No, I meant I have no idea how to reproduce it at all, either on Arch or Ubuntu. It happened for the first time a few days ago, and again 5 minutes ago (also on Arch, because my Ubuntu install isn't using ecryptfs at the moment).

I will try to set up ecryptfs on my Ubuntu install and try to trigger the bug. I've also moved my firefox profile outside ecryptfs on my Arch install to see if this is really related to ecryptfs or just firefox.

Here's the stack trace from the new crash. It's nearly identical: firefox, ecryptfs and ext4. The symptoms were exactly the same except that this time firefox was killed instead of frozen (but other applications would still freeze when writing to the disk).

[ 1525.865720] ------------[ cut here ]------------
[ 1525.865725] kernel BUG at fs/buffer.c:1278!
[ 1525.865726] invalid opcode: 0000 [#1] PREEMPT SMP 
[ 1525.865728] Modules linked in: xfs nilfs2 jfs btrfs raid6_pq libcrc32c xor fuse md5 ecb ecryptfs cbc sha256_ssse3 sha256_generic encrypted_keys hmac trusted tpm tpm_bios snd_hda_codec_hdmi snd_hda_codec_realtek nls_cp437 vfat fat nvidia(PO) x86_pkg_temp_thermal coretemp kvm_intel kvm crct10dif_pclmul crct10dif_common crc32_pclmul crc32c_intel ghash_clmulni_intel aesni_intel aes_x86_64 lrw gf128mul glue_helper ablk_helper cryptd mxm_wmi iTCO_wdt iTCO_vendor_support microcode hid_logitech_dj psmouse serio_raw evdev ip6t_REJECT pcspkr i2c_i801 xt_hl ip6t_rt drm nf_conntrack_ipv6 nf_defrag_ipv6 i2c_core ipt_REJECT xt_LOG snd_hda_intel snd_hda_codec fan xt_recent snd_hwdep snd_pcm snd_page_alloc xt_multiport e1000e snd_timer snd mei_me soundcore xt_limit xt_tcpudp ptp shpchp mei xt_addrtype pps_core wmi
[ 1525.865755]  processor lpc_ich video button thermal nf_conntrack_ipv4 nf_defrag_ipv4 xt_conntrack ip6table_filter ip6_tables nf_conntrack_netbios_ns nf_conntrack_broadcast nf_nat_ftp nf_nat nf_conntrack_ftp nf_conntrack iptable_filter ip_tables x_tables ext4 crc16 mbcache jbd2 hid_generic usbhid hid sr_mod cdrom sd_mod ahci libahci ehci_pci xhci_hcd libata ehci_hcd scsi_mod usbcore usb_common
[ 1525.865768] CPU: 0 PID: 839 Comm: firefox Tainted: P           O 3.12.6-1-ARCH #1
[ 1525.865769] Hardware name: Gigabyte Technology Co., Ltd. Z87X-D3H/Z87X-D3H-CF, BIOS F7 08/02/2013
[ 1525.865770] task: ffff88007fe64e30 ti: ffff8800c6862000 task.ti: ffff8800c6862000
[ 1525.865771] RIP: 0010:[<ffffffff811d6ab8>]  [<ffffffff811d6ab8>] __find_get_block+0x288/0x290
[ 1525.865775] RSP: 0018:ffff8800c68635a0  EFLAGS: 00010046
[ 1525.865776] RAX: 0000000000000092 RBX: ffff8802156393e8 RCX: ffff88007ff73800
[ 1525.865777] RDX: 0000000000001000 RSI: 0000000000d00068 RDI: ffff880225c28380
[ 1525.865777] RBP: ffff8800c68635f8 R08: 0000000000000003 R09: 0000000000000003
[ 1525.865778] R10: ffff88007ff73800 R11: 0000000000000001 R12: ffff880225c28380
[ 1525.865779] R13: 0000000000001000 R14: ffff88022eb20400 R15: 0000000000000010
[ 1525.865780] FS:  00007f1d7364b740(0000) GS:ffff88022ec00000(0000) knlGS:0000000000000000
[ 1525.865781] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 1525.865781] CR2: 00007f2729800018 CR3: 000000000280d000 CR4: 00000000001407f0
[ 1525.865782] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[ 1525.865783] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[ 1525.865783] Stack:
[ 1525.865784]  ffffffff811d6f75 ffff8800cd071300 ffff880221c1f000 ffff8800cd0b1900
[ 1525.865786]  ffff8800c68635d0 0000000000d00068 ffff8802156393e8 ffff8800c68636b0
