Re: File system corruption, btrfsck abort

[Qu Wenruo <quwenruo@cn.fujitsu.com>]

At 04/26/2017 01:50 AM, Christophe de Dinechin wrote:
> Hi,
> 
> 
> I”ve been trying to run btrfs as my primary work filesystem for about 3-4 months now on Fedora 25 systems. I ran a few times into filesystem corruptions. At least one I attributed to a damaged disk, but the last one is with a brand new 3T disk that reports no SMART errors. Worse yet, in at least three cases, the filesystem corruption caused btrfsck to crash.
> 
> The last filesystem corruption is documented here: https://bugzilla.redhat.com/show_bug.cgi?id=1444821. The dmesg log is in there.

According to the bugzilla, the btrfs-progs seems to be too old in btrfs 
standard.

What about using the latest btrfs-progs v4.10.2?

Furthermore for v4.10.2, btrfs check provides a new mode called lowmem.
You could try "btrfs check --mode=lowmem" to see if such problem can be 
avoided.

For the kernel bug, it seems to be related to wrongly inserted delayed 
ref, but I can totally be wrong.

Thanks,
Qu
> 
> The btrfsck crash is here: https://bugzilla.redhat.com/show_bug.cgi?id=1435567. I have two crash modes: either an abort or a SIGSEGV. I checked that both still happens on master as of today.
> 
> The cause of the abort is that we call set_extent_dirty from check_extent_refs with rec->max_size == 0. I’ve instrumented to try to see where we set this to 0 (see https://github.com/c3d/btrfs-progs/tree/rhbz1435567), and indeed, we do sometimes see max_size set to 0 in a few locations. My instrumentation shows this:
> 
> 78655 [1.792241:0x451fe0] MAX_SIZE_ZERO: Add extent rec 0x139eb80 max_size 16384 tmpl 0x7fffffffd120
> 78657 [1.792242:0x451cb8] MAX_SIZE_ZERO: Set max size 0 for rec 0x139ec50 from tmpl 0x7fffffffcf80
> 78660 [1.792244:0x451fe0] MAX_SIZE_ZERO: Add extent rec 0x139ed50 max_size 16384 tmpl 0x7fffffffd120
> 
> I don’t really know what to make of it.
> 
> The cause of the SIGSEGV is that we try to free a list entry that has its next set to NULL.
> 
> #0  list_del (entry=0x555555db0420) at /usr/src/debug/btrfs-progs-v4.10.1/kernel-lib/list.h:125
> #1  free_all_extent_backrefs (rec=0x555555db0350) at cmds-check.c:5386
> #2  maybe_free_extent_rec (extent_cache=0x7fffffffd990, rec=0x555555db0350) at cmds-check.c:5417
> #3  0x00005555555b308f in check_block (flags=<optimized out>, buf=0x55557b87cdf0, extent_cache=0x7fffffffd990, root=0x55555587d570) at cmds-check.c:5851
> #4  run_next_block (root=root@entry=0x55555587d570, bits=bits@entry=0x5555558841
> 
> I don’t know if the two problems are related, but they seem to be pretty consistent on this specific disk, so I think that we have a good opportunity to improve btrfsck to make it more robust to this specific form of corruption. But I don’t want to hapazardly modify a code I don’t really understand. So if anybody could make a suggestion on what the right strategy should be when we have max_size == 0, or how to avoid it in the first place.
> 
> I don’t know if this is relevant at all, but all the machines that failed that way were used to run VMs with KVM/QEMU. DIsk activity tends to be somewhat intense on occasions, since the VMs running there are part of a personal Jenkins ring that automatically builds various projects. Nominally, there are between three and five guests running (Windows XP, WIndows 10, macOS, Fedora25, Ubuntu 16.04).
> 
> 
> Thanks
> Christophe de Dinechin
> 
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