.kernel.org/powerpc/firmware-assisted-dump.html
> 
> Signed-off-by: Hari Bathini <hbathini@...ux.ibm.com>
> ---
>  include/linux/crash_dump.h | 18 ++++++++++++++++--
>  1 file changed, 16 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/crash_dump.h b/include/linux/crash_dump.h
> index 0f3a656293b0..1052a0faf0dd 100644
> --- a/include/linux/crash_dump.h
> +++ b/include/linux/crash_dump.h
> @@ -50,6 +50,7 @@ void vmcore_cleanup(void);
>  #define vmcore_elf64_check_arch(x) (elf_check_arch(x) || vmcore_elf_check_arch_cross(x))
>  #endif
>  
> +#ifndef is_kdump_kernel
>  /*
>   * is_kdump_kernel() checks whether this kernel is booting after a panic of
>   * previous kernel or not. This is determined by checking if previous kernel
> @@ -64,6 +65,19 @@ static inline bool is_kdump_kernel(void)
>  {
>  	return elfcorehdr_addr != ELFCORE_ADDR_MAX;
>  }
> +#endif
> +
> +/*
> + * Return true if this is a dump capture kernel, where vmcore needs to be
> + * exported, irrespective of the dump capture mechanism in use.
> + *
> + * Same as is_kdump_kernel() unless arch specific code defines is_kdump_kernel()
> + * differently while supporting other dump capturing mechanisms.
> + */
> +static inline bool is_crashdump_kernel(void)
> +{
> +	return elfcorehdr_addr != ELFCORE_ADDR_MAX;
> +}
>  
>  /* is_vmcore_usable() checks if the kernel is booting after a panic and
>   * the vmcore region is usable.
> @@ -75,7 +89,7 @@ static inline bool is_kdump_kernel(void)
>  
>  static inline int is_vmcore_usable(void)
>  {
> -	return is_kdump_kernel() && elfcorehdr_addr != ELFCORE_ADDR_ERR ? 1 : 0;
> +	return is_crashdump_kernel() && elfcorehdr_addr != ELFCORE_ADDR_ERR ? 1 : 0;
>  }
>  
>  /* vmcore_unusable() marks the vmcore as unusable,
> @@ -84,7 +98,7 @@ static inline int is_vmcore_usable(void)
>  
>  static inline void vmcore_unusable(void)
>  {
> -	if (is_kdump_kernel())
> +	if (is_crashdump_kernel())
>  		elfcorehdr_addr = ELFCORE_ADDR_ERR;
>  }
>  
> -- 
> 2.41.0
>