On Wed, Sep 27, 2023 at 02:11:30PM -0400, Eric DeVolder wrote: > When the kdump service is loaded, if a CPU or memory is hot > un/plugged, the crash elfcorehdr, 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 (eg. RHEL /usr/lib/udev/rules.d/ > 98-kexec.rules) to initiate an unload-then-reload of the *entire* kdump > image (eg. kernel, initrd, boot_params, purgatory and elfcorehdr) by > the userspace kexec utility. This occurrs just so the elfcorehdr can > be updated with the latest list of CPUs and memory regions. In a > previous post I have outlined the significant performance problems > related to offloading this activity to userspace. > > With the Linux kernel 6.6 commit below, the kernel now has the ability > to directly modify the elfcorehdr, eliminating the need to > unload-then-reload the entire kdump image when CPU or memory is hot > un/plugged or on/offlined. > > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=d6 > 8b4b6f307d155475cce541f2aee938032ed22e > > This kexec-tools patch series is for supporting hotplug with the > kexec_load() syscall; the kernel directly supports hotplug for the > kexec_file_load() syscall, requiring no userspace help. > > There are two basic obstacles/requirements for the kexec-tools to > overcome in order to support kernel hotplug rewriting of the > elfcorehdr. > > First, the buffer containing the elfcorehdr must be excluded from the > purgatory checksum/digest, which is computed at load time. Otherwise > kernel run-time changes to the elfcorehdr, as a result of hot un/plug, > would result in the checksum failing (specifically in purgatory at > panic kernel boot time), and kdump capture kernel failing to start. > To let the kernel know it is okay to modify the elfcorehdr, kexec > sets the KEXEC_UPDATE_ELFCOREHDR flag. > > NOTE: The kernel specifically does *NOT* attempt to recompute the > checksum/digest as that would ultimately require patching the in- > memory purgatory image with the updated checksum. As that purgatory > image is already fully linked, it is binary blob containing no ELF > information which would allow it to be re-linked or patched. Thus > excluding the elfcorehdr from the checksum/digests avoids all these > problems. > > Second, the size of the elfcorehdr buffer must be large enough > to accomodate growth of the number of CPUs and/or memory regions. > > To satisfy the first requirement, this patch series introduces the > --hotplug option to indicate to kexec-tools that kexec should exclude > the elfcorehdr buffer from the purgatory checksum/digest calculation > and set the KEXEC_UPDATE_ELFCOREHDR flag. > > To satisfy the second requirement, the size is obtained from the > /sys/kernel/crash_elfcorehdr_size node (new with the kernel series > cited above). > > To use this feature with kexec_load() syscall, invoke kexec with: > > kexec -c --hotplug ... > > Thanks! > eric Thanks Eric, applied. _______________________________________________ kexec mailing list kexec@xxxxxxxxxxxxxxxxxxx http://lists.infradead.org/mailman/listinfo/kexec
