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To:     syzbot <syzbot+38cd5310bb0818ffc964@syzkaller.appspotmail.com>,
        clm@fb.com, dsterba@suse.com, josef@toxicpanda.com,
        linux-btrfs@vger.kernel.org, linux-kernel@vger.kernel.org,
        syzkaller-bugs@googlegroups.com
References: <000000000000502c1a05c53fa2e1@google.com>
From:   Qu Wenruo <quwenruo.btrfs@gmx.com>
Subject: Re: [syzbot] general protection fault in detach_extent_buffer_page
Message-ID: <a6d02720-702b-0360-351e-b1470a6b105b@gmx.com>
Date:   Mon, 21 Jun 2021 16:53:12 +0800
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On 2021/6/21 =E4=B8=8B=E5=8D=881:06, syzbot wrote:
> Hello,
>
> syzbot found the following issue on:
>
> HEAD commit:    6b00bc63 Merge tag 'dmaengine-fix-5.13' of git://git.ker=
ne..
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=3D1591a33fd000=
00
> kernel config:  https://syzkaller.appspot.com/x/.config?x=3D7ca96a2d153c=
74b0
> dashboard link: https://syzkaller.appspot.com/bug?extid=3D38cd5310bb0818=
ffc964
> userspace arch: i386
>
> Unfortunately, I don't have any reproducer for this issue yet.
>
> IMPORTANT: if you fix the issue, please add the following tag to the com=
mit:
> Reported-by: syzbot+38cd5310bb0818ffc964@syzkaller.appspotmail.com
>
> general protection fault, probably for non-canonical address 0xdffffc000=
000002a: 0000 [#1] PREEMPT SMP KASAN
> KASAN: null-ptr-deref in range [0x0000000000000150-0x0000000000000157]
> CPU: 1 PID: 10005 Comm: syz-fuzzer Not tainted 5.13.0-rc6-syzkaller #0
> Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.14.0-2 04/01/=
2014
> RIP: 0010:__lock_acquire+0xcf0/0x5230 kernel/locking/lockdep.c:4772
> Code: 3d 0e 41 bf 01 00 00 00 0f 86 8c 00 00 00 89 05 06 49 3d 0e e9 81 =
00 00 00 48 b8 00 00 00 00 00 fc ff df 4c 89 f2 48 c1 ea 03 <80> 3c 02 00 =
0f 85 82 2f 00 00 49 81 3e c0 b3 42 8f 0f 84 da f3 ff
> RSP: 0000:ffffc9000d27e9e0 EFLAGS: 00010002
> RAX: dffffc0000000000 RBX: 0000000000000000 RCX: 0000000000000000
> RDX: 000000000000002a RSI: 0000000000000001 RDI: 0000000000000001
> RBP: 0000000000000000 R08: 0000000000000001 R09: 0000000000000000
> R10: fffffbfff1b92d2a R11: 0000000000000000 R12: ffff888017401c40
> R13: 0000000000000000 R14: 0000000000000150 R15: 0000000000000000
> FS:  000000c013afc090(0000) GS:ffff88802cb00000(0000) knlGS:000000000000=
0000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 00000000009a67f0 CR3: 000000001b446000 CR4: 0000000000150ee0
> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> Call Trace:
>   lock_acquire kernel/locking/lockdep.c:5512 [inline]
>   lock_acquire+0x1ab/0x740 kernel/locking/lockdep.c:5477
>   __raw_spin_lock include/linux/spinlock_api_smp.h:142 [inline]
>   _raw_spin_lock+0x2a/0x40 kernel/locking/spinlock.c:151
>   spin_lock include/linux/spinlock.h:354 [inline]
>   detach_extent_buffer_page+0x402/0xd20 fs/btrfs/extent_io.c:5446

This is the spin_lock() call for the if (mapped) branch.

It means, we got an extent buffer which has EXTENT_BUFFER_UNMAPPED bit
set, but its page->mapping is no long reliable.

I guess some racy for extent buffer page detaching is happening?

 From the code diff, the timing when we check EXTENT_BUFFER_UNMAPPED is
changed.

Previously it's checked out of the loop, but now we check it inside the
loop.
But it doesn't look possible as EXTENT_BUFFER_UNMAPPED is determined at
extent buffer alloc time.

