[PATCH v3 0/6] crash: Kernel handling of CPU and memory hot un/plug

[PATCH v3 0/6] crash: Kernel handling of CPU and memory hot un/plug

[Eric DeVolder]

When the kdump service is loaded, if a CPU or memory is hot
un/plugged, the crash elfcorehdr (for x86), which describes the CPUs
and memory in the system, must also be updated, else the resulting
vmcore is inaccurate (eg. missing either CPU context or memory
regions).

The current solution utilizes udev to initiate an unload-then-reload
of the kdump image (e. kernel, initrd, boot_params, puratory and
elfcorehdr) by the userspace kexec utility. In previous posts I have
outlined the significant performance problems related to offloading
this activity to userspace.

This patchset introduces a generic crash hot un/plug handler that
registers with the CPU and memory notifiers. Upon CPU or memory
changes, this generic handler is invoked and performs important
housekeeping, for example obtaining the appropriate lock, and then
invokes an architecture specific handler to do the appropriate
updates.

In the case of x86_64, the arch specific handler generates a new
elfcorehdr, and overwrites the old one in memory. No involvement
with userspace needed.

To realize the benefits/test this patchset, one must make a couple
of minor changes to userspace:

 - Disable the udev rule for updating kdump on hot un/plug changes
   Eg. on RHEL: rm -f /usr/lib/udev/rules.d/98-kexec.rules
   or other technique to neuter the rule.

 - Change to the kexec_file_load for loading the kdump kernel:
   Eg. on RHEL: in /usr/bin/kdumpctl, change to:
    standard_kexec_args="-p -d -s"
   which adds the -s to select kexec_file_load syscall.

This patchset supports kexec_load with a modified kexec userspace
utility, and a working changeset to the kexec userspace utility
is provided here (and to use, the above change to standard_kexec_args
would be, for example, to append --hotplug-size=262144 instead of -s).

 diff --git a/kexec/arch/i386/crashdump-x86.c b/kexec/arch/i386/crashdump-x86.c
 index 9826f6d..06adb7e 100644
 --- a/kexec/arch/i386/crashdump-x86.c
 +++ b/kexec/arch/i386/crashdump-x86.c
 @@ -48,6 +48,7 @@
  #include <x86/x86-linux.h>
  
  extern struct arch_options_t arch_options;
 +extern unsigned long long hotplug_size;
  
  static int get_kernel_page_offset(struct kexec_info *UNUSED(info),
  				  struct crash_elf_info *elf_info)
 @@ -975,6 +976,13 @@ int load_crashdump_segments(struct kexec_info *info, char* mod_cmdline,
  	} else {
  		memsz = bufsz;
  	}
 +
 +    /* If hotplug support enabled, use that size */
 +    if (hotplug_size) {
 +        memsz = hotplug_size;
 +    }
 +
 +    info->elfcorehdr =
  	elfcorehdr = add_buffer(info, tmp, bufsz, memsz, align, min_base,
  							max_addr, -1);
  	dbgprintf("Created elf header segment at 0x%lx\n", elfcorehdr);
 diff --git a/kexec/kexec.c b/kexec/kexec.c
 index f63b36b..9569d9a 100644
 --- a/kexec/kexec.c
 +++ b/kexec/kexec.c
 @@ -58,6 +58,7 @@
  
  unsigned long long mem_min = 0;
  unsigned long long mem_max = ULONG_MAX;
 +unsigned long long hotplug_size = 0;
  static unsigned long kexec_flags = 0;
  /* Flags for kexec file (fd) based syscall */
  static unsigned long kexec_file_flags = 0;
 @@ -672,6 +673,12 @@ static void update_purgatory(struct kexec_info *info)
  		if (info->segment[i].mem == (void *)info->rhdr.rel_addr) {
  			continue;
  		}
 +        /* Don't include elfcorehdr in the checksum, if hotplug
 +         * support enabled.
 +         */
 +        if (hotplug_size && (info->segment[i].mem == (void *)info->elfcorehdr)) {
 +			continue;
 +		}
  		sha256_update(&ctx, info->segment[i].buf,
  			      info->segment[i].bufsz);
  		nullsz = info->segment[i].memsz - info->segment[i].bufsz;
 @@ -1504,6 +1511,17 @@ int main(int argc, char *argv[])
  		case OPT_PRINT_CKR_SIZE:
  			print_crashkernel_region_size();
  			return 0;
 +		case OPT_HOTPLUG_SIZE:
 +            /* Reserved the specified size for hotplug growth */
 +			hotplug_size = strtoul(optarg, &endptr, 0);
 +			if (*endptr) {
 +				fprintf(stderr,
 +					"Bad option value in --hotplug-size=%s\n",
 +					optarg);
 +				usage();
 +				return 1;
 +			}
 +			break;
  		default:
  			break;
  		}
 diff --git a/kexec/kexec.h b/kexec/kexec.h
 index 595dd68..b30dda4 100644
 --- a/kexec/kexec.h
 +++ b/kexec/kexec.h
 @@ -169,6 +169,7 @@ struct kexec_info {
  	int command_line_len;
  
  	int skip_checks;
 +    unsigned long elfcorehdr;
   };
  
  struct arch_map_entry {
 @@ -231,7 +232,8 @@ extern int file_types;
  #define OPT_PRINT_CKR_SIZE	262
  #define OPT_LOAD_LIVE_UPDATE	263
  #define OPT_EXEC_LIVE_UPDATE	264
 -#define OPT_MAX			265
 +#define OPT_HOTPLUG_SIZE	265
 +#define OPT_MAX			266
  #define KEXEC_OPTIONS \
  	{ "help",		0, 0, OPT_HELP }, \
  	{ "version",		0, 0, OPT_VERSION }, \
 @@ -258,6 +260,7 @@ extern int file_types;
  	{ "debug",		0, 0, OPT_DEBUG }, \
  	{ "status",		0, 0, OPT_STATUS }, \
  	{ "print-ckr-size",     0, 0, OPT_PRINT_CKR_SIZE }, \
 +	{ "hotplug-size",     2, 0, OPT_HOTPLUG_SIZE }, \
  
  #define KEXEC_OPT_STR "h?vdfixyluet:pscaS"
 

Regards,
eric
---
v3: 10jan2022
 - Rebasing per Baoquan He request.
 - Changed memory notifier per David Hildenbrand.
 - Providing example kexec userspace change in cover letter.

RFC v2: 7dec2021
 https://lkml.org/lkml/2021/12/7/1088
 - Acting upon Baoquan He suggestion of removing elfcorehdr from
   the purgatory list of segments, removed purgatory code from
   patchset, and it is signficiantly simpler now.

RFC v1: 18nov2021
 https://lkml.org/lkml/2021/11/18/845
 - working patchset demonstrating kernel handling of hotplug
   updates to x86 elfcorehdr for kexec_file_load

RFC: 14dec2020
 https://lkml.org/lkml/2020/12/14/532
 - proposed concept of allowing kernel to handle hotplug update
   of elfcorehdr
---

Eric DeVolder (6):
  crash: fix minor typo/bug in debug message
  crash hp: Introduce CRASH_HOTPLUG configuration options
  crash hp: definitions and prototype changes
  crash hp: generic crash hotplug support infrastructure
  crash hp: kexec_file changes for crash hotplug support
  crash hp: Add x86 crash hotplug support

 arch/x86/Kconfig        |  26 ++++++++
 arch/x86/kernel/crash.c | 140 +++++++++++++++++++++++++++++++++++++++-
 include/linux/kexec.h   |  21 +++++-
 kernel/crash_core.c     | 117 +++++++++++++++++++++++++++++++++
 kernel/kexec_file.c     |  15 ++++-
 5 files changed, 313 insertions(+), 6 deletions(-)

-- 
2.27.0

[PATCH v3 1/6] crash: fix minor typo/bug in debug message

[Eric DeVolder]

The pr_debug() intends to display the memsz member, but the
parameter is actually the bufsz member (which is already
displayed). Correct this to display memsz value.

Signed-off-by: Eric DeVolder <eric.devolder@oracle.com>
Acked-by: Baoquan He <bhe@redhat.com>
---
 arch/x86/kernel/crash.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kernel/crash.c b/arch/x86/kernel/crash.c
index e8326a8d1c5d..9730c88530fc 100644
--- a/arch/x86/kernel/crash.c
+++ b/arch/x86/kernel/crash.c
@@ -407,7 +407,7 @@ int crash_load_segments(struct kimage *image)
 	}
 	image->elf_load_addr = kbuf.mem;
 	pr_debug("Loaded ELF headers at 0x%lx bufsz=0x%lx memsz=0x%lx\n",
-		 image->elf_load_addr, kbuf.bufsz, kbuf.bufsz);
+		 image->elf_load_addr, kbuf.bufsz, kbuf.memsz);
 
 	return ret;
 }
-- 
2.27.0

[PATCH v3 2/6] crash hp: Introduce CRASH_HOTPLUG configuration options

[Eric DeVolder]

Support for CPU and memory hotplug for crash is controlled by the
CRASH_HOTPLUG configuration option, introduced by this patch.

The CRASH_HOTPLUG_ELFCOREHDR_SZ related configuration option is
also introduced with this patch.

Signed-off-by: Eric DeVolder <eric.devolder@oracle.com>
---
 arch/x86/Kconfig | 26 ++++++++++++++++++++++++++
 1 file changed, 26 insertions(+)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 5c2ccb85f2ef..fcaae76fcfe9 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -2047,6 +2047,32 @@ config CRASH_DUMP
 	  (CONFIG_RELOCATABLE=y).
 	  For more details see Documentation/admin-guide/kdump/kdump.rst
 
+config CRASH_HOTPLUG
+	bool "kernel updates of crash elfcorehdr"
+	depends on CRASH_DUMP && (HOTPLUG_CPU || MEMORY_HOTPLUG) && KEXEC_FILE
+	help
+	  Enable the kernel to update the crash elfcorehdr (which contains
+	  the list of CPUs and memory regions) directly when hot plug/unplug
+	  of CPUs or memory. Otherwise userspace must monitor these hot
+	  plug/unplug change notifications via udev in order to
+	  unload-then-reload the crash kernel so that the list of CPUs and
+	  memory regions is kept up-to-date. Note that the udev CPU and
+	  memory change notifications still occur (however, userspace is not
+	  required to monitor for crash dump purposes).
+
+config CRASH_HOTPLUG_ELFCOREHDR_SZ
+	depends on CRASH_HOTPLUG
+	int
+	default 131072
+	help
+	  Specify the maximum size of the elfcorehdr buffer/segment.
+	  The 128KiB default is sized so that it can accommodate 2048
+	  Elf64_Phdr, where each Phdr represents either a CPU or a
+	  region of memory.
+	  For example, this size can accommodate hotplugging a machine
+	  with up to 1024 CPUs and up to 1024 memory regions (e.g. 1TiB
+	  with 1024 1GiB memory DIMMs).
+
 config KEXEC_JUMP
 	bool "kexec jump"
 	depends on KEXEC && HIBERNATION
-- 
2.27.0

[PATCH v3 3/6] crash hp: definitions and prototype changes

[Eric DeVolder]

This change adds members to struct kimage to facilitate crash
hotplug support.

This change also defines crash hotplug events and associated
prototypes.

