syzbot has bisected this bug to:

commit 9c61acffe2b8833152041f7b6a02d1d0a17fd378
Author: Song Liu [off-list ref]
Date:   Wed Oct 23 00:24:28 2019 +0000

     mm,thp: recheck each page before collapsing file THP

bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=15be4e14e00000
start commit:   49afce6d Add linux-next specific files for 20191031
git tree:       linux-next
final crash:    https://syzkaller.appspot.com/x/report.txt?x=17be4e14e00000
console output: https://syzkaller.appspot.com/x/log.txt?x=13be4e14e00000
kernel config:  https://syzkaller.appspot.com/x/.config?x=3c5f119b33031056
dashboard link: https://syzkaller.appspot.com/bug?extid=62ebe501c1ce9a91f68c
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=14c162f4e00000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=131b5eb8e00000

Reported-by: syzbot+62ebe501c1ce9a91f68c@syzkaller.appspotmail.com
Fixes: 9c61acffe2b8 ("mm,thp: recheck each page before collapsing file THP")

For information about bisection process see: https://goo.gl/tpsmEJ#bisection