Re: [PATCH v15 5/5] powerpc: add crash memory hotplug support

[Hari Bathini <hbathini@linux.ibm.com>]

On 11/01/24 4:21 pm, Sourabh Jain wrote:
> Extend the arch crash hotplug handler, as introduced by the patch title
> ("powerpc: add crash CPU hotplug support"), to also support memory
> add/remove events.
> 
> Elfcorehdr describes the memory of the crash kernel to capture the
> kernel; hence, it needs to be updated if memory resources change due to
> memory add/remove events. Therefore, arch_crash_handle_hotplug_event()
> is updated to recreate the elfcorehdr and replace it with the previous
> one on memory add/remove events.
> 
> The memblock list is used to prepare the elfcorehdr. In the case of
> memory hot removal, the memblock list is updated after the arch crash
> hotplug handler is triggered, as depicted in Figure 1. Thus, the
> hot-removed memory is explicitly removed from the crash memory ranges
> to ensure that the memory ranges added to elfcorehdr do not include the
> hot-removed memory.
> 
>      Memory remove
>            |
>            v
>      Offline pages
>            |
>            v
>   Initiate memory notify call <----> crash hotplug handler
>   chain for MEM_OFFLINE event
>            |
>            v
>   Update memblock list
> 
>   	Figure 1
> 
> There are two system calls, `kexec_file_load` and `kexec_load`, used to
> load the kdump image. A few changes have been made to ensure that the
> kernel can safely update the elfcorehdr component of the kdump image for
> both system calls.
> 
> For the kexec_file_load syscall, kdump image is prepared in the kernel.
> To support an increasing number of memory regions, the elfcorehdr is
> built with extra buffer space to ensure that it can accommodate
> additional memory ranges in future.
> 
> For the kexec_load syscall, the elfcorehdr is updated only if the
> KEXEC_CRASH_HOTPLUG_SUPPORT kexec flag is passed to the kernel by the
> kexec tool. Passing this flag to the kernel indicates that the
> elfcorehdr is built to accommodate additional memory ranges and the
> elfcorehdr segment is not considered for SHA calculation, making it safe
> to update.
> 
> The changes related to this feature are kept under the CRASH_HOTPLUG
> config, and it is enabled by default.
> 
> Signed-off-by: Sourabh Jain <sourabhjain@linux.ibm.com>
> Cc: Akhil Raj <lf32.dev@gmail.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Aneesh Kumar K.V <aneesh.kumar@kernel.org>
> Cc: Baoquan He <bhe@redhat.com>
> Cc: Borislav Petkov (AMD) <bp@alien8.de>
> Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
> Cc: Christophe Leroy <christophe.leroy@csgroup.eu>
> Cc: Dave Hansen <dave.hansen@linux.intel.com>
> Cc: Dave Young <dyoung@redhat.com>
> Cc: David Hildenbrand <david@redhat.com>
> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Cc: Hari Bathini <hbathini@linux.ibm.com>
> Cc: Laurent Dufour <laurent.dufour@fr.ibm.com>
> Cc: Mahesh Salgaonkar <mahesh@linux.ibm.com>
> Cc: Michael Ellerman <mpe@ellerman.id.au>
> Cc: Mimi Zohar <zohar@linux.ibm.com>
> Cc: Naveen N Rao <naveen@kernel.org>
> Cc: Oscar Salvador <osalvador@suse.de>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Valentin Schneider <vschneid@redhat.com>
> Cc: Vivek Goyal <vgoyal@redhat.com>
> Cc: kexec@lists.infradead.org
> Cc: x86@kernel.org
> ---
>   arch/powerpc/include/asm/kexec.h        |   5 +-
>   arch/powerpc/include/asm/kexec_ranges.h |   1 +
>   arch/powerpc/kexec/core_64.c            | 107 +++++++++++++++++++++++-
>   arch/powerpc/kexec/file_load_64.c       |  34 +++++++-
>   arch/powerpc/kexec/ranges.c             |  85 +++++++++++++++++++
>   5 files changed, 225 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/powerpc/include/asm/kexec.h b/arch/powerpc/include/asm/kexec.h
> index 943e58eb9bff..25ff5b7f1a28 100644
> --- a/arch/powerpc/include/asm/kexec.h
> +++ b/arch/powerpc/include/asm/kexec.h
