[PATCH v2] Document/kexec: Generalize crash hotplug description

[Petr Tesarik]

On Mon, 12 Aug 2024 09:46:51 +0530
Sourabh Jain <sourabhjain at linux.ibm.com> wrote:

> Commit 79365026f869 ("crash: add a new kexec flag for hotplug support")
> generalizes the crash hotplug support to allow architectures to update
> multiple kexec segments on CPU/Memory hotplug and not just elfcorehdr.
> Therefore, update the relevant kernel documentation to reflect the same.
> 
> No functional change.
> 
> Cc: Petr Tesarik <petr at tesarici.cz>
> Cc: Hari Bathini <hbathini at linux.ibm.com>
> Cc: kexec at lists.infradead.org
> Cc: linux-kernel at vger.kernel.org
> Cc: linuxppc-dev at lists.ozlabs.org
> Cc: x86 at kernel.org
> Signed-off-by: Sourabh Jain <sourabhjain at linux.ibm.com>

It's perfect now.

Reviewed-by: Petr Tesarik <ptesarik at suse.com>

Petr T

> ---
> 
> Changelog:
> 
> Since v1: https://lore.kernel.org/all/20240805050829.297171-1-sourabhjain@linux.ibm.com/
>   - Update crash_hotplug sysfs document as suggested by Petr T
>   - Update an error message in crash_handle_hotplug_event and
>     crash_check_hotplug_support function.
> 
> ---
>  .../ABI/testing/sysfs-devices-memory          |  6 ++--
>  .../ABI/testing/sysfs-devices-system-cpu      |  6 ++--
>  .../admin-guide/mm/memory-hotplug.rst         |  5 +--
>  Documentation/core-api/cpu_hotplug.rst        | 10 +++---
>  kernel/crash_core.c                           | 33 +++++++++++--------
>  5 files changed, 35 insertions(+), 25 deletions(-)
> 
> diff --git a/Documentation/ABI/testing/sysfs-devices-memory b/Documentation/ABI/testing/sysfs-devices-memory
> index a95e0f17c35a..cec65827e602 100644
> --- a/Documentation/ABI/testing/sysfs-devices-memory
> +++ b/Documentation/ABI/testing/sysfs-devices-memory
> @@ -115,6 +115,6 @@ What:		/sys/devices/system/memory/crash_hotplug
>  Date:		Aug 2023
>  Contact:	Linux kernel mailing list <linux-kernel at vger.kernel.org>
>  Description:
> -		(RO) indicates whether or not the kernel directly supports
> -		modifying the crash elfcorehdr for memory hot un/plug and/or
> -		on/offline changes.
> +		(RO) indicates whether or not the kernel updates relevant kexec
> +		segments on memory hot un/plug and/or on/offline events, avoiding the
> +		need to reload kdump kernel.
> diff --git a/Documentation/ABI/testing/sysfs-devices-system-cpu b/Documentation/ABI/testing/sysfs-devices-system-cpu
> index 325873385b71..1a31b7c71676 100644
> --- a/Documentation/ABI/testing/sysfs-devices-system-cpu
> +++ b/Documentation/ABI/testing/sysfs-devices-system-cpu
> @@ -703,9 +703,9 @@ What:		/sys/devices/system/cpu/crash_hotplug
>  Date:		Aug 2023
>  Contact:	Linux kernel mailing list <linux-kernel at vger.kernel.org>
>  Description:
> -		(RO) indicates whether or not the kernel directly supports
> -		modifying the crash elfcorehdr for CPU hot un/plug and/or
> -		on/offline changes.
> +		(RO) indicates whether or not the kernel updates relevant kexec
> +		segments on memory hot un/plug and/or on/offline events, avoiding the
> +		need to reload kdump kernel.
>  
>  What:		/sys/devices/system/cpu/enabled
>  Date:		Nov 2022
> diff --git a/Documentation/admin-guide/mm/memory-hotplug.rst b/Documentation/admin-guide/mm/memory-hotplug.rst
> index 098f14d83e99..cb2c080f400c 100644
> --- a/Documentation/admin-guide/mm/memory-hotplug.rst
> +++ b/Documentation/admin-guide/mm/memory-hotplug.rst
> @@ -294,8 +294,9 @@ The following files are currently defined:
>  ``crash_hotplug``      read-only: when changes to the system memory map
>  		       occur due to hot un/plug of memory, this file contains
>  		       '1' if the kernel updates the kdump capture kernel memory
> -		       map itself (via elfcorehdr), or '0' if userspace must update
> -		       the kdump capture kernel memory map.
> +		       map itself (via elfcorehdr and other relevant kexec
> +		       segments), or '0' if userspace must update the kdump
> +		       capture kernel memory map.
>  
>  		       Availability depends on the CONFIG_MEMORY_HOTPLUG kernel
>  		       configuration option.
> diff --git a/Documentation/core-api/cpu_hotplug.rst b/Documentation/core-api/cpu_hotplug.rst
> index dcb0e379e5e8..a21dbf261be7 100644
