linux-kernel - Re: linux-next test error

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Message-ID: <20180905085545.GD24902@quack2.suse.cz>
Date:   Wed, 5 Sep 2018 10:55:45 +0200
From:   Jan Kara <jack@...e.cz>
To:     syzbot <syzbot+87a05ae4accd500f5242@...kaller.appspotmail.com>
Cc:     ak@...ux.intel.com, akpm@...ux-foundation.org, jack@...e.cz,
        jrdr.linux@...il.com, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, mawilcox@...rosoft.com,
        mgorman@...hsingularity.net, syzkaller-bugs@...glegroups.com,
        tim.c.chen@...ux.intel.com, zwisler@...nel.org
Subject: Re: linux-next test error

On Wed 05-09-18 00:13:02, syzbot wrote:
> Hello,
> 
> syzbot found the following crash on:
> 
> HEAD commit:    387ac6229ecf Add linux-next specific files for 20180905
> git tree:       linux-next
> console output: https://syzkaller.appspot.com/x/log.txt?x=149c67a6400000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=ad5163873ecfbc32
> dashboard link: https://syzkaller.appspot.com/bug?extid=87a05ae4accd500f5242
> compiler:       gcc (GCC) 8.0.1 20180413 (experimental)
> 
> Unfortunately, I don't have any reproducer for this crash yet.
> 
> IMPORTANT: if you fix the bug, please add the following tag to the commit:
> Reported-by: syzbot+87a05ae4accd500f5242@...kaller.appspotmail.com
> 
> INFO: task hung in do_page_mkwriteINFO: task syz-fuzzer:4876 blocked for
> more than 140 seconds.
>       Not tainted 4.19.0-rc2-next-20180905+ #56
> "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> syz-fuzzer      D21704  4876   4871 0x00000000
> Call Trace:
>  context_switch kernel/sched/core.c:2825 [inline]
>  __schedule+0x87c/0x1df0 kernel/sched/core.c:3473
>  schedule+0xfb/0x450 kernel/sched/core.c:3517
>  io_schedule+0x1c/0x70 kernel/sched/core.c:5140
>  wait_on_page_bit_common mm/filemap.c:1100 [inline]
>  __lock_page+0x5b7/0x7a0 mm/filemap.c:1273
>  lock_page include/linux/pagemap.h:483 [inline]
>  do_page_mkwrite+0x429/0x520 mm/memory.c:2391

Waiting for page lock after ->page_mkwrite callback. Which means
->page_mkwrite did not return VM_FAULT_LOCKED but 0. Looking into
linux-next... indeed "fs: convert return type int to vm_fault_t" has busted
block_page_mkwrite(). It has to return VM_FAULT_LOCKED and not 0 now.
Souptick, can I ask you to run 'fstests' for at least common filesystems
like ext4, xfs, btrfs when you change generic filesystem code please? That
would catch a bug like this immediately. Thanks.

								Honza

-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR