And again I am sorry - it looks like it IS related to the start_sb. Because when I look at the stack track of the stuck flush-251:0 process (cat /proc/1428/stack) I see the following output:
[<ffffffff8117dae5>] sync_buffer+0x45/0x50
[<ffffffff8117da96>] __wait_on_buffer+0x26/0x30
[<ffffffff8117ea99>] sync_dirty_buffer+0x69/0xe0
[<ffffffff8122d24d>] jbd2_journal_update_superblock+0xad/0x170
[<ffffffff8122a757>] jbd2_cleanup_journal_tail+0x107/0x1e0
[<ffffffff8122af16>] jbd2_log_do_checkpoint+0x186/0x260
[<ffffffff8122b09e>] __jbd2_log_wait_for_space+0xae/0x1d0
[<ffffffff81226867>] start_this_handle+0x107/0x4b0
[<ffffffff81226de5>] jbd2_journal_start+0xb5/0x100
[<ffffffff81202a68>] ext4_journal_start_sb+0xf8/0x130
[<ffffffff811f06ff>] ext4_da_writepages+0x25f/0x640
[<ffffffff8110aab1>] do_writepages+0x21/0x40
[<ffffffff811759d6>] writeback_single_inode+0xe6/0x3f0
[<ffffffff81176135>] writeback_sb_inodes+0x195/0x280
[<ffffffff81176b8c>] wb_writeback+0x12c/0x2b0
[<ffffffff81176db9>] wb_do_writeback+0xa9/0x190
[<ffffffff81176ef3>] bdi_writeback_task+0x53/0x160
[<ffffffff8111a196>] bdi_start_fn+0x86/0x100
[<ffffffff8107ec56>] kthread+0x96/0xa0
[<ffffffff8100aee4>] kernel_thread_helper+0x4/0x10
[<ffffffffffffffff>] 0xffffffffffffffff
And again I am sorry - it looks like it IS related to the start_sb. Because when I look at the stack track of the stuck flush-251:0 process (cat /proc/1428/stack) I see the following output: ae5>] sync_buffer+ 0x45/0x50 a96>] __wait_ on_buffer+ 0x26/0x30 a99>] sync_dirty_ buffer+ 0x69/0xe0 24d>] jbd2_journal_ update_ superblock+ 0xad/0x170 757>] jbd2_cleanup_ journal_ tail+0x107/ 0x1e0 f16>] jbd2_log_ do_checkpoint+ 0x186/0x260 09e>] __jbd2_ log_wait_ for_space+ 0xae/0x1d0 867>] start_this_ handle+ 0x107/0x4b0 de5>] jbd2_journal_ start+0xb5/ 0x100 a68>] ext4_journal_ start_sb+ 0xf8/0x130 6ff>] ext4_da_ writepages+ 0x25f/0x640 ab1>] do_writepages+ 0x21/0x40 9d6>] writeback_ single_ inode+0xe6/ 0x3f0 135>] writeback_ sb_inodes+ 0x195/0x280 b8c>] wb_writeback+ 0x12c/0x2b0 db9>] wb_do_writeback +0xa9/0x190 ef3>] bdi_writeback_ task+0x53/ 0x160 196>] bdi_start_ fn+0x86/ 0x100 c56>] kthread+0x96/0xa0 ee4>] kernel_ thread_ helper+ 0x4/0x10 fff>] 0xffffffffffffffff
[<ffffffff8117d
[<ffffffff8117d
[<ffffffff8117e
[<ffffffff8122d
[<ffffffff8122a
[<ffffffff8122a
[<ffffffff8122b
[<ffffffff81226
[<ffffffff81226
[<ffffffff81202
[<ffffffff811f0
[<ffffffff8110a
[<ffffffff81175
[<ffffffff81176
[<ffffffff81176
[<ffffffff81176
[<ffffffff81176
[<ffffffff8111a
[<ffffffff8107e
[<ffffffff8100a
[<fffffffffffff