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From:   Yang Shi <shy828301@gmail.com>
Date:   Tue, 24 Aug 2021 15:19:07 -0700
Message-ID: <CAHbLzkrBY7L0B8_+arEJK9JjwVMw4ugeW_+JG4pDc8E8ZBgzYg@mail.gmail.com>
Subject: Re: [syzbot] kernel BUG in collapse_file
To:     syzbot <syzbot+f17e5ab118d29367bda9@syzkaller.appspotmail.com>,
        Linux MM <linux-mm@kvack.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Hugh Dickins <hughd@google.com>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
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On Tue, Aug 24, 2021 at 12:14 PM Yang Shi <shy828301@gmail.com> wrote:
>
> On Tue, Aug 24, 2021 at 12:13 PM Yang Shi <shy828301@gmail.com> wrote:
> >
> > On Tue, Aug 24, 2021 at 11:17 AM syzbot
> > <syzbot+f17e5ab118d29367bda9@syzkaller.appspotmail.com> wrote:
> > >
> > > Hello,
> > >
> > > syzbot found the following issue on:
> > >
> > > HEAD commit:    d6d09a694205 Merge tag 'for-5.14-rc6-tag' of git://git.ker..
> > > git tree:       upstream
> > > console output: https://syzkaller.appspot.com/x/log.txt?x=14924605300000
> > > kernel config:  https://syzkaller.appspot.com/x/.config?x=f61012d0b1cd846f
> > > dashboard link: https://syzkaller.appspot.com/bug?extid=f17e5ab118d29367bda9
> > > compiler:       Debian clang version 11.0.1-2, GNU ld (GNU Binutils for Debian) 2.35.1
> > >
> > > Unfortunately, I don't have any reproducer for this issue yet.
> > >
> > > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > > Reported-by: syzbot+f17e5ab118d29367bda9@syzkaller.appspotmail.com
> > >
> > >  unmap_region+0x2ab/0x300 mm/mmap.c:2680
> > >  __do_munmap+0x18eb/0x2050 mm/mmap.c:2911
> > >  __do_sys_mremap+0x5d9/0x1390 mm/mremap.c:969
> > >  do_syscall_x64 arch/x86/entry/common.c:50 [inline]
> > >  do_syscall_64+0x3d/0xb0 arch/x86/entry/common.c:80
> > >  entry_SYSCALL_64_after_hwframe+0x44/0xae
> > > ------------[ cut here ]------------
> > > kernel BUG at mm/khugepaged.c:1830!
> >
> > The BUG means the page is not fully unmapped due to some race, just
> > like what https://lore.kernel.org/linux-mm/00000000000030563005ca006ffd@google.com/
> > reported.
> >
> > But I really don't think it is worth VM_BUG since khugepaged can just back off.

In addition, I think we should be able to just replace
unmap_mapping_pages() to try_to_unmap() or unmap_page (of course, make
it non-static) since some races are closed by Hugh's patches.

>
> Forgot cc to other stakeholders.
