Re: btrfs-replace OOM on 2GB machine

[Austin S Hemmelgarn <ahferroin7@gmail.com>]

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On 2015-11-13 11:15, Georg Lukas wrote:
> Hi,
>
> while evaluating btrfs for production use I ended up with a degraded
> two-disk RAID1 with one disk missing, and wanted to perform a "btrfs
> replace" to rebuild the RAID1. However, the replace operation causes
> most of my userland to be OOM-killed and aborts eventually, at about
> 30% progress, on a box with 2GB of physical RAM.
>
> My setup is:
>
> Linux-4.3 with the following patches applied:
>   - http://www.spinics.net/lists/linux-btrfs/msg46123.html
>     (needed for degraded mount of RAID1)
>   - http://git.kernel.org/cgit/linux/kernel/git/mkp/linux.git/patch/?id=7c4fbd50bfece00abf529bc96ac989dd2bb83ca4
>     (needed for the Seagate SMRs)
>
> btrfs-progs v4.2.3
>
> A btrfs RAID1 initially built on two dm-crypt containers on top of two
> Seagate 8TB SMR disks. For testing purposes, I unmounted the fs,
> reformatted one of the two crypto containers, mounted the fs in degraded
> mode (which required Anand's patch), and tried different approaches to
> get it back to full operation (rebalance to m=d=single, remove the
> missing drive, finally a replace), all without success.
While it probably isn't related to the OOM issue, I would be 
particularly wary of using BTRFS on SMR disks, we've had multiple 
reports of serious issues with them (and IIRC, they were all the same 
model of 8TB Seagate SMR disks).
>
> The current status is as follows:
>
> # btrfs dev usage /media/archive/
> /dev/mapper/archive1, ID: 1
>     Device size:             7.28TiB
>     Data,single:           837.00GiB
>     Data,RAID0:              1.17TiB
>     Data,RAID1:            959.00GiB
>     Data,DUP:                2.17TiB
>     Metadata,single:         2.00GiB
>     Metadata,RAID1:          4.00GiB
>     Metadata,DUP:            5.00GiB
>     System,RAID1:           32.00MiB
>     System,DUP:            192.00MiB
>     Unallocated:             2.17TiB
>
> missing, ID: 2
>     Device size:               0.00B
>     Data,RAID0:              1.17TiB
>     Data,RAID1:            959.00GiB
>     Metadata,RAID1:          4.00GiB
>     System,RAID1:           32.00MiB
>     Unallocated:             5.17TiB
Hmm, it looks like things weren't all RAID1, you've got a little over 
1TiB of data that was RAID0, and that may be why you can't rebuild the 
FS.  This shouldn't be causing an OOM condition, but it definitely means 
things are not fully recoverable.
>
> I then start the replace:
>
> # btrfs replace start 2 /dev/mapper/archive2 /media/archive/
>
> That takes a while, OOM-kills half of my userspace in the process (it
> seems like the kernel is allocating and freeing large chunks of memory
> during the replace:
>
>               total       used       free     shared    buffers     cached
> Mem:          1.9G       1.6G       342M       784K       1.8M        14M
> -/+ buffers/cache:       1.6G       358M
> Swap:         4.0G        48M       4.0G
>
> (5 second pause)
>               total       used       free     shared    buffers     cached
> Mem:          1.9G       157M       1.8G       808K       6.6M        32M
> -/+ buffers/cache:       118M       1.8G
> Swap:         4.0G        46M       4.0G
>
> (another 5 seconds)
>               total       used       free     shared    buffers     cached