Signed-off-by: Eric DeVolder <eric.devolder@oracle.com>
---
 include/linux/kexec.h | 21 +++++++++++++++++++--
 1 file changed, 19 insertions(+), 2 deletions(-)

diff --git a/include/linux/kexec.h b/include/linux/kexec.h
index 0c994ae37729..068f853f1c65 100644
--- a/include/linux/kexec.h
+++ b/include/linux/kexec.h
@@ -221,8 +221,9 @@ struct crash_mem {
 extern int crash_exclude_mem_range(struct crash_mem *mem,
 				   unsigned long long mstart,
 				   unsigned long long mend);
-extern int crash_prepare_elf64_headers(struct crash_mem *mem, int kernel_map,
-				       void **addr, unsigned long *sz);
+extern int crash_prepare_elf64_headers(struct kimage *image,
+					struct crash_mem *mem, int kernel_map,
+					void **addr, unsigned long *sz);
 #endif /* CONFIG_KEXEC_FILE */
 
 #ifdef CONFIG_KEXEC_ELF
@@ -299,6 +300,13 @@ struct kimage {
 
 	/* Information for loading purgatory */
 	struct purgatory_info purgatory_info;
+
+#ifdef CONFIG_CRASH_HOTPLUG
+	bool hotplug_event;
+	int offlinecpu;
+	bool elf_index_valid;
+	int elf_index;
+#endif
 #endif
 
 #ifdef CONFIG_IMA_KEXEC
@@ -315,6 +323,15 @@ struct kimage {
 	unsigned long elf_load_addr;
 };
 
+#ifdef CONFIG_CRASH_HOTPLUG
+void arch_crash_hotplug_handler(struct kimage *image,
+	unsigned int hp_action, unsigned long a, unsigned long b);
+#define KEXEC_CRASH_HP_REMOVE_CPU   0
+#define KEXEC_CRASH_HP_ADD_CPU      1
+#define KEXEC_CRASH_HP_REMOVE_MEMORY 2
+#define KEXEC_CRASH_HP_ADD_MEMORY   3
+#endif /* CONFIG_CRASH_HOTPLUG */
+
 /* kexec interface functions */
 extern void machine_kexec(struct kimage *image);
 extern int machine_kexec_prepare(struct kimage *image);
-- 
2.27.0

[PATCH v3 4/6] crash hp: generic crash hotplug support infrastructure

[Eric DeVolder]

This patch introduces a generic crash hot plug/unplug infrastructure
for CPU and memory changes. Upon CPU and memory changes, a generic
crash_hotplug_handler() obtains the appropriate lock, does some
important house keeping and then dispatches the hot plug/unplug event
to the architecture specific arch_crash_hotplug_handler(), and when
that handler returns, the lock is released.

This patch modifies crash_core.c to implement a subsys_initcall()
function that installs handlers for hot plug/unplug events. If CPU
hotplug is enabled, then cpuhp_setup_state() is invoked to register a
handler for CPU changes. Similarly, if memory hotplug is enabled, then
register_memory_notifier() is invoked to install a handler for memory
changes. These handlers in turn invoke the common generic handler
crash_hotplug_handler().

On the CPU side, cpuhp_setup_state_nocalls() is invoked with parameter
CPUHP_AP_ONLINE_DYN. While this works, when a CPU is being unplugged,
the CPU still shows up in foreach_present_cpu() during the regeneration
of the new CPU list, thus the need to explicitly check and exclude the
soon-to-be offlined CPU in crash_prepare_elf64_headers().

On the memory side, each un/plugged memory block passes through the
handler. For example, if a 1GiB DIMM is hotplugged, that generate 8
memory events, one for each 128MiB memblock.

Signed-off-by: Eric DeVolder <eric.devolder@oracle.com>
---
 kernel/crash_core.c | 117 ++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 117 insertions(+)

diff --git a/kernel/crash_core.c b/kernel/crash_core.c
index 256cf6db573c..acdb9c7ad5f4 100644
--- a/kernel/crash_core.c
+++ b/kernel/crash_core.c
@@ -9,12 +9,16 @@
 #include <linux/init.h>
 #include <linux/utsname.h>
 #include <linux/vmalloc.h>
+#include <linux/memory.h>
+#include <linux/cpuhotplug.h>
 
 #include <asm/page.h>
 #include <asm/sections.h>
 
 #include <crypto/sha1.h>
 
+#include "kexec_internal.h"
+
 /* vmcoreinfo stuff */
 unsigned char *vmcoreinfo_data;
 size_t vmcoreinfo_size;
@@ -491,3 +495,116 @@ static int __init crash_save_vmcoreinfo_init(void)
 }
 
 subsys_initcall(crash_save_vmcoreinfo_init);
+
+#ifdef CONFIG_CRASH_HOTPLUG
+void __weak arch_crash_hotplug_handler(struct kimage *image,
+	unsigned int hp_action, unsigned long a, unsigned long b)
+{
+	pr_warn("crash hp: %s not implemented", __func__);
+}
+
+static void crash_hotplug_handler(unsigned int hp_action,
+	unsigned long a, unsigned long b)
+{
+	/* Obtain lock while changing crash information */
+	if (!mutex_trylock(&kexec_mutex))
+		return;
+
+	/* Check kdump is loaded */
+	if (kexec_crash_image) {
+		pr_debug("crash hp: hp_action %u, a %lu, b %lu", hp_action,
+			a, b);
+
+		/* Needed in order for the segments to be updated */
+		arch_kexec_unprotect_crashkres();
+
+		/* Flag to differentiate between normal load and hotplug */
+		kexec_crash_image->hotplug_event = true;
+
+		/*
+		 * Due to use of CPUHP_AP_ONLINE_DYN, upon unplug and during
+		 * this callback, the CPU is still in the for_each_present_cpu()
+		 * list. Must explicitly look to exclude this CPU when building
+		 * new list.
+		 */
+		kexec_crash_image->offlinecpu =
+			(hp_action == KEXEC_CRASH_HP_REMOVE_CPU) ?
+				(unsigned int)a : ~0U;
+
+		/* Now invoke arch-specific update handler */
+		arch_crash_hotplug_handler(kexec_crash_image, hp_action, a, b);
+
+		/* No longer handling a hotplug event */
+		kexec_crash_image->hotplug_event = false;
+
+		/* Change back to read-only */
+		arch_kexec_protect_crashkres();
+	}
+
+	/* Release lock now that update complete */
+	mutex_unlock(&kexec_mutex);
+}
+
+#if defined(CONFIG_MEMORY_HOTPLUG)
+static int crash_memhp_notifier(struct notifier_block *nb,
+	unsigned long val, void *v)
+{
+	struct memory_notify *mhp = v;
+	unsigned long start, end;
+
+	start = mhp->start_pfn << PAGE_SHIFT;
+	end = ((mhp->start_pfn + mhp->nr_pages) << PAGE_SHIFT) - 1;
+
+	switch (val) {
+	case MEM_ONLINE:
+		crash_hotplug_handler(KEXEC_CRASH_HP_ADD_MEMORY,
+			start, end-start);
+		break;
+
+	case MEM_OFFLINE:
+		crash_hotplug_handler(KEXEC_CRASH_HP_REMOVE_MEMORY,
+			start, end-start);
+		break;
+	}
+	return NOTIFY_OK;
+}
+
+static struct notifier_block crash_memhp_nb = {
+	.notifier_call = crash_memhp_notifier,
+	.priority = 0
+};
+#endif
+
+#if defined(CONFIG_HOTPLUG_CPU)
+static int crash_cpuhp_online(unsigned int cpu)
+{
+	crash_hotplug_handler(KEXEC_CRASH_HP_ADD_CPU, cpu, 0);
+	return 0;
+}
+
+static int crash_cpuhp_offline(unsigned int cpu)
+{
+	crash_hotplug_handler(KEXEC_CRASH_HP_REMOVE_CPU, cpu, 0);
+	return 0;
+}
+#endif
+
+static int __init crash_hotplug_init(void)
+{
+	int result = 0;
+
+#if defined(CONFIG_MEMORY_HOTPLUG)
+	register_memory_notifier(&crash_memhp_nb);
+#endif
+
+#if defined(CONFIG_HOTPLUG_CPU)
+	result = cpuhp_setup_state_nocalls(CPUHP_AP_ONLINE_DYN,
+				"crash/cpuhp",
+				crash_cpuhp_online, crash_cpuhp_offline);
+#endif
+
+	return result;
+}
+
+subsys_initcall(crash_hotplug_init);
+#endif /* CONFIG_CRASH_HOTPLUG */
-- 
2.27.0

[PATCH v3 5/6] crash hp: kexec_file changes for crash hotplug support

[Eric DeVolder]

Two important changes to note:

The kexec_calculate_store_digests() changed to specifically EXCLUDE
the elfcorehdr segment from its list of segments to check.
This is an important change as it allows, in a hotplug environment,
for the elfcorehdr segment (which contains the list of CPUs and
memory regions) to change dynamically without the need to update
purgatory (with the hash/digests of the segments it checks) as well.

The crash_prepare_elf64_headers() changed to look for the offline'd
CPU and exclude it. This due to the fact that the offline'd CPU is
still in the for_each_present_cpu() list at this point in time on
the cpu hotplug handler path.

Signed-off-by: Eric DeVolder <eric.devolder@oracle.com>
---
 kernel/kexec_file.c | 15 +++++++++++++--
 1 file changed, 13 insertions(+), 2 deletions(-)

diff --git a/kernel/kexec_file.c b/kernel/kexec_file.c
index 8347fc158d2b..339995d42169 100644
--- a/kernel/kexec_file.c
+++ b/kernel/kexec_file.c
@@ -765,6 +765,12 @@ static int kexec_calculate_store_digests(struct kimage *image)
 	for (j = i = 0; i < image->nr_segments; i++) {
 		struct kexec_segment *ksegment;
 
+#ifdef CONFIG_CRASH_HOTPLUG
+		/* This segment excluded to allow future changes via hotplug */
+		if (image->elf_index_valid && (j == image->elf_index))
+			continue;
+#endif
+
 		ksegment = &image->segment[i];
 		/*
 		 * Skip purgatory as it will be modified once we put digest
@@ -1260,8 +1266,8 @@ int crash_exclude_mem_range(struct crash_mem *mem,
 	return 0;
 }
 
-int crash_prepare_elf64_headers(struct crash_mem *mem, int kernel_map,
-			  void **addr, unsigned long *sz)
+int crash_prepare_elf64_headers(struct kimage *image, struct crash_mem *mem,
+	int kernel_map, void **addr, unsigned long *sz)
 {
 	Elf64_Ehdr *ehdr;
 	Elf64_Phdr *phdr;
@@ -1308,6 +1314,11 @@ int crash_prepare_elf64_headers(struct crash_mem *mem, int kernel_map,
 
 	/* Prepare one phdr of type PT_NOTE for each present CPU */
 	for_each_present_cpu(cpu) {
+#ifdef CONFIG_CRASH_HOTPLUG
+		/* Skip the soon-to-be offlined cpu */
+		if (image->hotplug_event && (cpu == image->offlinecpu))
+			continue;
+#endif
 		phdr->p_type = PT_NOTE;
 		notes_addr = per_cpu_ptr_to_phys(per_cpu_ptr(crash_notes, cpu));
 		phdr->p_offset = phdr->p_paddr = notes_addr;
-- 
2.27.0

[PATCH v3 6/6] crash hp: Add x86 crash hotplug support

[Eric DeVolder]

For x86_64, when CPU or memory is hot un/plugged, the crash
elfcorehdr, which describes the CPUs and memory in the system,
must also be updated.

To update the elfcorehdr for x86_64, a new elfcorehdr must be
generated from the available CPUs and memory. The new elfcorehdr
is prepared into a buffer, and if no errors occur, it is
installed over the top of the existing elfcorehdr.

In the patch 'crash hp: kexec_file changes for crash hotplug support'
the need to update purgatory due to the change in elfcorehdr was
eliminated.  As a result, no changes to purgatory or boot_params
(as the elfcorehdr= kernel command line parameter pointer
remains unchanged and correct) are needed, just elfcorehdr.

To accommodate a growing number of resources via hotplug, the
elfcorehdr segment must be sufficiently large enough to accommodate
changes, see the CRASH_HOTPLUG_ELFCOREHDR_SZ configure item.

NOTE that this supports both kexec_load and kexec_file_load. Support
for kexec_load is made possible by identifying the elfcorehdr segment
at load time and updating it as previously described. However, it is
the responsibility of the userspace kexec utility to ensure that:
 - the elfcorehdr segment is sufficiently large enough to accommodate
   hotplug changes, ala CRASH_HOTPLUG_ELFCOREHDR_SZ.
 - provides a purgatory that excludes the elfcorehdr from its list of
   run-time segments to check.
These changes to the userspace kexec utility are not yet available.