> @@ -116,8 +116,11 @@ int get_crash_memory_ranges(struct crash_mem **mem_ranges);
>   #ifdef CONFIG_CRASH_HOTPLUG
>   void arch_crash_handle_hotplug_event(struct kimage *image, void *arg);
>   #define arch_crash_handle_hotplug_event arch_crash_handle_hotplug_event
> -#endif /*CONFIG_CRASH_HOTPLUG */
>   
> +unsigned int arch_crash_get_elfcorehdr_size(void);
> +#define crash_get_elfcorehdr_size arch_crash_get_elfcorehdr_size
> +
> +#endif /*CONFIG_CRASH_HOTPLUG */
>   #endif /* CONFIG_PPC64 */
>   
>   #ifdef CONFIG_KEXEC_FILE
> diff --git a/arch/powerpc/include/asm/kexec_ranges.h b/arch/powerpc/include/asm/kexec_ranges.h
> index f83866a19e87..802abf580cf0 100644
> --- a/arch/powerpc/include/asm/kexec_ranges.h
> +++ b/arch/powerpc/include/asm/kexec_ranges.h
> @@ -7,6 +7,7 @@
>   void sort_memory_ranges(struct crash_mem *mrngs, bool merge);
>   struct crash_mem *realloc_mem_ranges(struct crash_mem **mem_ranges);
>   int add_mem_range(struct crash_mem **mem_ranges, u64 base, u64 size);
> +int remove_mem_range(struct crash_mem **mem_ranges, u64 base, u64 size);
>   int add_tce_mem_ranges(struct crash_mem **mem_ranges);
>   int add_initrd_mem_range(struct crash_mem **mem_ranges);
>   #ifdef CONFIG_PPC_64S_HASH_MMU
> diff --git a/arch/powerpc/kexec/core_64.c b/arch/powerpc/kexec/core_64.c
> index 43fcd78c2102..4673f150f973 100644
> --- a/arch/powerpc/kexec/core_64.c
> +++ b/arch/powerpc/kexec/core_64.c
> @@ -19,8 +19,11 @@
>   #include <linux/of.h>
>   #include <linux/libfdt.h>
>   #include <linux/memblock.h>
> +#include <linux/memory.h>
>   
>   #include <asm/page.h>
> +#include <asm/drmem.h>
> +#include <asm/mmzone.h>
>   #include <asm/current.h>
>   #include <asm/machdep.h>
>   #include <asm/cacheflush.h>
> @@ -546,6 +549,101 @@ int update_cpus_node(void *fdt)
>   #undef pr_fmt
>   #define pr_fmt(fmt) "crash hp: " fmt
>   
> +/*
> + * Advertise preferred elfcorehdr size to userspace via
> + * /sys/kernel/crash_elfcorehdr_size sysfs interface.
> + */
> +unsigned int arch_crash_get_elfcorehdr_size(void)
> +{
> +	unsigned int sz;
> +	unsigned long elf_phdr_cnt;
> +
> +	/* Program header for CPU notes and vmcoreinfo */
> +	elf_phdr_cnt = 2;
> +	if (IS_ENABLED(CONFIG_MEMORY_HOTPLUG))
> +		/* In the worst case, a Phdr is needed for every other LMB to be
> +		 * represented as an individual crash range.
> +		 */
> +		elf_phdr_cnt += memory_hotplug_max() / (2 * drmem_lmb_size());
> +
> +	/* Do not cross the max limit */
> +	if (elf_phdr_cnt > PN_XNUM)
> +		elf_phdr_cnt = PN_XNUM;
> +
> +	sz = sizeof(struct elfhdr) + (elf_phdr_cnt * sizeof(Elf64_Phdr));
> +	return sz;
> +}
> +
> +/**
> + * update_crash_elfcorehdr() - Recreate the elfcorehdr and replace it with old
> + *			       elfcorehdr in the kexec segment array.
> + * @image: the active struct kimage
> + * @mn: struct memory_notify data handler
> + */
> +static void update_crash_elfcorehdr(struct kimage *image, struct memory_notify *mn)
> +{
> +	int ret;
> +	struct crash_mem *cmem = NULL;
> +	struct kexec_segment *ksegment;
> +	void *ptr, *mem, *elfbuf = NULL;
> +	unsigned long elfsz, memsz, base_addr, size;
> +
> +	ksegment = &image->segment[image->elfcorehdr_index];
> +	mem = (void *) ksegment->mem;
> +	memsz = ksegment->memsz;
> +
> +	ret = get_crash_memory_ranges(&cmem);
> +	if (ret) {
> +		pr_err("Failed to get crash mem range\n");
> +		return;
> +	}
> +
> +	/*
> +	 * The hot unplugged memory is part of crash memory ranges,
> +	 * remove it here.
> +	 */
> +	if (image->hp_action == KEXEC_CRASH_HP_REMOVE_MEMORY) {
> +		base_addr = PFN_PHYS(mn->start_pfn);
> +		size = mn->nr_pages * PAGE_SIZE;
> +		ret = remove_mem_range(&cmem, base_addr, size);
> +		if (ret) {
> +			pr_err("Failed to remove hot-unplugged from crash memory ranges.\n");
> +			return;
> +		}
> +	}
> +
> +	ret = crash_prepare_elf64_headers(cmem, false, &elfbuf, &elfsz);
> +	if (ret) {
> +		pr_err("Failed to prepare elf header\n");
> +		return;
> +	}
> +