>
> >
> > > invalid opcode: 0000 [#1] PREEMPT SMP KASAN
> > > CPU: 0 PID: 1651 Comm: khugepaged Not tainted 5.14.0-rc6-syzkaller #0
> > > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
> > > RIP: 0010:collapse_file+0x3537/0x36e0 mm/khugepaged.c:1830
> > > Code: e8 1e 7a b2 ff 48 89 df 48 c7 c6 40 7b 3d 8a e8 ef 02 e8 ff 0f 0b e8 08 7a b2 ff 48 89 df 48 c7 c6 a0 7b 3d 8a e8 d9 02 e8 ff <0f> 0b e8 f2 79 b2 ff 4c 89 e7 48 c7 c6 a0 7e 3d 8a e8 c3 02 e8 ff
> > > RSP: 0018:ffffc900066df820 EFLAGS: 00010046
> > > RAX: 8601299b4ae62f00 RBX: ffffea0000e4e240 RCX: ffff888018fed4c0
> > > RDX: 0000000000000000 RSI: 000000000000ffff RDI: 000000000000ffff
> > > RBP: ffffc900066dfa90 R08: ffffffff81d08764 R09: ffffed1017383f2c
> > > R10: ffffed1017383f2c R11: 0000000000000000 R12: ffffc900066df9d8
> > > R13: dffffc0000000000 R14: ffffc900066dfa20 R15: 0000000000000001
> > > FS:  0000000000000000(0000) GS:ffff8880b9c00000(0000) knlGS:0000000000000000
> > > CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > > CR2: 0000000100000000 CR3: 000000000c48e000 CR4: 00000000001506f0
> > > DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> > > DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> > > Call Trace:
> > >  khugepaged_scan_file mm/khugepaged.c:2051 [inline]
> > >  khugepaged_scan_mm_slot+0x27be/0x2ad0 mm/khugepaged.c:2146
> > >  khugepaged_do_scan+0x2b1/0x640 mm/khugepaged.c:2230
> > >  khugepaged+0x105/0x890 mm/khugepaged.c:2275
> > >  kthread+0x453/0x480 kernel/kthread.c:319
> > >  ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:295
> > > Modules linked in:
> > > ---[ end trace 8de2a60ea4ac5483 ]---
> > > RIP: 0010:collapse_file+0x3537/0x36e0 mm/khugepaged.c:1830
> > > Code: e8 1e 7a b2 ff 48 89 df 48 c7 c6 40 7b 3d 8a e8 ef 02 e8 ff 0f 0b e8 08 7a b2 ff 48 89 df 48 c7 c6 a0 7b 3d 8a e8 d9 02 e8 ff <0f> 0b e8 f2 79 b2 ff 4c 89 e7 48 c7 c6 a0 7e 3d 8a e8 c3 02 e8 ff
> > > RSP: 0018:ffffc900066df820 EFLAGS: 00010046
> > > RAX: 8601299b4ae62f00 RBX: ffffea0000e4e240 RCX: ffff888018fed4c0
> > > RDX: 0000000000000000 RSI: 000000000000ffff RDI: 000000000000ffff
> > > RBP: ffffc900066dfa90 R08: ffffffff81d08764 R09: ffffed1017383f2c
> > > R10: ffffed1017383f2c R11: 0000000000000000 R12: ffffc900066df9d8
> > > R13: dffffc0000000000 R14: ffffc900066dfa20 R15: 0000000000000001
> > > FS:  0000000000000000(0000) GS:ffff8880b9c00000(0000) knlGS:0000000000000000
> > > CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > > CR2: 0000000100000000 CR3: 000000000c48e000 CR4: 00000000001506f0
> > > DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> > > DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> > > ----------------
> > > Code disassembly (best guess):
> > >    0:   e8 1e 7a b2 ff          callq  0xffb27a23
> > >    5:   48 89 df                mov    %rbx,%rdi
> > >    8:   48 c7 c6 40 7b 3d 8a    mov    $0xffffffff8a3d7b40,%rsi
> > >    f:   e8 ef 02 e8 ff          callq  0xffe80303
> > >   14:   0f 0b                   ud2
> > >   16:   e8 08 7a b2 ff          callq  0xffb27a23
> > >   1b:   48 89 df                mov    %rbx,%rdi
> > >   1e:   48 c7 c6 a0 7b 3d 8a    mov    $0xffffffff8a3d7ba0,%rsi
> > >   25:   e8 d9 02 e8 ff          callq  0xffe80303
> > >   2a:   0f 0b                   ud2     <-- trapping instruction
> > >   2c:   e8 f2 79 b2 ff          callq  0xffb27a23
> > >   31:   4c 89 e7                mov    %r12,%rdi
> > >   34:   48 c7 c6 a0 7e 3d 8a    mov    $0xffffffff8a3d7ea0,%rsi
> > >   3b:   e8 c3 02 e8 ff          callq  0xffe80303
> > >
> > >
> > > ---
> > > This report is generated by a bot. It may contain errors.
> > > See https://goo.gl/tpsmEJ for more information about syzbot.
> > > syzbot engineers can be reached at syzkaller@googlegroups.com.
> > >
> > > syzbot will keep track of this issue. See:
> > > https://goo.gl/tpsmEJ#status for how to communicate with syzbot.
> > >