> Mem:          1.9G       1.1G       835M       808K       6.7M        37M
> -/+ buffers/cache:       1.1G       879M
> Swap:         4.0G        46M       4.0G
>
> That seems to be kernel memory, as the swap is hardly used, despite
> default swappiness settings. Furthermore, /proc/meminfo and slabtop have
> no indication of how the memory is used; it just vanishes from the
> "available" pool.
This sounds to me like a memory leak in the kernel, but I'm not certain. 
  I'm going to try and reproduce this without the SMR patch (and 
obviously without the SMR drives themselves) in a VM.
>
> Eventually, the replace aborts:
>
> [64326.700731] BTRFS: btrfs_scrub_dev(<missing disk>, 2, /dev/mapper/archive2) failed -12
> [64326.700986] ------------[ cut here ]------------
> [64326.701024] WARNING: CPU: 1 PID: 36251 at fs/btrfs/dev-replace.c:428 btrfs_dev_replace_start+0x36b/0x390 [btrfs]()
> [64326.701062] Modules linked in: btrfs dm_crypt loop sha256_ssse3 sha256_generic hmac drbg ansi_cprng xts gf128mul algif_skcipher af_alg cpuid nfsd auth_rpcgss oid_registry nfs_acl nfs lockd grace fscache sunrpc xor raid6_pq intel_rapl iosf_mbi x86_pkg_temp_thermal iTCO_wdt intel_powerclamp iTCO_vendor_support kvm_intel kvm evdev crct10dif_pclmul crc32_pclmul cryptd snd_pcm snd_timer snd soundcore pcspkr psmouse serio_raw hpwdt hpilo lpc_ich mfd_core 8250_fintek shpchp acpi_power_meter button pcc_cpufreq acpi_cpufreq processor coretemp ipmi_watchdog dm_mod ipmi_si ipmi_poweroff ipmi_devintf ipmi_msghandler fuse autofs4 ext4 crc16 mbcache jbd2 sg sd_mod usb_storage hid_generic usbhid hid crc32c_intel uhci_hcd thermal ahci libahci libata scsi_mod tg3 ptp pps_core libphy ehci_pci ehci_hcd xhci_pci xhci_hcd
> [64326.701579]  usbcore usb_common [last unloaded: btrfs]
> [64326.701611] CPU: 1 PID: 36251 Comm: btrfs Tainted: G        W       4.3.0-gl+ #42
> [64326.701647] Hardware name: HP ProLiant MicroServer Gen8, BIOS J06 06/06/2014
> [64326.701671]  ffffffffa06e8b71 ffffffff8129eac3 0000000000000000 ffffffff8106891c
> [64326.701720]  00000000fffffff4 ffff880079079800 ffff880006f1a000 ffff880074e2e000
> [64326.701769]  ffff880006f1aec8 ffffffffa06da7db 00007ffc00000001 ffff880071c42400
> [64326.701818] Call Trace:
> [64326.701840]  [<ffffffff8129eac3>] ? dump_stack+0x40/0x5d
> [64326.701864]  [<ffffffff8106891c>] ? warn_slowpath_common+0x7c/0xb0
> [64326.701896]  [<ffffffffa06da7db>] ? btrfs_dev_replace_start+0x36b/0x390 [btrfs]
> [64326.701939]  [<ffffffffa06a6bbe>] ? btrfs_ioctl+0x1b6e/0x27b0 [btrfs]
> [64326.701964]  [<ffffffff8116b83a>] ? page_add_file_rmap+0x2a/0x50
> [64326.706074]  [<ffffffff81160379>] ? do_set_pte+0x99/0xc0
> [64326.706100]  [<ffffffff81135f49>] ? filemap_map_pages+0x219/0x220
> [64326.706123]  [<ffffffff81162127>] ? handle_mm_fault+0xdd7/0x16c0
> [64326.706149]  [<ffffffff811b0b0e>] ? do_vfs_ioctl+0x2be/0x490
> [64326.706174]  [<ffffffff811b0d51>] ? SyS_ioctl+0x71/0x80
> [64326.706198]  [<ffffffff815000ee>] ? entry_SYSCALL_64_fastpath+0x12/0x71
> [64326.706222] ---[ end trace 37fc29aa3c600bcf ]---
This actually looks like it's a different issue potentially, for some 
reason BTRFS is trying to scrub the missing disk (which won't work of 
course).
>
> I'm not sure how to proceed from here, or how to debug this issue. While
> the disks are not holding critical data, I'm sure it would benefit the
> community (and btrfs' reputation) if this issue could be sorted out.


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