Signed-off-by: Eric DeVolder <eric.devolder@oracle.com>
---
 arch/x86/kernel/crash.c | 138 +++++++++++++++++++++++++++++++++++++++-
 1 file changed, 137 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/crash.c b/arch/x86/kernel/crash.c
index 9730c88530fc..d185137b33d4 100644
--- a/arch/x86/kernel/crash.c
+++ b/arch/x86/kernel/crash.c
@@ -25,6 +25,7 @@
 #include <linux/slab.h>
 #include <linux/vmalloc.h>
 #include <linux/memblock.h>
+#include <linux/highmem.h>
 
 #include <asm/processor.h>
 #include <asm/hardirq.h>
@@ -265,7 +266,8 @@ static int prepare_elf_headers(struct kimage *image, void **addr,
 		goto out;
 
 	/* By default prepare 64bit headers */
-	ret =  crash_prepare_elf64_headers(cmem, IS_ENABLED(CONFIG_X86_64), addr, sz);
+	ret =  crash_prepare_elf64_headers(image, cmem,
+				IS_ENABLED(CONFIG_X86_64), addr, sz);
 
 out:
 	vfree(cmem);
@@ -397,7 +399,17 @@ int crash_load_segments(struct kimage *image)
 	image->elf_headers = kbuf.buffer;
 	image->elf_headers_sz = kbuf.bufsz;
 
+#ifdef CONFIG_CRASH_HOTPLUG
+	/* Ensure elfcorehdr segment large enough for hotplug changes */
+	kbuf.memsz = CONFIG_CRASH_HOTPLUG_ELFCOREHDR_SZ;
+	/* For marking as usable to crash kernel */
+	image->elf_headers_sz = kbuf.memsz;
+	/* Record the index of the elfcorehdr segment */
+	image->elf_index = image->nr_segments;
+	image->elf_index_valid = true;
+#else
 	kbuf.memsz = kbuf.bufsz;
+#endif
 	kbuf.buf_align = ELF_CORE_HEADER_ALIGN;
 	kbuf.mem = KEXEC_BUF_MEM_UNKNOWN;
 	ret = kexec_add_buffer(&kbuf);
@@ -412,3 +424,127 @@ int crash_load_segments(struct kimage *image)
 	return ret;
 }
 #endif /* CONFIG_KEXEC_FILE */
+
+#ifdef CONFIG_CRASH_HOTPLUG
+void *map_crash_pages(unsigned long paddr, unsigned long size)
+{
+	/*
+	 * NOTE: The addresses and sizes passed to this routine have
+	 * already been fully aligned on page boundaries. There is no
+	 * need for massaging the address or size.
+	 */
+	void *ptr = NULL;
+
+	/* NOTE: requires arch_kexec_[un]protect_crashkres() for write access */
+	if (size > 0) {
+		struct page *page = pfn_to_page(paddr >> PAGE_SHIFT);
+
+		ptr = kmap(page);
+	}
+
+	return ptr;
+}
+
+void unmap_crash_pages(void **ptr)
+{
+	if (ptr) {
+		if (*ptr)
+			kunmap(*ptr);
+		*ptr = NULL;
+	}
+}
+
+void arch_crash_hotplug_handler(struct kimage *image,
+	unsigned int hp_action, unsigned long a, unsigned long b)
+{
+	/*
+	 * To accurately reflect hot un/plug changes, the elfcorehdr (which
+	 * is passed to the crash kernel via the elfcorehdr= parameter)
+	 * must be updated with the new list of CPUs and memories. The new
+	 * elfcorehdr is prepared in a kernel buffer, and if no errors,
+	 * then it is written on top of the existing/old elfcorehdr.
+	 *
+	 * Due to the change to the elfcorehdr, purgatory must explicitly
+	 * exclude the elfcorehdr from the list of segments it checks.
+	 */
+	struct kexec_segment *ksegment;
+	unsigned char *ptr = NULL;
+	unsigned long elfsz = 0;
+	void *elfbuf = NULL;
+	unsigned long mem, memsz;
+	unsigned int n;
+
+	/*
+	 * When the struct kimage is alloced, it is wiped to zero, so
+	 * the elf_index_valid defaults to false. It is set on the
+	 * kexec_file_load path, or here for kexec_load.
+	 */
+	if (!image->elf_index_valid) {
+		for (n = 0; n < image->nr_segments; n++) {
+			mem = image->segment[n].mem;
+			memsz = image->segment[n].memsz;
+			ptr = map_crash_pages(mem, memsz);
+			if (ptr) {
+				/* The segment containing elfcorehdr */
+				if ((ptr[0] == 0x7F) &&
+					(ptr[1] == 'E') &&
+					(ptr[2] == 'L') &&
+					(ptr[3] == 'F')) {
+					image->elf_index = (int)n;
+					image->elf_index_valid = true;
+				}
+			}
+			unmap_crash_pages((void **)&ptr);
+		}
+	}
+
+	/* Must have valid elfcorehdr index */
+	if (!image->elf_index_valid) {
+		pr_err("crash hp: unable to locate elfcorehdr segment");
+		goto out;
+	}
+
+	/*
+	 * Create the new elfcorehdr reflecting the changes to CPU and/or
+	 * memory resources. The elfcorehdr segment memsz must be
+	 * sufficiently large to accommodate increases due to hotplug
+	 * activity. See CRASH_HOTPLUG_ELFCOREHDR_SZ.
+	 */
+	if (prepare_elf_headers(image, &elfbuf, &elfsz)) {
+		pr_err("crash hp: unable to prepare elfcore headers");
+		goto out;
+	}
+	ksegment = &image->segment[image->elf_index];
+	memsz = ksegment->memsz;
+	if (elfsz > memsz) {
+		pr_err("crash hp: update elfcorehdr elfsz %lu > memsz %lu",
+			elfsz, memsz);
+		goto out;
+	}
+
+	/*
+	 * At this point, we are all but assured of success.
+	 * Copy new elfcorehdr into destination.
+	 */
+	ksegment = &image->segment[image->elf_index];
+	mem = ksegment->mem;
+	memsz = ksegment->memsz;
+	ptr = map_crash_pages(mem, memsz);
+	if (ptr) {
+		/* Temporarily invalidate the crash image while it is replaced */
+		xchg(&kexec_crash_image, NULL);
+		/* Write the new elfcorehdr into memory */
+		memcpy((void *)ptr, elfbuf, elfsz);
+		/* The crash image is now valid once again */
+		xchg(&kexec_crash_image, image);
+	}
+	unmap_crash_pages((void **)&ptr);
+	pr_debug("crash hp: re-loaded elfcorehdr at 0x%lx\n", mem);
+
+//FIX??? somekind of cache flush perhaps?
+
+out:
+	if (elfbuf)
+		vfree(elfbuf);
+}
+#endif /* CONFIG_CRASH_HOTPLUG */
-- 
2.27.0

Re: [PATCH v3 3/6] crash hp: definitions and prototype changes

[Baoquan He]

On 01/10/22 at 02:57pm, Eric DeVolder wrote:
> This change adds members to struct kimage to facilitate crash
> hotplug support.
> 
> This change also defines crash hotplug events and associated
> prototypes.
> 
> Signed-off-by: Eric DeVolder <eric.devolder@oracle.com>
> ---
>  include/linux/kexec.h | 21 +++++++++++++++++++--
>  1 file changed, 19 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/kexec.h b/include/linux/kexec.h
> index 0c994ae37729..068f853f1c65 100644
> --- a/include/linux/kexec.h
> +++ b/include/linux/kexec.h
> @@ -221,8 +221,9 @@ struct crash_mem {
>  extern int crash_exclude_mem_range(struct crash_mem *mem,
>  				   unsigned long long mstart,
>  				   unsigned long long mend);
> -extern int crash_prepare_elf64_headers(struct crash_mem *mem, int kernel_map,
> -				       void **addr, unsigned long *sz);
> +extern int crash_prepare_elf64_headers(struct kimage *image,
> +					struct crash_mem *mem, int kernel_map,
> +					void **addr, unsigned long *sz);
>  #endif /* CONFIG_KEXEC_FILE */
>  
>  #ifdef CONFIG_KEXEC_ELF
> @@ -299,6 +300,13 @@ struct kimage {
>  
>  	/* Information for loading purgatory */
>  	struct purgatory_info purgatory_info;
> +
> +#ifdef CONFIG_CRASH_HOTPLUG
> +	bool hotplug_event;
> +	int offlinecpu;
> +	bool elf_index_valid;
> +	int elf_index;

Do we really need elf_index_valid? Can we initialize elf_index to , e.g '-1',
then check if the value is valid?

> +#endif
>  #endif
>  
>  #ifdef CONFIG_IMA_KEXEC
> @@ -315,6 +323,15 @@ struct kimage {
>  	unsigned long elf_load_addr;
>  };
>  
> +#ifdef CONFIG_CRASH_HOTPLUG
> +void arch_crash_hotplug_handler(struct kimage *image,
> +	unsigned int hp_action, unsigned long a, unsigned long b);
> +#define KEXEC_CRASH_HP_REMOVE_CPU   0
> +#define KEXEC_CRASH_HP_ADD_CPU      1
> +#define KEXEC_CRASH_HP_REMOVE_MEMORY 2
> +#define KEXEC_CRASH_HP_ADD_MEMORY   3
> +#endif /* CONFIG_CRASH_HOTPLUG */
> +
>  /* kexec interface functions */
>  extern void machine_kexec(struct kimage *image);
>  extern int machine_kexec_prepare(struct kimage *image);
> -- 
> 2.27.0
> 

Re: [PATCH v3 6/6] crash hp: Add x86 crash hotplug support

[Baoquan He]

On 01/10/22 at 02:57pm, Eric DeVolder wrote:
> For x86_64, when CPU or memory is hot un/plugged, the crash
> elfcorehdr, which describes the CPUs and memory in the system,
> must also be updated.
> 
> To update the elfcorehdr for x86_64, a new elfcorehdr must be
> generated from the available CPUs and memory. The new elfcorehdr
> is prepared into a buffer, and if no errors occur, it is
> installed over the top of the existing elfcorehdr.
> 
> In the patch 'crash hp: kexec_file changes for crash hotplug support'
> the need to update purgatory due to the change in elfcorehdr was
> eliminated.  As a result, no changes to purgatory or boot_params
> (as the elfcorehdr= kernel command line parameter pointer
> remains unchanged and correct) are needed, just elfcorehdr.
> 
> To accommodate a growing number of resources via hotplug, the
> elfcorehdr segment must be sufficiently large enough to accommodate
> changes, see the CRASH_HOTPLUG_ELFCOREHDR_SZ configure item.
> 
> NOTE that this supports both kexec_load and kexec_file_load. Support
> for kexec_load is made possible by identifying the elfcorehdr segment
> at load time and updating it as previously described. However, it is
> the responsibility of the userspace kexec utility to ensure that:
>  - the elfcorehdr segment is sufficiently large enough to accommodate
>    hotplug changes, ala CRASH_HOTPLUG_ELFCOREHDR_SZ.
>  - provides a purgatory that excludes the elfcorehdr from its list of
>    run-time segments to check.
> These changes to the userspace kexec utility are not yet available.
> 
> Signed-off-by: Eric DeVolder <eric.devolder@oracle.com>
> ---
>  arch/x86/kernel/crash.c | 138 +++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 137 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kernel/crash.c b/arch/x86/kernel/crash.c
> index 9730c88530fc..d185137b33d4 100644
> --- a/arch/x86/kernel/crash.c
> +++ b/arch/x86/kernel/crash.c
> @@ -25,6 +25,7 @@
>  #include <linux/slab.h>
>  #include <linux/vmalloc.h>
>  #include <linux/memblock.h>
> +#include <linux/highmem.h>
>  
>  #include <asm/processor.h>
>  #include <asm/hardirq.h>
> @@ -265,7 +266,8 @@ static int prepare_elf_headers(struct kimage *image, void **addr,
>  		goto out;
>  
>  	/* By default prepare 64bit headers */
> -	ret =  crash_prepare_elf64_headers(cmem, IS_ENABLED(CONFIG_X86_64), addr, sz);
> +	ret =  crash_prepare_elf64_headers(image, cmem,
> +				IS_ENABLED(CONFIG_X86_64), addr, sz);
>  
>  out:
>  	vfree(cmem);
> @@ -397,7 +399,17 @@ int crash_load_segments(struct kimage *image)
>  	image->elf_headers = kbuf.buffer;
>  	image->elf_headers_sz = kbuf.bufsz;
>  
> +#ifdef CONFIG_CRASH_HOTPLUG
> +	/* Ensure elfcorehdr segment large enough for hotplug changes */
> +	kbuf.memsz = CONFIG_CRASH_HOTPLUG_ELFCOREHDR_SZ;

I would define a default value for the size, meantime provide a Kconfig
option to allow user to customize.

