Re: [Crash-utility] determining a "valid" vmcore

Andrew Hecox wrote: > On Thu, 2008-02-07 at 11:27 -0500, Dave Anderson wrote: >> Andrew Hecox wrote: >>> On Thu, 2008-02-07 at 10:32 -0500, Dave Anderson wrote: >>>> Andrew Hecox wrote: >>>>> hello, >>>>> >>>>> I'm looking at a customer issue where diskdumpmsg is unable to read a >>>>> vmcore file. It is not clear if this a problem with the vmcore file or >>>>> diskdumpmsg. I can load the vmcore with crash and in my naive usage of >>>>> it, can see no problems. However, I'm new to the tool so that doesn't >>>>> give me a lot of confidence. >>>>> >>>>> Does anyone have any suggestions on how or if I can use crash to help >>>>> determine if there's corruption in the vmcore file? Or any other way of >>>>> approaching the problem? >>>>> >>>>> Thanks much, >>>>> >>>>> Andrew >>>>> >>>> I'm not sure what you expect the crash utility to do -- if it comes >>>> up to a prompt with no error or warning messages, it means that the >>>> ELF header contains what appears to be valid usable information, >>>> and that the minimum kernel memory contents required to set up the >>>> crash utility's notion of the running system are all in place. That's >>>> not to say that there is no chance that the vmcore contains some >>>> corruption that was not recognized. >>>> >>> Thanks. Any other suggestions on how to determine if a vmcore is "valid" >>> or is that not even a reasonable question to try and ask? The problem >>> I'm trying to solve is described better below: >>> >>>> With respect to diskdumpmsg, as I understand it, it was fairly recently >>>> changed from a perl script to a C file so that it could be run >>>> earlier in time so as to be able to use the swap partition. Looking >>>> at main() in the diskdumpmsg.c file (version 1.4.1-2), there are numerous >>>> error types and associated error messages. What do you mean when you >>>> say that "diskdumpmsg is unable to read a vmcore file"? >>> Specifically: >>> >>> - user reported a floating point exception from diskdump on startup >>> - the result was reproducible locally but only with their vmcore file >>> - fpe occurred in get_logbuf: >>> log_end %= log_buf_len; >>> - log_buf_len had been set to 0 in read_buffer >>> if (!page_is_dumpable(pfn, dump->device)) { >>> memset(buf, 0, copy_len); >>> } else { >>> - I don't know enough to say if the page really wasn't dumpable. >>> static inline bool page_is_dumpable(unsigned int nr, DumpDevice *device) >>> { >>> return device->dumpable_bitmap[nr>>3] & (1 << (nr & 7)); >>> } >>> - I wrote a patch with one way to avoid the FPE (attached) and sent it >>> to SEG. >>> >>> Now I'm trying to determine if the vmcore file should be readable by >>> diskdumpmsg. In other words, is this a problem in diskdumpmsg post-crash >>> or a problem with the vmcore file prior to it getting to diskdumpmsg. >>> Unfortunately, I don't understand the problem domain very well at all, >>> hence the probably naive questions :) >>> >>> Any suggestions are appreciated. >>> >>> -Andrew >> So it appears that the page containing the log_buf_len symbol is not >> readable or contained in the dumpfile. BTW, is this a compressed >> dumpfile or an ELF formatted dumpfile? And what "dump_level" did >> they configure? >> > > compressed, level is 19. > >> Anyway, back to the log_buf_len symbol read, what happens when you >> enter the "log" command while in a crash session? It attempts to >> read that symbol immediately. >> > > I get what appears to be a full and valid dump of the kernel message > buffer. > The crash utility has the same page_is_dumpable() function, which I presume looks at precisely the same bitmap data from the dumpfile. And that must be working, given that the "log" command works as expected. One difference is that diskdumpmsg uses /boot/System.map-<release> for the symbol values, whereas crash uses the vmlinux file. It might be of interest to determine whether the value of "log_buf_len" used by diskdumpmsg is the same symbol value as used by crash.

Dave