[ 1525.865787]  ffff88007ff73000 ffff88022eb20400 0000000000000010 ffff8800c6863660
[ 1525.865789] Call Trace:
[ 1525.865791]  [<ffffffff811d6f75>] ? __getblk+0x25/0x2e0
[ 1525.865800]  [<ffffffffa0284465>] __ext4_get_inode_loc+0x105/0x440 [ext4]
[ 1525.865805]  [<ffffffffa0286ffd>] ext4_get_inode_loc+0x1d/0x20 [ext4]
[ 1525.865810]  [<ffffffffa0288866>] ext4_reserve_inode_write+0x26/0xa0 [ext4]
[ 1525.865816]  [<ffffffffa028bed0>] ? ext4_dirty_inode+0x40/0x60 [ext4]
[ 1525.865821]  [<ffffffffa028891f>] ext4_mark_inode_dirty+0x3f/0x230 [ext4]
[ 1525.865827]  [<ffffffffa028bed0>] ext4_dirty_inode+0x40/0x60 [ext4]
[ 1525.865829]  [<ffffffff811ce24a>] __mark_inode_dirty+0x3a/0x2f0
[ 1525.865832]  [<ffffffff811bec21>] update_time+0x81/0xd0
[ 1525.865834]  [<ffffffff811bee30>] file_update_time+0x80/0xd0
[ 1525.865837]  [<ffffffff8113778c>] __generic_file_aio_write+0x16c/0x3b0
[ 1525.865841]  [<ffffffff8101517f>] ? math_state_restore+0x9f/0x190
[ 1525.865843]  [<ffffffff81137a28>] generic_file_aio_write+0x58/0xa0
[ 1525.865848]  [<ffffffffa0280159>] ext4_file_write+0x99/0x3e0 [ext4]
[ 1525.865851]  [<ffffffffa070d083>] ? __ablk_encrypt+0x43/0x50 [ablk_helper]
[ 1525.865853]  [<ffffffff811524bd>] ? kzfree+0x2d/0x30
[ 1525.865856]  [<ffffffff811a430a>] do_sync_write+0x5a/0x90
[ 1525.865858]  [<ffffffff811a49cd>] vfs_write+0xbd/0x1e0
[ 1525.865860]  [<ffffffff811d0e38>] kernel_write+0x38/0x50
[ 1525.865864]  [<ffffffffa006bce9>] ecryptfs_write_lower+0x29/0x50 [ecryptfs]
[ 1525.865867]  [<ffffffffa006d0d3>] ecryptfs_encrypt_page+0xe3/0x180 [ecryptfs]
[ 1525.865870]  [<ffffffffa006b194>] ecryptfs_writepage+0x14/0x60 [ecryptfs]
[ 1525.865872]  [<ffffffff8113fc73>] __writepage+0x13/0x50
[ 1525.865874]  [<ffffffff8114020b>] write_cache_pages+0x21b/0x510
[ 1525.865877]  [<ffffffff8109045b>] ? ttwu_stat+0x9b/0x110
[ 1525.865879]  [<ffffffff8113fc60>] ? mapping_tagged+0x20/0x20
[ 1525.865891]  [<ffffffff811356cb>] ? filemap_fdatawait_range+0x17b/0x1a0
[ 1525.865892]  [<ffffffff81140540>] generic_writepages+0x40/0x60
[ 1525.865894]  [<ffffffff81141d75>] do_writepages+0x35/0x40
[ 1525.865896]  [<ffffffff81136c49>] __filemap_fdatawrite_range+0x59/0x60
[ 1525.865898]  [<ffffffff81136ccc>] filemap_write_and_wait+0x2c/0x60
[ 1525.865901]  [<ffffffffa006ae12>] ecryptfs_put_lower_file+0x32/0x60 [ecryptfs]
[ 1525.865903]  [<ffffffffa00681f2>] ecryptfs_release+0x12/0x30 [ecryptfs]
[ 1525.865905]  [<ffffffff811a62e1>] __fput+0xa1/0x230
[ 1525.865907]  [<ffffffff811a64be>] ____fput+0xe/0x10
[ 1525.865910]  [<ffffffff81081d2c>] task_work_run+0xbc/0xe0
[ 1525.865912]  [<ffffffff8106507c>] do_exit+0x2bc/0xab0
[ 1525.865914]  [<ffffffff81071a63>] ? __sigqueue_free.part.17+0x33/0x40
[ 1525.865915]  [<ffffffff810658ef>] do_group_exit+0x3f/0xa0
[ 1525.865916]  [<ffffffff8107488d>] get_signal_to_deliver+0x2ad/0x610
[ 1525.865918]  [<ffffffff810144d8>] do_signal+0x48/0x940
[ 1525.865920]  [<ffffffff81014e38>] do_notify_resume+0x68/0xa0
[ 1525.865922]  [<ffffffff814f5bbc>] retint_signal+0x48/0x8c
[ 1525.865923] Code: 85 f6 0f 84 4e fe ff ff 4c 89 f7 e8 73 fb ff ff e9 41 fe ff ff 4c 89 e7 e8 66 fb ff ff e9 35 ff ff ff 45 31 f6 eb d1 0f 0b 0f 0b <0f> 0b 66 0f 1f 44 00 00 0f 1f 44 00 00 55 48 89 e5 41 54 53 48 
[ 1525.865937] RIP  [<ffffffff811d6ab8>] __find_get_block+0x288/0x290
[ 1525.865938]  RSP <ffff8800c68635a0>
[ 1525.865940] ---[ end trace 89c91966e7e1a370 ]---
[ 1525.865951] Fixing recursive fault but reboot is needed!