> +	/* For marking as usable to crash kernel */
> +	image->elf_headers_sz = kbuf.memsz;
> +	/* Record the index of the elfcorehdr segment */
> +	image->elf_index = image->nr_segments;
> +	image->elf_index_valid = true;
> +#else
>  	kbuf.memsz = kbuf.bufsz;
> +#endif
>  	kbuf.buf_align = ELF_CORE_HEADER_ALIGN;
>  	kbuf.mem = KEXEC_BUF_MEM_UNKNOWN;
>  	ret = kexec_add_buffer(&kbuf);
> @@ -412,3 +424,127 @@ int crash_load_segments(struct kimage *image)
>  	return ret;
>  }
>  #endif /* CONFIG_KEXEC_FILE */
> +
> +#ifdef CONFIG_CRASH_HOTPLUG

These two helper function should be carved out into a separate patch as
a preparatory one. I am considering how to rearrange and split the
patches, will reply to cover letter.

> +void *map_crash_pages(unsigned long paddr, unsigned long size)
> +{
> +	/*
> +	 * NOTE: The addresses and sizes passed to this routine have
> +	 * already been fully aligned on page boundaries. There is no
> +	 * need for massaging the address or size.
> +	 */
> +	void *ptr = NULL;
> +
> +	/* NOTE: requires arch_kexec_[un]protect_crashkres() for write access */
> +	if (size > 0) {
> +		struct page *page = pfn_to_page(paddr >> PAGE_SHIFT);
> +
> +		ptr = kmap(page);
> +	}
> +
> +	return ptr;
> +}
> +
> +void unmap_crash_pages(void **ptr)
> +{
> +	if (ptr) {
> +		if (*ptr)
> +			kunmap(*ptr);
> +		*ptr = NULL;
> +	}
> +}
> +
> +void arch_crash_hotplug_handler(struct kimage *image,
> +	unsigned int hp_action, unsigned long a, unsigned long b)
> +{
> +	/*
> +	 * To accurately reflect hot un/plug changes, the elfcorehdr (which
> +	 * is passed to the crash kernel via the elfcorehdr= parameter)
> +	 * must be updated with the new list of CPUs and memories. The new
> +	 * elfcorehdr is prepared in a kernel buffer, and if no errors,
> +	 * then it is written on top of the existing/old elfcorehdr.
> +	 *
> +	 * Due to the change to the elfcorehdr, purgatory must explicitly
> +	 * exclude the elfcorehdr from the list of segments it checks.
> +	 */

Please move this code comment to above function as kernel-doc if you
this it benefits the entire function. Otherwise should move them above
the code block they are explaining. For this place, I think moving them
to above arch_crash_hotplug_handler() is better.

> +	struct kexec_segment *ksegment;
> +	unsigned char *ptr = NULL;
> +	unsigned long elfsz = 0;
> +	void *elfbuf = NULL;
> +	unsigned long mem, memsz;
> +	unsigned int n;
> +
> +	/*
> +	 * When the struct kimage is alloced, it is wiped to zero, so
> +	 * the elf_index_valid defaults to false. It is set on the
> +	 * kexec_file_load path, or here for kexec_load.
> +	 */

I think this kexec loading part should be taken out and post after this
whole patchset being accepted. At least, it's worth to put them in a
separate patch.

> +	if (!image->elf_index_valid) {
> +		for (n = 0; n < image->nr_segments; n++) {
> +			mem = image->segment[n].mem;
> +			memsz = image->segment[n].memsz;
> +			ptr = map_crash_pages(mem, memsz);
> +			if (ptr) {
> +				/* The segment containing elfcorehdr */
> +				if ((ptr[0] == 0x7F) &&
> +					(ptr[1] == 'E') &&
> +					(ptr[2] == 'L') &&
> +					(ptr[3] == 'F')) {

                                      Is this for safety checking later?

> +					image->elf_index = (int)n;
> +					image->elf_index_valid = true;
> +				}
> +			}
> +			unmap_crash_pages((void **)&ptr);
> +		}
> +	}
> +
> +	/* Must have valid elfcorehdr index */
> +	if (!image->elf_index_valid) {
> +		pr_err("crash hp: unable to locate elfcorehdr segment");
> +		goto out;
> +	}
> +
> +	/*
> +	 * Create the new elfcorehdr reflecting the changes to CPU and/or
> +	 * memory resources. The elfcorehdr segment memsz must be
> +	 * sufficiently large to accommodate increases due to hotplug
> +	 * activity. See CRASH_HOTPLUG_ELFCOREHDR_SZ.
> +	 */
> +	if (prepare_elf_headers(image, &elfbuf, &elfsz)) {
> +		pr_err("crash hp: unable to prepare elfcore headers");
> +		goto out;
> +	}
> +	ksegment = &image->segment[image->elf_index];
> +	memsz = ksegment->memsz;
> +	if (elfsz > memsz) {
> +		pr_err("crash hp: update elfcorehdr elfsz %lu > memsz %lu",
> +			elfsz, memsz);
> +		goto out;
> +	}
> +
> +	/*
> +	 * At this point, we are all but assured of success.
> +	 * Copy new elfcorehdr into destination.
> +	 */
> +	ksegment = &image->segment[image->elf_index];
> +	mem = ksegment->mem;
> +	memsz = ksegment->memsz;

The ksegment and memsz have repeated assignment.

> +	ptr = map_crash_pages(mem, memsz);
> +	if (ptr) {
> +		/* Temporarily invalidate the crash image while it is replaced */
> +		xchg(&kexec_crash_image, NULL);
> +		/* Write the new elfcorehdr into memory */
> +		memcpy((void *)ptr, elfbuf, elfsz);
> +		/* The crash image is now valid once again */
> +		xchg(&kexec_crash_image, image);
> +	}
> +	unmap_crash_pages((void **)&ptr);
> +	pr_debug("crash hp: re-loaded elfcorehdr at 0x%lx\n", mem);
> +
> +//FIX??? somekind of cache flush perhaps?

You might mean memcpy_flushcache() on x86.


> +
> +out:
> +	if (elfbuf)
> +		vfree(elfbuf);
> +}
> +#endif /* CONFIG_CRASH_HOTPLUG */
> -- 
> 2.27.0
> 

Re: [PATCH v3 3/6] crash hp: definitions and prototype changes

[Eric DeVolder]

Baoquan,
Thanks for looking at this. Inline responses below.
eric

On 1/19/22 02:26, Baoquan He wrote:
> On 01/10/22 at 02:57pm, Eric DeVolder wrote:
>> This change adds members to struct kimage to facilitate crash
>> hotplug support.
>>
>> This change also defines crash hotplug events and associated
>> prototypes.
>>
>> Signed-off-by: Eric DeVolder <eric.devolder@oracle.com>
>> ---
>>   include/linux/kexec.h | 21 +++++++++++++++++++--
>>   1 file changed, 19 insertions(+), 2 deletions(-)
>>
>> diff --git a/include/linux/kexec.h b/include/linux/kexec.h
>> index 0c994ae37729..068f853f1c65 100644
>> --- a/include/linux/kexec.h
>> +++ b/include/linux/kexec.h
>> @@ -221,8 +221,9 @@ struct crash_mem {
>>   extern int crash_exclude_mem_range(struct crash_mem *mem,
>>   				   unsigned long long mstart,
>>   				   unsigned long long mend);
>> -extern int crash_prepare_elf64_headers(struct crash_mem *mem, int kernel_map,
>> -				       void **addr, unsigned long *sz);
>> +extern int crash_prepare_elf64_headers(struct kimage *image,
>> +					struct crash_mem *mem, int kernel_map,
>> +					void **addr, unsigned long *sz);
>>   #endif /* CONFIG_KEXEC_FILE */
>>   
>>   #ifdef CONFIG_KEXEC_ELF
>> @@ -299,6 +300,13 @@ struct kimage {
>>   
>>   	/* Information for loading purgatory */
>>   	struct purgatory_info purgatory_info;
>> +
>> +#ifdef CONFIG_CRASH_HOTPLUG
>> +	bool hotplug_event;
>> +	int offlinecpu;
>> +	bool elf_index_valid;
>> +	int elf_index;
> 
> Do we really need elf_index_valid? Can we initialize elf_index to , e.g '-1',
> then check if the value is valid?

These members become part of struct kimage, and when the kimage is allocated, it is automatically 
zero'd. Wrt/ elf_index, 0 is a valid index, and so it needs to be qualified. I initially had used 
-1, but that required code and was fragile as I had to find the right place to do that. Using the 
boolean elf_index_valid, the problems with -1 vanish, and for free! I also found when examining the 
code that reading 'elf_index_valid' was better than 'elf_index != -1', more clear.

Let me know what you think.
eric

> 
>> +#endif
>>   #endif
>>   
>>   #ifdef CONFIG_IMA_KEXEC
>> @@ -315,6 +323,15 @@ struct kimage {
>>   	unsigned long elf_load_addr;
>>   };
>>   
>> +#ifdef CONFIG_CRASH_HOTPLUG
>> +void arch_crash_hotplug_handler(struct kimage *image,
>> +	unsigned int hp_action, unsigned long a, unsigned long b);
>> +#define KEXEC_CRASH_HP_REMOVE_CPU   0
>> +#define KEXEC_CRASH_HP_ADD_CPU      1
>> +#define KEXEC_CRASH_HP_REMOVE_MEMORY 2
>> +#define KEXEC_CRASH_HP_ADD_MEMORY   3
>> +#endif /* CONFIG_CRASH_HOTPLUG */
>> +
>>   /* kexec interface functions */
>>   extern void machine_kexec(struct kimage *image);
>>   extern int machine_kexec_prepare(struct kimage *image);
>> -- 
>> 2.27.0
>>
> 

Re: [PATCH v3 6/6] crash hp: Add x86 crash hotplug support

[Eric DeVolder]

Baoquan,
Thanks for the feedback, inline responses below!
eric


On 1/19/22 04:23, Baoquan He wrote:
> On 01/10/22 at 02:57pm, Eric DeVolder wrote:
>> For x86_64, when CPU or memory is hot un/plugged, the crash
>> elfcorehdr, which describes the CPUs and memory in the system,
>> must also be updated.
>>
>> To update the elfcorehdr for x86_64, a new elfcorehdr must be
>> generated from the available CPUs and memory. The new elfcorehdr
>> is prepared into a buffer, and if no errors occur, it is
>> installed over the top of the existing elfcorehdr.
>>
>> In the patch 'crash hp: kexec_file changes for crash hotplug support'
>> the need to update purgatory due to the change in elfcorehdr was
>> eliminated.  As a result, no changes to purgatory or boot_params
>> (as the elfcorehdr= kernel command line parameter pointer
>> remains unchanged and correct) are needed, just elfcorehdr.
>>
>> To accommodate a growing number of resources via hotplug, the
>> elfcorehdr segment must be sufficiently large enough to accommodate
>> changes, see the CRASH_HOTPLUG_ELFCOREHDR_SZ configure item.
>>
>> NOTE that this supports both kexec_load and kexec_file_load. Support
>> for kexec_load is made possible by identifying the elfcorehdr segment
>> at load time and updating it as previously described. However, it is
>> the responsibility of the userspace kexec utility to ensure that:
>>   - the elfcorehdr segment is sufficiently large enough to accommodate
>>     hotplug changes, ala CRASH_HOTPLUG_ELFCOREHDR_SZ.
>>   - provides a purgatory that excludes the elfcorehdr from its list of
>>     run-time segments to check.
>> These changes to the userspace kexec utility are not yet available.
>>
>> Signed-off-by: Eric DeVolder <eric.devolder@oracle.com>
>> ---
>>   arch/x86/kernel/crash.c | 138 +++++++++++++++++++++++++++++++++++++++-
>>   1 file changed, 137 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/x86/kernel/crash.c b/arch/x86/kernel/crash.c
>> index 9730c88530fc..d185137b33d4 100644
>> --- a/arch/x86/kernel/crash.c
>> +++ b/arch/x86/kernel/crash.c
>> @@ -25,6 +25,7 @@
>>   #include <linux/slab.h>
>>   #include <linux/vmalloc.h>
>>   #include <linux/memblock.h>
>> +#include <linux/highmem.h>
>>   
>>   #include <asm/processor.h>
>>   #include <asm/hardirq.h>
>> @@ -265,7 +266,8 @@ static int prepare_elf_headers(struct kimage *image, void **addr,
>>   		goto out;
>>   
>>   	/* By default prepare 64bit headers */
>> -	ret =  crash_prepare_elf64_headers(cmem, IS_ENABLED(CONFIG_X86_64), addr, sz);
>> +	ret =  crash_prepare_elf64_headers(image, cmem,
>> +				IS_ENABLED(CONFIG_X86_64), addr, sz);
>>   
>>   out:
>>   	vfree(cmem);
>> @@ -397,7 +399,17 @@ int crash_load_segments(struct kimage *image)
>>   	image->elf_headers = kbuf.buffer;
>>   	image->elf_headers_sz = kbuf.bufsz;
>>   
>> +#ifdef CONFIG_CRASH_HOTPLUG
>> +	/* Ensure elfcorehdr segment large enough for hotplug changes */
>> +	kbuf.memsz = CONFIG_CRASH_HOTPLUG_ELFCOREHDR_SZ;
> 
> I would define a default value for the size, meantime provide a Kconfig
> option to allow user to customize.

