Re: INFO: task hung in mpage_prepare_extent_to_map

[Song Liu]

> On Oct 28, 2019, at 1:14 PM, Johannes Weiner <hannes@xxxxxxxxxxx> wrote:
> 
> On Mon, Oct 28, 2019 at 12:52:09PM -0700, syzbot wrote:
>> Hello,
>> 
>> syzbot found the following crash on:
>> 
>> HEAD commit:    12d61c69 Add linux-next specific files for 20191024
>> git tree:       linux-next
>> console output: https://urldefense.proofpoint.com/v2/url?u=https-3A__syzkaller.appspot.com_x_log.txt-3Fx-3D15a0fa97600000&d=DwIBAg&c=5VD0RTtNlTh3ycd41b3MUw&r=dR8692q0_uaizy0jkrBJQM5k2hfm4CiFxYT8KaysFrg&m=YEaOe5RP2hLXAC4tKPLehAQsea0_3k3tI4DL32BcA-8&s=6-TXLGQxJcK1GdwMwa51423Y221rRncNiC_T09O0OLc&e= 
>> kernel config:  https://urldefense.proofpoint.com/v2/url?u=https-3A__syzkaller.appspot.com_x_.config-3Fx-3Dafb75fd8c9fd5ed8&d=DwIBAg&c=5VD0RTtNlTh3ycd41b3MUw&r=dR8692q0_uaizy0jkrBJQM5k2hfm4CiFxYT8KaysFrg&m=YEaOe5RP2hLXAC4tKPLehAQsea0_3k3tI4DL32BcA-8&s=GuFgLJZOb7jtjZ5mDbkVT_zqtiVW4Py13e6Oq5CFxgY&e= 
>> dashboard link: https://urldefense.proofpoint.com/v2/url?u=https-3A__syzkaller.appspot.com_bug-3Fextid-3Defb9e48b9fbdc49bb34a&d=DwIBAg&c=5VD0RTtNlTh3ycd41b3MUw&r=dR8692q0_uaizy0jkrBJQM5k2hfm4CiFxYT8KaysFrg&m=YEaOe5RP2hLXAC4tKPLehAQsea0_3k3tI4DL32BcA-8&s=pF1hv-zGR8F378weGq9zxCE5ibI2_73qweMB_KuaZLM&e= 
>> compiler:       gcc (GCC) 9.0.0 20181231 (experimental)
>> syz repro:      https://urldefense.proofpoint.com/v2/url?u=https-3A__syzkaller.appspot.com_x_repro.syz-3Fx-3D13a63dc4e00000&d=DwIBAg&c=5VD0RTtNlTh3ycd41b3MUw&r=dR8692q0_uaizy0jkrBJQM5k2hfm4CiFxYT8KaysFrg&m=YEaOe5RP2hLXAC4tKPLehAQsea0_3k3tI4DL32BcA-8&s=mI7ZOgrDWeG-p6vn2d_kj65a5g8J7exXJ2MIUUF84-w&e= 
>> 
>> The bug was bisected to:
>> 
>> commit 9c61acffe2b8833152041f7b6a02d1d0a17fd378
>> Author: Song Liu <songliubraving@xxxxxx>
>> Date:   Wed Oct 23 00:24:28 2019 +0000
>> 
>>    mm,thp: recheck each page before collapsing file THP
>> 
>> bisection log:  https://urldefense.proofpoint.com/v2/url?u=https-3A__syzkaller.appspot.com_x_bisect.txt-3Fx-3D13eb6ec0e00000&d=DwIBAg&c=5VD0RTtNlTh3ycd41b3MUw&r=dR8692q0_uaizy0jkrBJQM5k2hfm4CiFxYT8KaysFrg&m=YEaOe5RP2hLXAC4tKPLehAQsea0_3k3tI4DL32BcA-8&s=YtSUy5Dtjo6tek7CvwzMTPL40BJwOC6rEom-AkVx0SM&e= 
>> final crash:    https://urldefense.proofpoint.com/v2/url?u=https-3A__syzkaller.appspot.com_x_report.txt-3Fx-3D101b6ec0e00000&d=DwIBAg&c=5VD0RTtNlTh3ycd41b3MUw&r=dR8692q0_uaizy0jkrBJQM5k2hfm4CiFxYT8KaysFrg&m=YEaOe5RP2hLXAC4tKPLehAQsea0_3k3tI4DL32BcA-8&s=BvPJx3QSPHgsN12jSZci_MqW_VxYp-MZpQtogZjlJOo&e= 
>> console output: https://urldefense.proofpoint.com/v2/url?u=https-3A__syzkaller.appspot.com_x_log.txt-3Fx-3D17eb6ec0e00000&d=DwIBAg&c=5VD0RTtNlTh3ycd41b3MUw&r=dR8692q0_uaizy0jkrBJQM5k2hfm4CiFxYT8KaysFrg&m=YEaOe5RP2hLXAC4tKPLehAQsea0_3k3tI4DL32BcA-8&s=YPvxWpQDpk9MI9W6QCtxME64wmxL2CZ5ZtEkCn0nI0c&e= 
>> 
>> IMPORTANT: if you fix the bug, please add the following tag to the commit:
>> Reported-by: syzbot+efb9e48b9fbdc49bb34a@xxxxxxxxxxxxxxxxxxxxxxxxx
>> Fixes: 9c61acffe2b8 ("mm,thp: recheck each page before collapsing file THP")
>> 
>> INFO: task khugepaged:1084 blocked for more than 143 seconds.
>>      Not tainted 5.4.0-rc4-next-20191024 #0
>> "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
>> khugepaged      D27568  1084      2 0x80004000
>> Call Trace:
>> context_switch kernel/sched/core.c:3384 [inline]
>> __schedule+0x94a/0x1e70 kernel/sched/core.c:4069
>> schedule+0xd9/0x260 kernel/sched/core.c:4136
>> io_schedule+0x1c/0x70 kernel/sched/core.c:5780
>> wait_on_page_bit_common mm/filemap.c:1175 [inline]
>> __lock_page+0x422/0xab0 mm/filemap.c:1383
>> lock_page include/linux/pagemap.h:480 [inline]
>> mpage_prepare_extent_to_map+0xb3f/0xf90 fs/ext4/inode.c:2668
>> ext4_writepages+0xb6a/0x2e70 fs/ext4/inode.c:2866
>> ? 0xffffffff81000000
>> do_writepages+0xfa/0x2a0 mm/page-writeback.c:2344
>> __filemap_fdatawrite_range+0x2bc/0x3b0 mm/filemap.c:421
>> __filemap_fdatawrite mm/filemap.c:429 [inline]
>> filemap_flush+0x24/0x30 mm/filemap.c:456
> 
> This is a double locking deadlock. The page lock is already held when
> we call into filemap_flush() here, and does another lock_page() in
> write_cache_pages().
> 
> To fix it, we have to either initiate flushing before acquiring the
> page lock, or simply skip over dirty pages.
> 
> Maybe doing vfs_fsync_range() from the madvise(HUGEPAGE) call isn't a
> bad idea after all? (I had discussed this with Song off-list before.)

Thanks syzbot and Johannes!

I just sent a quick fix, that just removes filemap_flush(). 

I will work on a better mechanism to flush the file. 

Thanks,
Song