Re: INFO: task hung in generic_file_write_iter

[Tetsuo Handa]

On 2018/08/06 19:09, Jan Kara wrote:
> On Tue 31-07-18 00:07:22, Tetsuo Handa wrote:
>> On 2018/07/21 5:06, Andrew Morton wrote:
>>> On Fri, 20 Jul 2018 19:36:23 +0900 Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx> wrote:
>>>
>>>>>
>>>>> This report is stalling after mount() completed and process used remap_file_pages().
>>>>> I think that we might need to use debug printk(). But I don't know what to examine.
>>>>>
>>>>
>>>> Andrew, can you pick up this debug printk() patch?
>>>> I guess we can get the result within one week.
>>>
>>> Sure, let's toss it in -next for a while.
>>>
>>>> >From 8f55e00b21fefffbc6abd9085ac503c52a302464 Mon Sep 17 00:00:00 2001
>>>> From: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
>>>> Date: Fri, 20 Jul 2018 19:29:06 +0900
>>>> Subject: [PATCH] fs/buffer.c: add debug print for __getblk_gfp() stall problem
>>>>
>>>> Among syzbot's unresolved hung task reports, 18 out of 65 reports contain
>>>> __getblk_gfp() line in the backtrace. Since there is a comment block that
>>>> says that __getblk_gfp() will lock up the machine if try_to_free_buffers()
>>>> attempt from grow_dev_page() is failing, let's start from checking whether
>>>> syzbot is hitting that case. This change will be removed after the bug is
>>>> fixed.
>>>
>>> I'm not sure that grow_dev_page() is hanging.  It has often been
>>> suspected, but always is proven innocent.  Lets see.
>>
>> syzbot reproduced this problem ( https://syzkaller.appspot.com/text?tag=CrashLog&x=11f2fc44400000 ) .
>> It says that grow_dev_page() is returning 1 but __find_get_block() is failing forever. Any idea?
> 
> Looks like some kind of a race where device block size gets changed while
> getblk() runs (and creates buffers for underlying page). I don't have time
> to nail it down at this moment can have a look into it later unless someone
> beats me to it.

I feel that the frequency of hitting this problem was decreased
by merging loop module's ioctl() serialization patches. But this
problem is still there, and syzbot got a new line in
https://syzkaller.appspot.com/text?tag=CrashLog&x=177f889f400000 .

  [  615.881781] __loop_clr_fd: partition scan of loop5 failed (rc=-22)
  [  619.059920] syz-executor4(2193): getblk(): executed=cd bh_count=0 bh_state=29
  [  622.069808] syz-executor4(2193): getblk(): executed=9 bh_count=0 bh_state=0
  [  625.080013] syz-executor4(2193): getblk(): executed=9 bh_count=0 bh_state=0
  [  628.089900] syz-executor4(2193): getblk(): executed=9 bh_count=0 bh_state=0

I guess that loop module is somehow related to this problem.