In patch 2/6 of this series, "crash hp: Introduce CRASH_HOTPLUG configuration options", I provide 
the following:

+config CRASH_HOTPLUG_ELFCOREHDR_SZ
+   depends on CRASH_HOTPLUG
+   int
+   default 131072
+   help
+     Specify the maximum size of the elfcorehdr buffer/segment.

which defines a default value of 128KiB, and can be overriden at configure time.

Are you asking for a different technique?

> 
>> +	/* For marking as usable to crash kernel */
>> +	image->elf_headers_sz = kbuf.memsz;
>> +	/* Record the index of the elfcorehdr segment */
>> +	image->elf_index = image->nr_segments;
>> +	image->elf_index_valid = true;
>> +#else
>>   	kbuf.memsz = kbuf.bufsz;
>> +#endif
>>   	kbuf.buf_align = ELF_CORE_HEADER_ALIGN;
>>   	kbuf.mem = KEXEC_BUF_MEM_UNKNOWN;
>>   	ret = kexec_add_buffer(&kbuf);
>> @@ -412,3 +424,127 @@ int crash_load_segments(struct kimage *image)
>>   	return ret;
>>   }
>>   #endif /* CONFIG_KEXEC_FILE */
>> +
>> +#ifdef CONFIG_CRASH_HOTPLUG
> 
> These two helper function should be carved out into a separate patch as
> a preparatory one. I am considering how to rearrange and split the
> patches, will reply to cover letter.

OK, I look forward to that insight!

> 
>> +void *map_crash_pages(unsigned long paddr, unsigned long size)
>> +{
>> +	/*
>> +	 * NOTE: The addresses and sizes passed to this routine have
>> +	 * already been fully aligned on page boundaries. There is no
>> +	 * need for massaging the address or size.
>> +	 */
>> +	void *ptr = NULL;
>> +
>> +	/* NOTE: requires arch_kexec_[un]protect_crashkres() for write access */
>> +	if (size > 0) {
>> +		struct page *page = pfn_to_page(paddr >> PAGE_SHIFT);
>> +
>> +		ptr = kmap(page);
>> +	}
>> +
>> +	return ptr;
>> +}
>> +
>> +void unmap_crash_pages(void **ptr)
>> +{
>> +	if (ptr) {
>> +		if (*ptr)
>> +			kunmap(*ptr);
>> +		*ptr = NULL;
>> +	}
>> +}
>> +
>> +void arch_crash_hotplug_handler(struct kimage *image,
>> +	unsigned int hp_action, unsigned long a, unsigned long b)
>> +{
>> +	/*
>> +	 * To accurately reflect hot un/plug changes, the elfcorehdr (which
>> +	 * is passed to the crash kernel via the elfcorehdr= parameter)
>> +	 * must be updated with the new list of CPUs and memories. The new
>> +	 * elfcorehdr is prepared in a kernel buffer, and if no errors,
>> +	 * then it is written on top of the existing/old elfcorehdr.
>> +	 *
>> +	 * Due to the change to the elfcorehdr, purgatory must explicitly
>> +	 * exclude the elfcorehdr from the list of segments it checks.
>> +	 */
> 
> Please move this code comment to above function as kernel-doc if you
> this it benefits the entire function. Otherwise should move them above
> the code block they are explaining. For this place, I think moving them
> to above arch_crash_hotplug_handler() is better.

ok, I will do that!

> 
>> +	struct kexec_segment *ksegment;
>> +	unsigned char *ptr = NULL;
>> +	unsigned long elfsz = 0;
>> +	void *elfbuf = NULL;
>> +	unsigned long mem, memsz;
>> +	unsigned int n;
>> +
>> +	/*
>> +	 * When the struct kimage is alloced, it is wiped to zero, so
>> +	 * the elf_index_valid defaults to false. It is set on the
>> +	 * kexec_file_load path, or here for kexec_load.
>> +	 */
> 
> I think this kexec loading part should be taken out and post after this
> whole patchset being accepted. At least, it's worth to put them in a
> separate patch.

This little bit of code that identifies the incoming elfcorehdr is all that is needed to support 
kexec_load (and the userspace changes of course). I'm happy to split as a separate patch, but I 
would think that be maintaining it with this series, then when it is accepted, both the kexec_load 
and kexec_file_load paths would be supported? Your call.

> 
>> +	if (!image->elf_index_valid) {
>> +		for (n = 0; n < image->nr_segments; n++) {
>> +			mem = image->segment[n].mem;
>> +			memsz = image->segment[n].memsz;
>> +			ptr = map_crash_pages(mem, memsz);
>> +			if (ptr) {
>> +				/* The segment containing elfcorehdr */
>> +				if ((ptr[0] == 0x7F) &&
>> +					(ptr[1] == 'E') &&
>> +					(ptr[2] == 'L') &&
>> +					(ptr[3] == 'F')) {
> 
>                                        Is this for safety checking later?
No, this code is here to support the kexec_load path; the incoming elfcorehdr has to be identified 
in order to make changes to it later.
> 
>> +					image->elf_index = (int)n;
>> +					image->elf_index_valid = true;
>> +				}
>> +			}
>> +			unmap_crash_pages((void **)&ptr);
>> +		}
>> +	}
>> +
>> +	/* Must have valid elfcorehdr index */
>> +	if (!image->elf_index_valid) {
>> +		pr_err("crash hp: unable to locate elfcorehdr segment");
>> +		goto out;
>> +	}
>> +
>> +	/*
>> +	 * Create the new elfcorehdr reflecting the changes to CPU and/or
>> +	 * memory resources. The elfcorehdr segment memsz must be
>> +	 * sufficiently large to accommodate increases due to hotplug
>> +	 * activity. See CRASH_HOTPLUG_ELFCOREHDR_SZ.
>> +	 */
>> +	if (prepare_elf_headers(image, &elfbuf, &elfsz)) {
>> +		pr_err("crash hp: unable to prepare elfcore headers");
>> +		goto out;
>> +	}
>> +	ksegment = &image->segment[image->elf_index];
>> +	memsz = ksegment->memsz;
>> +	if (elfsz > memsz) {
>> +		pr_err("crash hp: update elfcorehdr elfsz %lu > memsz %lu",
>> +			elfsz, memsz);
>> +		goto out;
>> +	}
>> +
>> +	/*
>> +	 * At this point, we are all but assured of success.
>> +	 * Copy new elfcorehdr into destination.
>> +	 */
>> +	ksegment = &image->segment[image->elf_index];
>> +	mem = ksegment->mem;
>> +	memsz = ksegment->memsz;
> 
> The ksegment and memsz have repeated assignment.

Ah, good catch! I will correct.

> 
>> +	ptr = map_crash_pages(mem, memsz);
>> +	if (ptr) {
>> +		/* Temporarily invalidate the crash image while it is replaced */
>> +		xchg(&kexec_crash_image, NULL);
>> +		/* Write the new elfcorehdr into memory */
>> +		memcpy((void *)ptr, elfbuf, elfsz);
>> +		/* The crash image is now valid once again */
>> +		xchg(&kexec_crash_image, image);
>> +	}
>> +	unmap_crash_pages((void **)&ptr);
>> +	pr_debug("crash hp: re-loaded elfcorehdr at 0x%lx\n", mem);
>> +
>> +//FIX??? somekind of cache flush perhaps?
> 
> You might mean memcpy_flushcache() on x86.

Thanks, I'll look into this and hopefully use it above.

> 
> 
>> +
>> +out:
>> +	if (elfbuf)
>> +		vfree(elfbuf);
>> +}
>> +#endif /* CONFIG_CRASH_HOTPLUG */
>> -- 
>> 2.27.0
>>
> 

Re: [PATCH v3 3/6] crash hp: definitions and prototype changes

[Baoquan He]

On 01/21/22 at 07:48am, Eric DeVolder wrote:
...... 
> > > diff --git a/include/linux/kexec.h b/include/linux/kexec.h
> > > index 0c994ae37729..068f853f1c65 100644
> > > --- a/include/linux/kexec.h
> > > +++ b/include/linux/kexec.h
> > > @@ -221,8 +221,9 @@ struct crash_mem {
> > >   extern int crash_exclude_mem_range(struct crash_mem *mem,
> > >   				   unsigned long long mstart,
> > >   				   unsigned long long mend);
> > > -extern int crash_prepare_elf64_headers(struct crash_mem *mem, int kernel_map,
> > > -				       void **addr, unsigned long *sz);
> > > +extern int crash_prepare_elf64_headers(struct kimage *image,
> > > +					struct crash_mem *mem, int kernel_map,
> > > +					void **addr, unsigned long *sz);
> > >   #endif /* CONFIG_KEXEC_FILE */
> > >   #ifdef CONFIG_KEXEC_ELF
> > > @@ -299,6 +300,13 @@ struct kimage {
> > >   	/* Information for loading purgatory */
> > >   	struct purgatory_info purgatory_info;
> > > +
> > > +#ifdef CONFIG_CRASH_HOTPLUG
> > > +	bool hotplug_event;
> > > +	int offlinecpu;
> > > +	bool elf_index_valid;
> > > +	int elf_index;
> > 
> > Do we really need elf_index_valid? Can we initialize elf_index to , e.g '-1',
> > then check if the value is valid?
> 
> These members become part of struct kimage, and when the kimage is
> allocated, it is automatically zero'd. Wrt/ elf_index, 0 is a valid index,
> and so it needs to be qualified. I initially had used -1, but that required
> code and was fragile as I had to find the right place to do that. Using the
> boolean elf_index_valid, the problems with -1 vanish, and for free! I also
> found when examining the code that reading 'elf_index_valid' was better than
> 'elf_index != -1', more clear.
> 
> Let me know what you think.

OK, I am fine with it. Will see if other people have comment.

Re: [PATCH v3 6/6] crash hp: Add x86 crash hotplug support

[Baoquan He]

On 01/21/22 at 08:06am, Eric DeVolder wrote:
......
> > >   arch/x86/kernel/crash.c | 138 +++++++++++++++++++++++++++++++++++++++-
> > >   1 file changed, 137 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/arch/x86/kernel/crash.c b/arch/x86/kernel/crash.c
> > > index 9730c88530fc..d185137b33d4 100644
> > > --- a/arch/x86/kernel/crash.c
> > > +++ b/arch/x86/kernel/crash.c
> > > @@ -25,6 +25,7 @@
> > >   #include <linux/slab.h>
> > >   #include <linux/vmalloc.h>
> > >   #include <linux/memblock.h>
> > > +#include <linux/highmem.h>
> > >   #include <asm/processor.h>
> > >   #include <asm/hardirq.h>
> > > @@ -265,7 +266,8 @@ static int prepare_elf_headers(struct kimage *image, void **addr,
> > >   		goto out;
> > >   	/* By default prepare 64bit headers */
> > > -	ret =  crash_prepare_elf64_headers(cmem, IS_ENABLED(CONFIG_X86_64), addr, sz);
> > > +	ret =  crash_prepare_elf64_headers(image, cmem,
> > > +				IS_ENABLED(CONFIG_X86_64), addr, sz);
> > >   out:
> > >   	vfree(cmem);
> > > @@ -397,7 +399,17 @@ int crash_load_segments(struct kimage *image)
> > >   	image->elf_headers = kbuf.buffer;
> > >   	image->elf_headers_sz = kbuf.bufsz;
> > > +#ifdef CONFIG_CRASH_HOTPLUG
> > > +	/* Ensure elfcorehdr segment large enough for hotplug changes */
> > > +	kbuf.memsz = CONFIG_CRASH_HOTPLUG_ELFCOREHDR_SZ;
> > 
> > I would define a default value for the size, meantime provide a Kconfig
> > option to allow user to customize.
> 
> In patch 2/6 of this series, "crash hp: Introduce CRASH_HOTPLUG
> configuration options", I provide the following:
> 
> +config CRASH_HOTPLUG_ELFCOREHDR_SZ
> +   depends on CRASH_HOTPLUG
> +   int
> +   default 131072
> +   help
> +     Specify the maximum size of the elfcorehdr buffer/segment.
> 
> which defines a default value of 128KiB, and can be overriden at configure time.
> 
> Are you asking for a different technique?