> --- a/Documentation/core-api/cpu_hotplug.rst
> +++ b/Documentation/core-api/cpu_hotplug.rst
> @@ -737,8 +737,9 @@ can process the event further.
>  
>  When changes to the CPUs in the system occur, the sysfs file
>  /sys/devices/system/cpu/crash_hotplug contains '1' if the kernel
> -updates the kdump capture kernel list of CPUs itself (via elfcorehdr),
> -or '0' if userspace must update the kdump capture kernel list of CPUs.
> +updates the kdump capture kernel list of CPUs itself (via elfcorehdr and
> +other relevant kexec segment), or '0' if userspace must update the kdump
> +capture kernel list of CPUs.
>  
>  The availability depends on the CONFIG_HOTPLUG_CPU kernel configuration
>  option.
> @@ -750,8 +751,9 @@ file can be used in a udev rule as follows:
>   SUBSYSTEM=="cpu", ATTRS{crash_hotplug}=="1", GOTO="kdump_reload_end"
>  
>  For a CPU hot un/plug event, if the architecture supports kernel updates
> -of the elfcorehdr (which contains the list of CPUs), then the rule skips
> -the unload-then-reload of the kdump capture kernel.
> +of the elfcorehdr (which contains the list of CPUs) and other relevant
> +kexec segments, then the rule skips the unload-then-reload of the kdump
> +capture kernel.
>  
>  Kernel Inline Documentations Reference
>  ======================================
> diff --git a/kernel/crash_core.c b/kernel/crash_core.c
> index 63cf89393c6e..c1048893f4b6 100644
> --- a/kernel/crash_core.c
> +++ b/kernel/crash_core.c
> @@ -505,7 +505,7 @@ int crash_check_hotplug_support(void)
>  	crash_hotplug_lock();
>  	/* Obtain lock while reading crash information */
>  	if (!kexec_trylock()) {
> -		pr_info("kexec_trylock() failed, elfcorehdr may be inaccurate\n");
> +		pr_info("kexec_trylock() failed, kdump image may be inaccurate\n");
>  		crash_hotplug_unlock();
>  		return 0;
>  	}
> @@ -520,18 +520,25 @@ int crash_check_hotplug_support(void)
>  }
>  
>  /*
> - * To accurately reflect hot un/plug changes of cpu and memory resources
> - * (including onling and offlining of those resources), the elfcorehdr
> - * (which is passed to the crash kernel via the elfcorehdr= parameter)
> - * must be updated with the new list of CPUs and memories.
> + * To accurately reflect hot un/plug changes of CPU and Memory resources
> + * (including onling and offlining of those resources), the relevant
> + * kexec segments must be updated with latest CPU and Memory resources.
>   *
> - * In order to make changes to elfcorehdr, two conditions are needed:
> - * First, the segment containing the elfcorehdr must be large enough
> - * to permit a growing number of resources; the elfcorehdr memory size
> - * is based on NR_CPUS_DEFAULT and CRASH_MAX_MEMORY_RANGES.
> - * Second, purgatory must explicitly exclude the elfcorehdr from the
> - * list of segments it checks (since the elfcorehdr changes and thus
> - * would require an update to purgatory itself to update the digest).
> + * Architectures must ensure two things for all segments that need
> + * updating during hotplug events:
> + *
> + * 1. Segments must be large enough to accommodate a growing number of
> + *    resources.
> + * 2. Exclude the segments from SHA verification.
> + *
> + * For example, on most architectures, the elfcorehdr (which is passed
> + * to the crash kernel via the elfcorehdr= parameter) must include the
> + * new list of CPUs and memory. To make changes to the elfcorehdr, it
> + * should be large enough to permit a growing number of CPU and Memory
> + * resources. One can estimate the elfcorehdr memory size based on
> + * NR_CPUS_DEFAULT and CRASH_MAX_MEMORY_RANGES. The elfcorehdr is
> + * excluded from SHA verification by default if the architecture
> + * supports crash hotplug.
>   */
>  static void crash_handle_hotplug_event(unsigned int hp_action, unsigned int cpu, void *arg)
>  {
> @@ -540,7 +547,7 @@ static void crash_handle_hotplug_event(unsigned int hp_action, unsigned int cpu,
>  	crash_hotplug_lock();
>  	/* Obtain lock while changing crash information */
>  	if (!kexec_trylock()) {
> -		pr_info("kexec_trylock() failed, elfcorehdr may be inaccurate\n");
> +		pr_info("kexec_trylock() failed, kdump image may be inaccurate\n");
>  		crash_hotplug_unlock();
>  		return;
>  	}