From: Sandeep Joshi <sanjos100@gmail.com>
Subject: process hangs in ext4_sync_file
Date: Mon, 21 Oct 2013 18:09:02 +0530
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I am seeing a problem reported 4 years earlier
https://lkml.org/lkml/2009/3/12/226
(same stack as seen by Alexander)

The problem is reproducible.  Let me know if you need any info in
addition to that seen below.

I have multiple threads in a process doing heavy IO on a ext4
filesystem mounted with (discard, noatime) on a SSD or HDD.

This is on Linux 3.8.0-29-generic #42~precise1-Ubuntu SMP Wed Aug 14
16:19:23 UTC 2013 x86_64 x86_64 x86_64 GNU/Linux

For upto minutes at a time, one of the threads seems to hang in sync to disk.

When I check the thread stack in /proc, I find that the stack is one
of the following two

<ffffffff81134a4e>] sleep_on_page+0xe/0x20
[<ffffffff81134c88>] wait_on_page_bit+0x78/0x80
[<ffffffff81134d9c>] filemap_fdatawait_range+0x10c/0x1a0
[<ffffffff811367d8>] filemap_write_and_wait_range+0x68/0x80
[<ffffffff81236a4f>] ext4_sync_file+0x6f/0x2b0
[<ffffffff811cba9b>] vfs_fsync+0x2b/0x40
[<ffffffff81168fb3>] sys_msync+0x143/0x1d0
[<ffffffff816fc8dd>] system_call_fastpath+0x1a/0x1f
[<ffffffffffffffff>] 0xffffffffffffffff


OR


[<ffffffff812947f5>] jbd2_log_wait_commit+0xb5/0x130
[<ffffffff81297213>] jbd2_complete_transaction+0x53/0x90
[<ffffffff81236bcd>] ext4_sync_file+0x1ed/0x2b0
[<ffffffff811cba9b>] vfs_fsync+0x2b/0x40
[<ffffffff81168fb3>] sys_msync+0x143/0x1d0
[<ffffffff816fc8dd>] system_call_fastpath+0x1a/0x1f
[<ffffffffffffffff>] 0xffffffffffffffff

Any clues?

-Sandeep