I thought to define a global variable, like

/* Defaults to ahve 128K elfcorehdr buffer which contains 2048 entries.*/
unsigned long crash_hotplug_elfcorehdr_size = 0x20000;

Then initialize it in crash_hotplug_init() if CONFIG_CRASH_HOTPLUG_ELFCOREHDR_SZ
is enabled.

Seems using the config directly is also OK. Let's keep it and see if
other people have comment.

> 
> > 
> > > +	/* For marking as usable to crash kernel */
> > > +	image->elf_headers_sz = kbuf.memsz;
> > > +	/* Record the index of the elfcorehdr segment */
> > > +	image->elf_index = image->nr_segments;
> > > +	image->elf_index_valid = true;
> > > +#else
> > >   	kbuf.memsz = kbuf.bufsz;
> > > +#endif
> > >   	kbuf.buf_align = ELF_CORE_HEADER_ALIGN;
> > >   	kbuf.mem = KEXEC_BUF_MEM_UNKNOWN;
> > >   	ret = kexec_add_buffer(&kbuf);
> > > @@ -412,3 +424,127 @@ int crash_load_segments(struct kimage *image)
> > >   	return ret;
> > >   }
> > >   #endif /* CONFIG_KEXEC_FILE */
> > > +
> > > +#ifdef CONFIG_CRASH_HOTPLUG
> > 
> > These two helper function should be carved out into a separate patch as
> > a preparatory one. I am considering how to rearrange and split the
> > patches, will reply to cover letter.
> 
> OK, I look forward to that insight!
> 
> > 
> > > +void *map_crash_pages(unsigned long paddr, unsigned long size)
> > > +{
> > > +	/*
> > > +	 * NOTE: The addresses and sizes passed to this routine have
> > > +	 * already been fully aligned on page boundaries. There is no
> > > +	 * need for massaging the address or size.
> > > +	 */
> > > +	void *ptr = NULL;
> > > +
> > > +	/* NOTE: requires arch_kexec_[un]protect_crashkres() for write access */
> > > +	if (size > 0) {
> > > +		struct page *page = pfn_to_page(paddr >> PAGE_SHIFT);
> > > +
> > > +		ptr = kmap(page);
> > > +	}
> > > +
> > > +	return ptr;
> > > +}
> > > +
> > > +void unmap_crash_pages(void **ptr)
> > > +{
> > > +	if (ptr) {
> > > +		if (*ptr)
> > > +			kunmap(*ptr);
> > > +		*ptr = NULL;
> > > +	}
> > > +}
> > > +
> > > +void arch_crash_hotplug_handler(struct kimage *image,
> > > +	unsigned int hp_action, unsigned long a, unsigned long b)
> > > +{
> > > +	/*
> > > +	 * To accurately reflect hot un/plug changes, the elfcorehdr (which
> > > +	 * is passed to the crash kernel via the elfcorehdr= parameter)
> > > +	 * must be updated with the new list of CPUs and memories. The new
> > > +	 * elfcorehdr is prepared in a kernel buffer, and if no errors,
> > > +	 * then it is written on top of the existing/old elfcorehdr.
> > > +	 *
> > > +	 * Due to the change to the elfcorehdr, purgatory must explicitly
> > > +	 * exclude the elfcorehdr from the list of segments it checks.
> > > +	 */
> > 
> > Please move this code comment to above function as kernel-doc if you
> > this it benefits the entire function. Otherwise should move them above
> > the code block they are explaining. For this place, I think moving them
> > to above arch_crash_hotplug_handler() is better.
> 
> ok, I will do that!
> 
> > 
> > > +	struct kexec_segment *ksegment;
> > > +	unsigned char *ptr = NULL;
> > > +	unsigned long elfsz = 0;
> > > +	void *elfbuf = NULL;
> > > +	unsigned long mem, memsz;
> > > +	unsigned int n;
> > > +
> > > +	/*
> > > +	 * When the struct kimage is alloced, it is wiped to zero, so
> > > +	 * the elf_index_valid defaults to false. It is set on the
> > > +	 * kexec_file_load path, or here for kexec_load.
> > > +	 */
> > 
> > I think this kexec loading part should be taken out and post after this
> > whole patchset being accepted. At least, it's worth to put them in a
> > separate patch.
> 
> This little bit of code that identifies the incoming elfcorehdr is all that
> is needed to support kexec_load (and the userspace changes of course). I'm
> happy to split as a separate patch, but I would think that be maintaining it
> with this series, then when it is accepted, both the kexec_load and
> kexec_file_load paths would be supported? Your call.

Hmm, at first, let's split it out from this patch since it's an
independent action to kdump. I would suggest we don't carry it in this
series. After this series is done, you can post another patchset
including this part as kernel patch, and also the code change in
kexec_tools as user space patch.

......

Re: [PATCH v3 6/6] crash hp: Add x86 crash hotplug support

[Eric DeVolder]

Baoquan,
Thanks for looking at this! See inline response below.
eric

On 1/26/22 03:12, Baoquan He wrote:
> On 01/21/22 at 08:06am, Eric DeVolder wrote:
> ......
>>>>    arch/x86/kernel/crash.c | 138 +++++++++++++++++++++++++++++++++++++++-
>>>>    1 file changed, 137 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/arch/x86/kernel/crash.c b/arch/x86/kernel/crash.c
>>>> index 9730c88530fc..d185137b33d4 100644
>>>> --- a/arch/x86/kernel/crash.c
>>>> +++ b/arch/x86/kernel/crash.c
>>>> @@ -25,6 +25,7 @@
>>>>    #include <linux/slab.h>
>>>>    #include <linux/vmalloc.h>
>>>>    #include <linux/memblock.h>
>>>> +#include <linux/highmem.h>
>>>>    #include <asm/processor.h>
>>>>    #include <asm/hardirq.h>
>>>> @@ -265,7 +266,8 @@ static int prepare_elf_headers(struct kimage *image, void **addr,
>>>>    		goto out;
>>>>    	/* By default prepare 64bit headers */
>>>> -	ret =  crash_prepare_elf64_headers(cmem, IS_ENABLED(CONFIG_X86_64), addr, sz);
>>>> +	ret =  crash_prepare_elf64_headers(image, cmem,
>>>> +				IS_ENABLED(CONFIG_X86_64), addr, sz);
>>>>    out:
>>>>    	vfree(cmem);
>>>> @@ -397,7 +399,17 @@ int crash_load_segments(struct kimage *image)
>>>>    	image->elf_headers = kbuf.buffer;
>>>>    	image->elf_headers_sz = kbuf.bufsz;
>>>> +#ifdef CONFIG_CRASH_HOTPLUG
>>>> +	/* Ensure elfcorehdr segment large enough for hotplug changes */
>>>> +	kbuf.memsz = CONFIG_CRASH_HOTPLUG_ELFCOREHDR_SZ;
>>>
>>> I would define a default value for the size, meantime provide a Kconfig
>>> option to allow user to customize.
>>
>> In patch 2/6 of this series, "crash hp: Introduce CRASH_HOTPLUG
>> configuration options", I provide the following:
>>
>> +config CRASH_HOTPLUG_ELFCOREHDR_SZ
>> +   depends on CRASH_HOTPLUG
>> +   int
>> +   default 131072
>> +   help
>> +     Specify the maximum size of the elfcorehdr buffer/segment.
>>
>> which defines a default value of 128KiB, and can be overriden at configure time.
>>
>> Are you asking for a different technique?
> 
> I thought to define a global variable, like
> 
> /* Defaults to ahve 128K elfcorehdr buffer which contains 2048 entries.*/
> unsigned long crash_hotplug_elfcorehdr_size = 0x20000;
> 
> Then initialize it in crash_hotplug_init() if CONFIG_CRASH_HOTPLUG_ELFCOREHDR_SZ
> is enabled.
> 
> Seems using the config directly is also OK. Let's keep it and see if
> other people have comment.

OK, I will leave alone for the time being.

> 
>>
>>>
>>>> +	/* For marking as usable to crash kernel */
>>>> +	image->elf_headers_sz = kbuf.memsz;
>>>> +	/* Record the index of the elfcorehdr segment */
>>>> +	image->elf_index = image->nr_segments;
>>>> +	image->elf_index_valid = true;
>>>> +#else
>>>>    	kbuf.memsz = kbuf.bufsz;
>>>> +#endif
>>>>    	kbuf.buf_align = ELF_CORE_HEADER_ALIGN;
>>>>    	kbuf.mem = KEXEC_BUF_MEM_UNKNOWN;
>>>>    	ret = kexec_add_buffer(&kbuf);
>>>> @@ -412,3 +424,127 @@ int crash_load_segments(struct kimage *image)
>>>>    	return ret;
>>>>    }
>>>>    #endif /* CONFIG_KEXEC_FILE */
>>>> +
>>>> +#ifdef CONFIG_CRASH_HOTPLUG
>>>
>>> These two helper function should be carved out into a separate patch as
>>> a preparatory one. I am considering how to rearrange and split the
>>> patches, will reply to cover letter.
>>
>> OK, I look forward to that insight!
>>
>>>
>>>> +void *map_crash_pages(unsigned long paddr, unsigned long size)
>>>> +{
>>>> +	/*
>>>> +	 * NOTE: The addresses and sizes passed to this routine have
>>>> +	 * already been fully aligned on page boundaries. There is no
>>>> +	 * need for massaging the address or size.
>>>> +	 */
>>>> +	void *ptr = NULL;
>>>> +
>>>> +	/* NOTE: requires arch_kexec_[un]protect_crashkres() for write access */
>>>> +	if (size > 0) {
>>>> +		struct page *page = pfn_to_page(paddr >> PAGE_SHIFT);
>>>> +
>>>> +		ptr = kmap(page);
>>>> +	}
>>>> +
>>>> +	return ptr;
>>>> +}
>>>> +
>>>> +void unmap_crash_pages(void **ptr)
>>>> +{
>>>> +	if (ptr) {
>>>> +		if (*ptr)
>>>> +			kunmap(*ptr);
>>>> +		*ptr = NULL;
>>>> +	}
>>>> +}
>>>> +
>>>> +void arch_crash_hotplug_handler(struct kimage *image,
>>>> +	unsigned int hp_action, unsigned long a, unsigned long b)
>>>> +{
>>>> +	/*
>>>> +	 * To accurately reflect hot un/plug changes, the elfcorehdr (which
>>>> +	 * is passed to the crash kernel via the elfcorehdr= parameter)
>>>> +	 * must be updated with the new list of CPUs and memories. The new
>>>> +	 * elfcorehdr is prepared in a kernel buffer, and if no errors,
>>>> +	 * then it is written on top of the existing/old elfcorehdr.
>>>> +	 *
>>>> +	 * Due to the change to the elfcorehdr, purgatory must explicitly
>>>> +	 * exclude the elfcorehdr from the list of segments it checks.
>>>> +	 */
>>>
>>> Please move this code comment to above function as kernel-doc if you
>>> this it benefits the entire function. Otherwise should move them above
>>> the code block they are explaining. For this place, I think moving them
>>> to above arch_crash_hotplug_handler() is better.
>>
>> ok, I will do that!
>>
>>>
>>>> +	struct kexec_segment *ksegment;
>>>> +	unsigned char *ptr = NULL;
>>>> +	unsigned long elfsz = 0;
>>>> +	void *elfbuf = NULL;
>>>> +	unsigned long mem, memsz;
>>>> +	unsigned int n;
>>>> +
>>>> +	/*
>>>> +	 * When the struct kimage is alloced, it is wiped to zero, so
>>>> +	 * the elf_index_valid defaults to false. It is set on the
>>>> +	 * kexec_file_load path, or here for kexec_load.
>>>> +	 */
>>>
>>> I think this kexec loading part should be taken out and post after this
>>> whole patchset being accepted. At least, it's worth to put them in a
>>> separate patch.
>>
>> This little bit of code that identifies the incoming elfcorehdr is all that
>> is needed to support kexec_load (and the userspace changes of course). I'm
>> happy to split as a separate patch, but I would think that be maintaining it
>> with this series, then when it is accepted, both the kexec_load and
>> kexec_file_load paths would be supported? Your call.
> 
> Hmm, at first, let's split it out from this patch since it's an
> independent action to kdump. I would suggest we don't carry it in this
> series. After this series is done, you can post another patchset
> including this part as kernel patch, and also the code change in
> kexec_tools as user space patch.
> 
> ......
> 

OK, I'll remove the bit of code that supports kexec_load, so it can be introduced
later coincident with the changes to kexec-tools.

In a previous message you mentioned making changes to the order of the patches,
was this it, or is there more to come?

Thanks!
eric

Re: [PATCH v3 0/6] crash: Kernel handling of CPU and memory hot un/plug

[Baoquan He]

Hi Eric,

On 01/10/22 at 02:57pm, Eric DeVolder wrote:
> When the kdump service is loaded, if a CPU or memory is hot
> un/plugged, the crash elfcorehdr (for x86), which describes the CPUs
> and memory in the system, must also be updated, else the resulting
> vmcore is inaccurate (eg. missing either CPU context or memory
> regions).
> 
> The current solution utilizes udev to initiate an unload-then-reload
> of the kdump image (e. kernel, initrd, boot_params, puratory and
> elfcorehdr) by the userspace kexec utility. In previous posts I have
> outlined the significant performance problems related to offloading
> this activity to userspace.
> 
> This patchset introduces a generic crash hot un/plug handler that
> registers with the CPU and memory notifiers. Upon CPU or memory
> changes, this generic handler is invoked and performs important
> housekeeping, for example obtaining the appropriate lock, and then
> invokes an architecture specific handler to do the appropriate
> updates.
> 
> In the case of x86_64, the arch specific handler generates a new
> elfcorehdr, and overwrites the old one in memory. No involvement
> with userspace needed.

About patch splitting, it is always a headache to me when I post
patches. Both too coarse or too small granularity are not good. But we
should obey two rules:
1) commits had better be bisectable, so that later we find out which
   commit cause issue or regression easier. Here, your patch 5/6 and 6/6 are
   obviously breaking the rule. Because crash_prepare_elf64_headers()'s
   protype has been changed, its invokation is not modified accodingly.

2) Each patch only cover one action, bad better not stack too much in
one patch. Surely, we should avoid to scatter one thing into several
patch, it's not easy to reivew. E.g in this patchset, kexec_load can be
taken out, for this we have discussed. Skipping the elfcorehdr digest
can be taken out, its code change is simple, while it's worth to tell
the reason in a patch.

So my suggestion to rearrange the patchset as below, please take it as a
reference.
1)crash: fix minor typo/bug in debug message
2)crash hp: Introduce CRASH_HOTPLUG configuration options
3)crash hp: definitions and prototype changes
  Note: this patch need cover those invokation of crash_prepare_elf64_headers.
  Otherwise it's not bisectable.
4)crash hp: introduce helper function map_crash_pages/unmap_crash_pages
5)crash hp: generic crash hotplug support infrastructure
6)crash hp: exclude elfcorehdr from the segment checksum calculation
7)crash hp: exclude hot removed cpu from cpu notes of elfcorehdr
  this includes the assigment part in crash_hotplug_handler() and 
  usage part in crash_prepare_elf64_headers().
8)crash hp/x86: add the crash hp support for x86

Above patch splitting and patch subject may not be appropriate, please
adust or change with your understanding.

> 
> To realize the benefits/test this patchset, one must make a couple
> of minor changes to userspace:
> 
>  - Disable the udev rule for updating kdump on hot un/plug changes
>    Eg. on RHEL: rm -f /usr/lib/udev/rules.d/98-kexec.rules
>    or other technique to neuter the rule.
> 
>  - Change to the kexec_file_load for loading the kdump kernel:
>    Eg. on RHEL: in /usr/bin/kdumpctl, change to:
>     standard_kexec_args="-p -d -s"
>    which adds the -s to select kexec_file_load syscall.
> 
> This patchset supports kexec_load with a modified kexec userspace
> utility, and a working changeset to the kexec userspace utility
> is provided here (and to use, the above change to standard_kexec_args
> would be, for example, to append --hotplug-size=262144 instead of -s).
> 
>  diff --git a/kexec/arch/i386/crashdump-x86.c b/kexec/arch/i386/crashdump-x86.c
>  index 9826f6d..06adb7e 100644
>  --- a/kexec/arch/i386/crashdump-x86.c
>  +++ b/kexec/arch/i386/crashdump-x86.c
>  @@ -48,6 +48,7 @@
>   #include <x86/x86-linux.h>
>   
>   extern struct arch_options_t arch_options;
>  +extern unsigned long long hotplug_size;
>   
>   static int get_kernel_page_offset(struct kexec_info *UNUSED(info),
>   				  struct crash_elf_info *elf_info)
>  @@ -975,6 +976,13 @@ int load_crashdump_segments(struct kexec_info *info, char* mod_cmdline,
>   	} else {
>   		memsz = bufsz;
>   	}
>  +
>  +    /* If hotplug support enabled, use that size */
>  +    if (hotplug_size) {
>  +        memsz = hotplug_size;
>  +    }
>  +
>  +    info->elfcorehdr =
>   	elfcorehdr = add_buffer(info, tmp, bufsz, memsz, align, min_base,
>   							max_addr, -1);
>   	dbgprintf("Created elf header segment at 0x%lx\n", elfcorehdr);
>  diff --git a/kexec/kexec.c b/kexec/kexec.c
>  index f63b36b..9569d9a 100644
>  --- a/kexec/kexec.c
>  +++ b/kexec/kexec.c
>  @@ -58,6 +58,7 @@
>   
>   unsigned long long mem_min = 0;
>   unsigned long long mem_max = ULONG_MAX;
>  +unsigned long long hotplug_size = 0;
>   static unsigned long kexec_flags = 0;
>   /* Flags for kexec file (fd) based syscall */
>   static unsigned long kexec_file_flags = 0;
>  @@ -672,6 +673,12 @@ static void update_purgatory(struct kexec_info *info)
>   		if (info->segment[i].mem == (void *)info->rhdr.rel_addr) {
>   			continue;
>   		}
>  +        /* Don't include elfcorehdr in the checksum, if hotplug
>  +         * support enabled.
>  +         */
>  +        if (hotplug_size && (info->segment[i].mem == (void *)info->elfcorehdr)) {
>  +			continue;
>  +		}
>   		sha256_update(&ctx, info->segment[i].buf,
>   			      info->segment[i].bufsz);
>   		nullsz = info->segment[i].memsz - info->segment[i].bufsz;
>  @@ -1504,6 +1511,17 @@ int main(int argc, char *argv[])
>   		case OPT_PRINT_CKR_SIZE:
>   			print_crashkernel_region_size();
>   			return 0;
>  +		case OPT_HOTPLUG_SIZE:
>  +            /* Reserved the specified size for hotplug growth */
>  +			hotplug_size = strtoul(optarg, &endptr, 0);
>  +			if (*endptr) {
>  +				fprintf(stderr,
>  +					"Bad option value in --hotplug-size=%s\n",
>  +					optarg);
>  +				usage();
>  +				return 1;
>  +			}
>  +			break;
>   		default:
>   			break;
>   		}
>  diff --git a/kexec/kexec.h b/kexec/kexec.h
>  index 595dd68..b30dda4 100644
>  --- a/kexec/kexec.h
>  +++ b/kexec/kexec.h
>  @@ -169,6 +169,7 @@ struct kexec_info {
>   	int command_line_len;
>   
>   	int skip_checks;
>  +    unsigned long elfcorehdr;
>    };
>   
>   struct arch_map_entry {
>  @@ -231,7 +232,8 @@ extern int file_types;
>   #define OPT_PRINT_CKR_SIZE	262
>   #define OPT_LOAD_LIVE_UPDATE	263
>   #define OPT_EXEC_LIVE_UPDATE	264
>  -#define OPT_MAX			265
>  +#define OPT_HOTPLUG_SIZE	265
>  +#define OPT_MAX			266
>   #define KEXEC_OPTIONS \
>   	{ "help",		0, 0, OPT_HELP }, \
>   	{ "version",		0, 0, OPT_VERSION }, \
>  @@ -258,6 +260,7 @@ extern int file_types;
>   	{ "debug",		0, 0, OPT_DEBUG }, \
>   	{ "status",		0, 0, OPT_STATUS }, \
>   	{ "print-ckr-size",     0, 0, OPT_PRINT_CKR_SIZE }, \
>  +	{ "hotplug-size",     2, 0, OPT_HOTPLUG_SIZE }, \
>   
>   #define KEXEC_OPT_STR "h?vdfixyluet:pscaS"
>  
> 
> Regards,
> eric
> ---
> v3: 10jan2022
>  - Rebasing per Baoquan He request.
>  - Changed memory notifier per David Hildenbrand.
>  - Providing example kexec userspace change in cover letter.
> 
> RFC v2: 7dec2021
>  https://lkml.org/lkml/2021/12/7/1088
>  - Acting upon Baoquan He suggestion of removing elfcorehdr from
>    the purgatory list of segments, removed purgatory code from
>    patchset, and it is signficiantly simpler now.
> 
> RFC v1: 18nov2021
>  https://lkml.org/lkml/2021/11/18/845
>  - working patchset demonstrating kernel handling of hotplug
>    updates to x86 elfcorehdr for kexec_file_load
> 
> RFC: 14dec2020
>  https://lkml.org/lkml/2020/12/14/532
>  - proposed concept of allowing kernel to handle hotplug update
>    of elfcorehdr
> ---
> 
> Eric DeVolder (6):
>   crash: fix minor typo/bug in debug message
>   crash hp: Introduce CRASH_HOTPLUG configuration options
>   crash hp: definitions and prototype changes
>   crash hp: generic crash hotplug support infrastructure
>   crash hp: kexec_file changes for crash hotplug support
>   crash hp: Add x86 crash hotplug support
> 
>  arch/x86/Kconfig        |  26 ++++++++
>  arch/x86/kernel/crash.c | 140 +++++++++++++++++++++++++++++++++++++++-
>  include/linux/kexec.h   |  21 +++++-
>  kernel/crash_core.c     | 117 +++++++++++++++++++++++++++++++++
>  kernel/kexec_file.c     |  15 ++++-
>  5 files changed, 313 insertions(+), 6 deletions(-)
> 
> -- 
> 2.27.0
> 

Re: [PATCH v3 6/6] crash hp: Add x86 crash hotplug support

[Baoquan He]

On 01/26/22 at 11:32am, Eric DeVolder wrote:
..snip.... 
> > > > > +void arch_crash_hotplug_handler(struct kimage *image,
> > > > > +	unsigned int hp_action, unsigned long a, unsigned long b)
> > > > > +{
> > > > > +	/*
> > > > > +	 * To accurately reflect hot un/plug changes, the elfcorehdr (which
> > > > > +	 * is passed to the crash kernel via the elfcorehdr= parameter)
> > > > > +	 * must be updated with the new list of CPUs and memories. The new
> > > > > +	 * elfcorehdr is prepared in a kernel buffer, and if no errors,
> > > > > +	 * then it is written on top of the existing/old elfcorehdr.
> > > > > +	 *
> > > > > +	 * Due to the change to the elfcorehdr, purgatory must explicitly
> > > > > +	 * exclude the elfcorehdr from the list of segments it checks.
> > > > > +	 */
> > > > 
> > > > Please move this code comment to above function as kernel-doc if you
> > > > this it benefits the entire function. Otherwise should move them above
> > > > the code block they are explaining. For this place, I think moving them
> > > > to above arch_crash_hotplug_handler() is better.
> > > 
> > > ok, I will do that!
> > > 
> > > > 
> > > > > +	struct kexec_segment *ksegment;
> > > > > +	unsigned char *ptr = NULL;
> > > > > +	unsigned long elfsz = 0;
> > > > > +	void *elfbuf = NULL;
> > > > > +	unsigned long mem, memsz;
> > > > > +	unsigned int n;
> > > > > +
> > > > > +	/*
> > > > > +	 * When the struct kimage is alloced, it is wiped to zero, so
> > > > > +	 * the elf_index_valid defaults to false. It is set on the
> > > > > +	 * kexec_file_load path, or here for kexec_load.
> > > > > +	 */
> > > > 
> > > > I think this kexec loading part should be taken out and post after this
> > > > whole patchset being accepted. At least, it's worth to put them in a
> > > > separate patch.
> > > 
> > > This little bit of code that identifies the incoming elfcorehdr is all that
> > > is needed to support kexec_load (and the userspace changes of course). I'm
> > > happy to split as a separate patch, but I would think that be maintaining it
> > > with this series, then when it is accepted, both the kexec_load and
> > > kexec_file_load paths would be supported? Your call.
> > 
> > Hmm, at first, let's split it out from this patch since it's an
> > independent action to kdump. I would suggest we don't carry it in this
> > series. After this series is done, you can post another patchset
> > including this part as kernel patch, and also the code change in
> > kexec_tools as user space patch.
> > 
> > ......
> > 
> 
> OK, I'll remove the bit of code that supports kexec_load, so it can be introduced
> later coincident with the changes to kexec-tools.
> 
> In a previous message you mentioned making changes to the order of the patches,
> was this it, or is there more to come?

