> About the second one:
> 
> > ==The Second Issue (aka "The Busy Looping sync()" case) ==
> > The box is different from first, so conditions are a bit different.
> > - /dev/root on / type btrfs (rw,noatime,autodefrag)
> >   (note autodefrag!)
> > - 15% full 594GB filesystem (usual nonmixed mode)
> > 
> >    $ liferea
> >    <wait it to calm down. it does a lot of SQLite reads/writes>
> >    $ sync
> >    Got CPU is 100% loaded <hung>
> 
> Still reproducible with 2 patches above + $SUBJ one. strace says it hangs in
> strace() syscall. Stack trace is odd:
> # cat /proc/`pidof sync`/stack
> [<ffffffffffffffff>] 0xffffffffffffffff

"got" stack of the looper with such hack:
    $ while :; do sudo echo "-"; cat /proc/2304/stack | fgrep -v '[<ffffffffffffffff>] 0xffffffffffffffff'; done | uniq

[<ffffffff8109d39d>] find_get_pages_tag+0x14d/0x1a0
[<ffffffff81076981>] __lock_acquire+0x5a1/0xbd0
[<ffffffff8109e7bd>] filemap_fdatawait_range+0x9d/0x1b0
[<ffffffff8109e8f2>] filemap_fdatawait+0x22/0x30
[<ffffffff81106a04>] sync_inodes_sb+0x1c4/0x250
[<ffffffff8110b398>] __sync_filesystem+0x78/0x80
[<ffffffff8110b3b7>] sync_one_sb+0x17/0x20
[<ffffffff810e3fbe>] iterate_supers+0x7e/0xe0
[<ffffffff8110b400>] sys_sync+0x40/0x60
[<ffffffff81456afb>] system_call_fastpath+0x16/0x1b
-
[<ffffffff811069f1>] sync_inodes_sb+0x1b1/0x250
[<ffffffff8110b398>] __sync_filesystem+0x78/0x80
[<ffffffff8110b3b7>] sync_one_sb+0x17/0x20
[<ffffffff810e3fbe>] iterate_supers+0x7e/0xe0
[<ffffffff8110b400>] sys_sync+0x40/0x60
[<ffffffff81456afb>] system_call_fastpath+0x16/0x1b
-
[<ffffffff811069f1>] sync_inodes_sb+0x1b1/0x250
[<ffffffff8110b398>] __sync_filesystem+0x78/0x80
[<ffffffff8110b3b7>] sync_one_sb+0x17/0x20
[<ffffffff810e3fbe>] iterate_supers+0x7e/0xe0
[<ffffffff8110b400>] sys_sync+0x40/0x60
[<ffffffff81456afb>] system_call_fastpath+0x16/0x1b
-
[<ffffffff814564b6>] retint_kernel+0x26/0x30
[<ffffffff8109d398>] find_get_pages_tag+0x148/0x1a0
[<ffffffff810a7a60>] pagevec_lookup_tag+0x20/0x30
[<ffffffff8109e7bd>] filemap_fdatawait_range+0x9d/0x1b0
[<ffffffff8109e8f2>] filemap_fdatawait+0x22/0x30
[<ffffffff81106a04>] sync_inodes_sb+0x1c4/0x250
[<ffffffff8110b398>] __sync_filesystem+0x78/0x80
[<ffffffff8110b3b7>] sync_one_sb+0x17/0x20
[<ffffffff810e3fbe>] iterate_supers+0x7e/0xe0
[<ffffffff8110b400>] sys_sync+0x40/0x60
[<ffffffff81456afb>] system_call_fastpath+0x16/0x1b

Looks like dirty inode count (or dirty page count) is affected by
the $SUBJ patch.

-- 

  Sergei