Thursday Jul 24, 2014

Crashdump restructuring in Solaris

In Solaris 11.2 the crashdump restructuring project changed the way how dump data are stored. Data which are not pure kernel pages now go into separate files. Together with my colleague Sriman we made it to happen.

The first noticeable change was a change in the layout of the crash directory. The files are stored under /var/crash/data/uuid/ directory. The long hexadecimal string (uuid) was added to better align with FMA - it's actually uuid (universally unique ID) of the crash event which can be found in fmadm faulty output. Actually, if you look at FMA panic events from earlier versions you can see that the resource string for the event was already designed this way, it's just materialized with this project.

For example, after 2 panic events the /var/crash directory will look like this:

0 -> data/404778fb-88da-4188-f222-8da174d44fa4
1 -> data/6e50417e-95fc-4ab4-e9a8-abbe80bc6b48
bounds
data/
  404778fb-88da-4188-f222-8da174d44fa4/
    vmcore-zfs.0
    vmcore.0
    vmdump-zfs.0
    vmdump.0
  6e50417e-95fc-4ab4-e9a8-abbe80bc6b48/
    vmdump-zfs.1
    vmdump.1

The 0, 1 symlinks maintain the sequential ordering of the old layout.

The example reflects a configuration when savecore is not automatically run after boot (i.e. dumpadm -n is in effect) and the administrator has extracted the first crash dump by hand (running savecore 0 in /var/crash/0/ directory). If you take a look at the console after the system rebooted after panic there are commands which you can copy-n-paste to the terminal to perform the extraction.

The other change in the above example is that there is new vmcore-zfs.N file. This is not the only new file which can appear. Depending on dumpadm(1M) configuration there can be files like:

  • vmcore.N - core kernel pages
  • vmcore-zfs.N - ZFS metadata (ZIO buffers)
  • vmcore-proc.N - process pages
  • vmcore-other.N - other pages (ZFS data, free pages)

By splitting the dump into multiple files it is possible to transfer just vmcore.N file for analysis to quickly assess what caused the panic and transfer the rest of the files later on.

If any of the "auxiliary" files is missing, mdb will report it:

root@va64-v20zl-prg06:/var/crash/0# mdb 0
mdb: failed to locate file ./vmcore-zfs.0. Contents of ZFS metadata (ZIO buffers) 
will not be available
Loading modules: [ unix genunix specfs dtrace mac cpu.generic 
cpu_ms.AuthenticAMD.15 uppc pcplusmp zvpsm scsi_vhci zfs mpt sd ip hook neti arp
 usba kssl sockfs lofs idm cpc nfs fcip fctl ufs logindmux ptm sppp ipc ]
> ::status
debugging crash dump vmcore.0 (64-bit) from va64-v20zl-prg06
operating system: 5.12 dump.on12 (i86pc)
usr/src version: 23877:f2e76e2d0329:on12_51+48
usr/closed version: 1961:cad017e4c7e4:on12_51+4
image uuid: 404778fb-88da-4188-f222-8da174d44fa4
panic message: forced crash dump initiated at user request
complete: yes, all pages present as configured
dump content: kernel [LOADED,UNVERIFIED] (core kernel pages)
              zfs [MISSING] (ZFS metadata (ZIO buffers))
panicking PID: 101575 (not dumped)

Another choice is not to dump some of the sections at all. E.g. to dump only pages kernel and currently running process at the time of panic but not ZFS metadata the system can be configured as:

dumpadm -c curproc-zfs

Also, the unix.N file is no longer extracted automatically (it can be done with -u option for savecore(1M) if you need the file for some reason) since it is embedded in vmcore.N file; mdb will find it automatically.

How to load the files into mdb with all these files around ? The easiest way how to access an extracted dump is to use just the suffix, i.e.

  mdb N

which will pick up all the files based on metadata in the main vmcore.N file. This worked even before this change, except there was just one file (2 if counting unix.N).

It is still possible to specify the files by hand, just remember to put the main file (vmcore.N) as the first argument:

  mdb vmcore.N vmcore-zfs.N ...

The other change (which is hard to notice unless you're dealing with lots of crash dump files) was that we laid out the infrastructure in kernel/mdb/libkvm to be properly backwards compatible w.r.t. on-disk crash dump format. As a result mdb will automatically load crash dump files produced in earlier Solaris versions. Currently it supports 3 latest versions.

Tuesday Apr 07, 2009

Collateral salutariness

Each build of (Open)Solaris is tested with a variety of test suites on variety of platforms and I wanted nc test suite to participate in these runs.
Eoin Hughes from PIT team (which runs those tests) was kind enough to workaround couple of bugs (which are fixed now) in the test suite so it can be run in PIT environment. Later on, I got a report from Eoin that as a result of nc test suite run CR 6793191 (watchmalloc triggers system panic on sockfs copyin) was caught. This bug is manifested by a panic:

Panic message (this particular panic is on a DomU, although this happens across the board):
panic[cpu0]/thread=ffffff0150ce1540: copyin_noerr: argument not in kernel address space

ffffff000416dcf0 unix:bcopy_ck_size+102 ()
ffffff000416ddb0 genunix:watch_xcopyin+151 ()
ffffff000416dde0 genunix:watch_copyin+1d ()
ffffff000416de50 sockfs:copyin_name+91 ()
ffffff000416deb0 sockfs:bind+90 ()
ffffff000416df00 unix:brand_sys_syscall32+328 ()

The bug is actually a regression caused by CR 6292199 (bcopy and kcopy should'nt use rep, smov) and was fixed by an engineer from Intel in OpenSolaris/Nevada code base.

This is instance of an event which I like so much - unintended positive consequence elsewhere. In contrast with so called collateral damage this is something which is beneficial in other areas. I've written nc test suite to test primarily nc(1) command but here it proved to be useful for testing other areas of the system as well. In this case it was thanks to the fact that the test suite is run with memory leak checking by default (see NC_PRELOADS variable in src/suites/net/nc/include/vars file).

And yes, CR 6793191 is fixed by now.

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