"K.Prasad" <prasad at linux.vnet.ibm.com> writes: > Hi All, > We've been trying to study and improve the kdump behaviour when > a panic is triggered due to an unrecoverable memory error causing a > machine check exception (MCE) followed by a kernel panic. > > In this context we foresee a few issues in capturing kdump and would > like to receive comments about the ways to handle them. > > Probable Issues when capturing coredump through kdump following a memory > error > --------------------------- > - First, a coredump of the memory from the crashing kernel isn't really > helpful in debugging the crash that was caused due to a faulty memory. > Collecting the same has some of the problems illustrated below. It should > therefore suffice to let the user know the reason of the crash > rather than provide a complete dump of the memory. > > For this, a 'slim' yet crash-tool readable coredump containing: > - message about the cause (such as crash due to unrecoverable memory error) > in the coredump's elf-note section. > - and no data from the memory of the 'crashing' kernel (their elf > sections can be reduced to zero length). > may be suitable. > > - Alternatively, if the kdump kernel decides to capture the coredump, > its attempts to read the faulty memory location may lead to subsequent > faults in the context of kdump kernel with fatal consequences. This > may either be avoided by: > > a) Pass the address of the corrupt memory location to the kdump kernel > and skip reading that location while creating the vmcore. This needs > an instance of 'struct mce' (from the 'crashing' kernel), which > already contains the faulty memory address (in the physical address > form, which should be confirmed using the IA32_MCi_MISC[8:6] bits stored > in 'misc' field of 'struct mce') to be populated inside the elf > (-notes?) section. > > b) Use modified copy applications (such as a modified 'cp' command) > that can map the /dev/oldmem into user-space and then initiate the > creation of vmcore. In this method, the user-space process performing > the copy will receive a SIGBUS while consuming the faulty memory (through > INT18 -> do_machine_check) but it must be modified to be resilient to the > signal, while intelligently skipping to the subsequent memory location > for further copying. Meanwhile the data for the faulty memory location > can be represented using 'zero-ed' data and the vmcore enhanced to > indicate the cause of the crash as one resulting from a fatal MCE. > > Any thoughts/suggestions? In practice this all works for me. I have received several crash dumps where there was an mce error. I admit I have my userspace configured to just grab the dmesg from the kernel log and not do a full crash dump. So in that sense I am already a slim crash dump. But in practice with real hardware errors it is working today without kernel changes. Eric
