Livelock when running xfstests generic/127 on ext4 with 3.15

[Matthew Wilcox <willy@linux.intel.com>]

I didn't see this with 3.14, but I'm not sure what's changed.

When running generic/127, fsx ends up taking 30% CPU time with a kthread
taking 70% CPU time for hours.  It might be making forward progress,
but if it is, it's incredibly slow.

I can usually catch fsx waiting for the kthread:

# ./check generic/127
FSTYP         -- ext4
PLATFORM      -- Linux/x86_64 walter 3.15.0
MKFS_OPTIONS  -- /dev/ram1
MOUNT_OPTIONS -- -o acl,user_xattr /dev/ram1 /mnt/ram1

generic/127 19s ...

$ sudo cat /proc/4795/stack 
[<ffffffff8120bee9>] writeback_inodes_sb_nr+0xa9/0xe0
[<ffffffff8120bfae>] try_to_writeback_inodes_sb_nr+0x5e/0x80
[<ffffffff8120bff5>] try_to_writeback_inodes_sb+0x25/0x30
[<ffffffffa01bae2a>] ext4_nonda_switch+0x8a/0x90 [ext4]
[<ffffffffa01c49a5>] ext4_page_mkwrite+0x265/0x440 [ext4]
[<ffffffff811936ed>] do_page_mkwrite+0x3d/0x70
[<ffffffff81195887>] do_wp_page+0x627/0x770
[<ffffffff811981a1>] handle_mm_fault+0x781/0xf00
[<ffffffff815a8996>] __do_page_fault+0x186/0x570
[<ffffffff815a8da2>] do_page_fault+0x22/0x30
[<ffffffff815a5038>] page_fault+0x28/0x30
[<ffffffffffffffff>] 0xffffffffffffffff


My setup is a 1GB ram disk:

modprobe brd rd_size=1048576 rd_nr=2

local.config:

TEST_DEV=/dev/ram0
TEST_DIR=/mnt/ram0
SCRATCH_DEV=/dev/ram1
SCRATCH_MNT=/mnt/ram1


Hardware is an Intel(R) Core(TM) i7-4770 CPU @ 3.40GHz with 4GB RAM,
in case it matters.  But I think what matters is that I'm running it on
a "tiny" 1GB filesystem, since this code is only invoked whenever the
number of dirty clusters is large relative to the number of free clusters.

df shows:
/dev/ram1         999320     1284    929224   1% /mnt/ram1
/dev/ram0         999320   646088    284420  70% /mnt/ram0

So it's not *unreasonably* full.