Document/kexec: generalize crash hotplug description

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.

Link: https://lkml.kernel.org/r/20240812041651.703156-1-sourabhjain@linux.ibm.com
Signed-off-by: Sourabh Jain <sourabhjain@linux.ibm.com>
Reviewed-by: Petr Tesarik <ptesarik@suse.com>
Acked-by: Baoquan He <bhe@redhat.com>
Cc: Hari Bathini <hbathini@linux.ibm.com>
Cc: Petr Tesarik <petr@tesarici.cz>
Cc: Sourabh Jain <sourabhjain@linux.ibm.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/Documentation/ABI/testing/sysfs-devices-memory b/Documentation/ABI/testing/sysfs-devices-memory
index a95e0f17c35ade3a8c4623160773cf6060d16feb..cec65827e6020d8724e0ada6eeeaaeb85343b584 100644 (file)
--- 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@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 de725ca3be825ba0e289afbf6d44d97f3760a477..206079d3bd5b127b0ccddfd4be50c978563a0b46 100644 (file)
--- a/Documentation/ABI/testing/sysfs-devices-system-cpu
+++ b/Documentation/ABI/testing/sysfs-devices-system-cpu
@@ -704,9 +704,9 @@ What:               /sys/devices/system/cpu/crash_hotplug
 Date:          Aug 2023
 Contact:       Linux kernel mailing list <linux-kernel@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 098f14d83e991ecabe04548c3a192e4c82b85dc6..cb2c080f400ce40d9019763faa62c1a016496cca 100644 (file)
--- 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 dcb0e379e5e8fbb5293d2be94cf8361be0c5c5e6..a21dbf261be7f5000855394e01ecb62594d3ec76 100644 (file)
--- 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 63cf89393c6eb7c4b5d2d3fe2188a1e46377d952..c1048893f4b6821eeae2e82254f1dc5c6f3e972d 100644 (file)
--- 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;
        }