> +	/*
> +	 * It is unlikely that kernel hit this because elfcorehdr kexec
> +	 * segment (memsz) is built with addition space to accommodate growing
> +	 * number of crash memory ranges while loading the kdump kernel. It is
> +	 * Just to avoid any unforeseen case.

nitpick..

s/Just/just/

> +	 */
> +	if (elfsz > memsz) {
> +		pr_err("Updated crash elfcorehdr elfsz %lu > memsz %lu", elfsz, memsz);
> +		goto out;
> +	}
> +
> +	ptr = __va(mem);
> +	if (ptr) {
> +		/* Temporarily invalidate the crash image while it is replaced */
> +		xchg(&kexec_crash_image, NULL);
> +
> +		/* Replace the old elfcorehdr with newly prepared elfcorehdr */
> +		memcpy((void *)ptr, elfbuf, elfsz);
> +
> +		/* The crash image is now valid once again */
> +		xchg(&kexec_crash_image, image);
> +	}
> +out:
> +	vfree(elfbuf);
> +}
> +
>   /**
>    * arch_crash_handle_hotplug_event - Handle crash CPU/Memory hotplug events to update the
>    *				     necessary kexec segments based on the hotplug event.
> @@ -556,7 +654,7 @@ int update_cpus_node(void *fdt)
>    * CPU addition: Update the FDT segment to include the newly added CPU.
>    * CPU removal: No action is needed, with the assumption that it's okay to have offline CPUs
>    *		as part of the FDT.
> - * Memory addition/removal: No action is taken as this is not yet supported.
> + * Memory addition/removal: Recreate the elfcorehdr segment
>    */
>   void arch_crash_handle_hotplug_event(struct kimage *image, void *arg)
>   {
> @@ -570,7 +668,6 @@ void arch_crash_handle_hotplug_event(struct kimage *image, void *arg)
>   		return;
>   

>   	} else if (hp_action == KEXEC_CRASH_HP_ADD_CPU) {
> -

>   		void *fdt, *ptr;
>   		unsigned long mem;
>   		int i, fdt_index = -1;
> @@ -605,8 +702,10 @@ void arch_crash_handle_hotplug_event(struct kimage *image, void *arg)
>   

>   	} else if (hp_action == KEXEC_CRASH_HP_REMOVE_MEMORY ||
>   		   hp_action == KEXEC_CRASH_HP_ADD_MEMORY) {

> -		pr_info_once("Crash update is not supported for memory hotplug\n");
> -		return;
> +		struct memory_notify *mn;
> +
> +		mn = (struct memory_notify *)arg;
> +		update_crash_elfcorehdr(image, mn);
>   	}

Use switch case statement for hotplug actions for better readability..

Thanks
Hari