Yeah, replied to cover letter, please check it there.

Re: [PATCH v3 0/6] crash: Kernel handling of CPU and memory hot un/plug

[Eric DeVolder]

Baoquan,
Thanks for the feedback! I think I've incorporated all suggestions in this message and others in v3. 
As a result, I just posted v4.
Thank you!
eric

On 1/27/22 00:51, Baoquan He wrote:
> Hi Eric,
> 
> On 01/10/22 at 02:57pm, Eric DeVolder wrote:
>> When the kdump service is loaded, if a CPU or memory is hot
>> un/plugged, the crash elfcorehdr (for x86), which describes the CPUs
>> and memory in the system, must also be updated, else the resulting
>> vmcore is inaccurate (eg. missing either CPU context or memory
>> regions).
>>
>> The current solution utilizes udev to initiate an unload-then-reload
>> of the kdump image (e. kernel, initrd, boot_params, puratory and
>> elfcorehdr) by the userspace kexec utility. In previous posts I have
>> outlined the significant performance problems related to offloading
>> this activity to userspace.
>>
>> This patchset introduces a generic crash hot un/plug handler that
>> registers with the CPU and memory notifiers. Upon CPU or memory
>> changes, this generic handler is invoked and performs important
>> housekeeping, for example obtaining the appropriate lock, and then
>> invokes an architecture specific handler to do the appropriate
>> updates.
>>
>> In the case of x86_64, the arch specific handler generates a new
>> elfcorehdr, and overwrites the old one in memory. No involvement
>> with userspace needed.
> 
> About patch splitting, it is always a headache to me when I post
> patches. Both too coarse or too small granularity are not good. But we
> should obey two rules:
> 1) commits had better be bisectable, so that later we find out which
>     commit cause issue or regression easier. Here, your patch 5/6 and 6/6 are
>     obviously breaking the rule. Because crash_prepare_elf64_headers()'s
>     protype has been changed, its invokation is not modified accodingly.
> 
> 2) Each patch only cover one action, bad better not stack too much in
> one patch. Surely, we should avoid to scatter one thing into several
> patch, it's not easy to reivew. E.g in this patchset, kexec_load can be
> taken out, for this we have discussed. Skipping the elfcorehdr digest
> can be taken out, its code change is simple, while it's worth to tell
> the reason in a patch.
> 
> So my suggestion to rearrange the patchset as below, please take it as a
> reference.
> 1)crash: fix minor typo/bug in debug message
> 2)crash hp: Introduce CRASH_HOTPLUG configuration options
> 3)crash hp: definitions and prototype changes
>    Note: this patch need cover those invokation of crash_prepare_elf64_headers.
>    Otherwise it's not bisectable.
> 4)crash hp: introduce helper function map_crash_pages/unmap_crash_pages
> 5)crash hp: generic crash hotplug support infrastructure
> 6)crash hp: exclude elfcorehdr from the segment checksum calculation
> 7)crash hp: exclude hot removed cpu from cpu notes of elfcorehdr
>    this includes the assigment part in crash_hotplug_handler() and
>    usage part in crash_prepare_elf64_headers().
> 8)crash hp/x86: add the crash hp support for x86
> 
> Above patch splitting and patch subject may not be appropriate, please
> adust or change with your understanding.
> 
>>
>> To realize the benefits/test this patchset, one must make a couple
>> of minor changes to userspace:
>>
>>   - Disable the udev rule for updating kdump on hot un/plug changes
>>     Eg. on RHEL: rm -f /usr/lib/udev/rules.d/98-kexec.rules
>>     or other technique to neuter the rule.
>>
>>   - Change to the kexec_file_load for loading the kdump kernel:
>>     Eg. on RHEL: in /usr/bin/kdumpctl, change to:
>>      standard_kexec_args="-p -d -s"
>>     which adds the -s to select kexec_file_load syscall.
>>
>> This patchset supports kexec_load with a modified kexec userspace
>> utility, and a working changeset to the kexec userspace utility
>> is provided here (and to use, the above change to standard_kexec_args
>> would be, for example, to append --hotplug-size=262144 instead of -s).
>>
>>   diff --git a/kexec/arch/i386/crashdump-x86.c b/kexec/arch/i386/crashdump-x86.c
>>   index 9826f6d..06adb7e 100644
>>   --- a/kexec/arch/i386/crashdump-x86.c
>>   +++ b/kexec/arch/i386/crashdump-x86.c
>>   @@ -48,6 +48,7 @@
>>    #include <x86/x86-linux.h>
>>    
>>    extern struct arch_options_t arch_options;
>>   +extern unsigned long long hotplug_size;
>>    
>>    static int get_kernel_page_offset(struct kexec_info *UNUSED(info),
>>    				  struct crash_elf_info *elf_info)
>>   @@ -975,6 +976,13 @@ int load_crashdump_segments(struct kexec_info *info, char* mod_cmdline,
>>    	} else {
>>    		memsz = bufsz;
>>    	}
>>   +
>>   +    /* If hotplug support enabled, use that size */
>>   +    if (hotplug_size) {
>>   +        memsz = hotplug_size;
>>   +    }
>>   +
>>   +    info->elfcorehdr =
>>    	elfcorehdr = add_buffer(info, tmp, bufsz, memsz, align, min_base,
>>    							max_addr, -1);
>>    	dbgprintf("Created elf header segment at 0x%lx\n", elfcorehdr);
>>   diff --git a/kexec/kexec.c b/kexec/kexec.c
>>   index f63b36b..9569d9a 100644
>>   --- a/kexec/kexec.c
>>   +++ b/kexec/kexec.c
>>   @@ -58,6 +58,7 @@
>>    
>>    unsigned long long mem_min = 0;
>>    unsigned long long mem_max = ULONG_MAX;
>>   +unsigned long long hotplug_size = 0;
>>    static unsigned long kexec_flags = 0;
>>    /* Flags for kexec file (fd) based syscall */
>>    static unsigned long kexec_file_flags = 0;
>>   @@ -672,6 +673,12 @@ static void update_purgatory(struct kexec_info *info)
>>    		if (info->segment[i].mem == (void *)info->rhdr.rel_addr) {
>>    			continue;
>>    		}
>>   +        /* Don't include elfcorehdr in the checksum, if hotplug
>>   +         * support enabled.
>>   +         */
>>   +        if (hotplug_size && (info->segment[i].mem == (void *)info->elfcorehdr)) {
>>   +			continue;
>>   +		}
>>    		sha256_update(&ctx, info->segment[i].buf,
>>    			      info->segment[i].bufsz);
>>    		nullsz = info->segment[i].memsz - info->segment[i].bufsz;
>>   @@ -1504,6 +1511,17 @@ int main(int argc, char *argv[])
>>    		case OPT_PRINT_CKR_SIZE:
>>    			print_crashkernel_region_size();
>>    			return 0;
>>   +		case OPT_HOTPLUG_SIZE:
>>   +            /* Reserved the specified size for hotplug growth */
>>   +			hotplug_size = strtoul(optarg, &endptr, 0);
>>   +			if (*endptr) {
>>   +				fprintf(stderr,
>>   +					"Bad option value in --hotplug-size=%s\n",
>>   +					optarg);
>>   +				usage();
>>   +				return 1;
>>   +			}
>>   +			break;
>>    		default:
>>    			break;
>>    		}
>>   diff --git a/kexec/kexec.h b/kexec/kexec.h
>>   index 595dd68..b30dda4 100644
>>   --- a/kexec/kexec.h
>>   +++ b/kexec/kexec.h
>>   @@ -169,6 +169,7 @@ struct kexec_info {
>>    	int command_line_len;
>>    
>>    	int skip_checks;
>>   +    unsigned long elfcorehdr;
>>     };
>>    
>>    struct arch_map_entry {
>>   @@ -231,7 +232,8 @@ extern int file_types;
>>    #define OPT_PRINT_CKR_SIZE	262
>>    #define OPT_LOAD_LIVE_UPDATE	263
>>    #define OPT_EXEC_LIVE_UPDATE	264
>>   -#define OPT_MAX			265
>>   +#define OPT_HOTPLUG_SIZE	265
>>   +#define OPT_MAX			266
>>    #define KEXEC_OPTIONS \
>>    	{ "help",		0, 0, OPT_HELP }, \
>>    	{ "version",		0, 0, OPT_VERSION }, \
>>   @@ -258,6 +260,7 @@ extern int file_types;
>>    	{ "debug",		0, 0, OPT_DEBUG }, \
>>    	{ "status",		0, 0, OPT_STATUS }, \
>>    	{ "print-ckr-size",     0, 0, OPT_PRINT_CKR_SIZE }, \
>>   +	{ "hotplug-size",     2, 0, OPT_HOTPLUG_SIZE }, \
>>    
>>    #define KEXEC_OPT_STR "h?vdfixyluet:pscaS"
>>   
>>
>> Regards,
>> eric
>> ---
>> v3: 10jan2022
>>   - Rebasing per Baoquan He request.
>>   - Changed memory notifier per David Hildenbrand.
>>   - Providing example kexec userspace change in cover letter.
>>
>> RFC v2: 7dec2021
>>   https://lkml.org/lkml/2021/12/7/1088
>>   - Acting upon Baoquan He suggestion of removing elfcorehdr from
>>     the purgatory list of segments, removed purgatory code from
>>     patchset, and it is signficiantly simpler now.
>>
>> RFC v1: 18nov2021
>>   https://lkml.org/lkml/2021/11/18/845
>>   - working patchset demonstrating kernel handling of hotplug
>>     updates to x86 elfcorehdr for kexec_file_load
>>
>> RFC: 14dec2020
>>   https://lkml.org/lkml/2020/12/14/532
>>   - proposed concept of allowing kernel to handle hotplug update
>>     of elfcorehdr
>> ---
>>
>> Eric DeVolder (6):
>>    crash: fix minor typo/bug in debug message
>>    crash hp: Introduce CRASH_HOTPLUG configuration options
>>    crash hp: definitions and prototype changes
>>    crash hp: generic crash hotplug support infrastructure
>>    crash hp: kexec_file changes for crash hotplug support
>>    crash hp: Add x86 crash hotplug support
>>
>>   arch/x86/Kconfig        |  26 ++++++++
>>   arch/x86/kernel/crash.c | 140 +++++++++++++++++++++++++++++++++++++++-
>>   include/linux/kexec.h   |  21 +++++-
>>   kernel/crash_core.c     | 117 +++++++++++++++++++++++++++++++++
>>   kernel/kexec_file.c     |  15 ++++-
>>   5 files changed, 313 insertions(+), 6 deletions(-)
>>
>> -- 
>> 2.27.0
>>
>