Re: INFO: task hung in generic_file_write_iter

[Jan Kara]

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.

								Honza
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR