Messages in this thread | | | Date | Fri, 21 Jan 2022 08:06:50 -0600 | Subject | Re: [PATCH v3 6/6] crash hp: Add x86 crash hotplug support | From | Eric DeVolder <> |
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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 >> >
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