Thus it's super weird.

Have you guys hit similar crash?

Thanks,
Qu

>   btrfs_release_extent_buffer_pages+0xf1/0x320 fs/btrfs/extent_io.c:5515
>   release_extent_buffer+0x242/0x2b0 fs/btrfs/extent_io.c:5985
>   try_release_extent_buffer+0x706/0x900 fs/btrfs/extent_io.c:7032
>   btree_releasepage+0x1fe/0x310 fs/btrfs/disk-io.c:1023
>   try_to_release_page+0x27b/0x3e0 mm/filemap.c:3856
>   shrink_page_list+0x3cb6/0x6060 mm/vmscan.c:1599
>   shrink_inactive_list+0x347/0xca0 mm/vmscan.c:2145
>   shrink_list mm/vmscan.c:2367 [inline]
>   shrink_lruvec+0x7f9/0x14f0 mm/vmscan.c:2662
>   shrink_node_memcgs mm/vmscan.c:2850 [inline]
>   shrink_node+0x868/0x1de0 mm/vmscan.c:2967
>   shrink_zones mm/vmscan.c:3170 [inline]
>   do_try_to_free_pages+0x388/0x14b0 mm/vmscan.c:3225
>   try_to_free_pages+0x29f/0x750 mm/vmscan.c:3464
>   __perform_reclaim mm/page_alloc.c:4430 [inline]
>   __alloc_pages_direct_reclaim mm/page_alloc.c:4451 [inline]
>   __alloc_pages_slowpath.constprop.0+0x84e/0x2140 mm/page_alloc.c:4855
>   __alloc_pages+0x422/0x500 mm/page_alloc.c:5213
>   alloc_pages+0x18c/0x2a0 mm/mempolicy.c:2272
>   __page_cache_alloc mm/filemap.c:1005 [inline]
>   __page_cache_alloc+0x303/0x3a0 mm/filemap.c:990
>   pagecache_get_page+0x38f/0x18d0 mm/filemap.c:1885
>   filemap_fault+0x166c/0x25b0 mm/filemap.c:2992
>   ext4_filemap_fault+0x87/0xc0 fs/ext4/inode.c:6194
>   __do_fault+0x10d/0x4d0 mm/memory.c:3680
>   do_read_fault mm/memory.c:3984 [inline]
>   do_fault mm/memory.c:4112 [inline]
>   handle_pte_fault mm/memory.c:4371 [inline]
>   __handle_mm_fault+0x2c5f/0x52c0 mm/memory.c:4506
>   handle_mm_fault+0x1bc/0x7e0 mm/memory.c:4604
>   do_user_addr_fault+0x483/0x1210 arch/x86/mm/fault.c:1390
>   handle_page_fault arch/x86/mm/fault.c:1475 [inline]
>   exc_page_fault+0x9e/0x180 arch/x86/mm/fault.c:1531
>   asm_exc_page_fault+0x1e/0x30 arch/x86/include/asm/idtentry.h:577
> RIP: 0033:0x455709
> Code: 24 40 8b 44 24 60 85 c0 7d 1b c7 44 24 70 ff ff ff ff 48 c7 44 24 =
78 00 00 00 00 48 8b 6c 24 40 48 83 c4 48 c3 48 8b 4c 24 50 <39> 41 20 7e =
db 48 8d 51 27 48 63 c0 48 8d 04 82 48 8d 40 01 8b 00
> RSP: 002b:000000c00d74b848 EFLAGS: 00010246
> RAX: 0000000000000000 RBX: 000000000099b3a0 RCX: 0000000000a03f30
> RDX: 0000000000495e89 RSI: 0000000000495e89 RDI: 0000000000495f60
> RBP: 000000c00d74b888 R08: 00000000000005fe R09: 000000c00038a600
> R10: 0000000000000001 R11: 0000000000000001 R12: 0000000000005608
> R13: 0000000000000200 R14: 000000000094b641 R15: 0000000000000000
> Modules linked in:
> ---[ end trace c2509809fbde7be8 ]---
> RIP: 0010:__lock_acquire+0xcf0/0x5230 kernel/locking/lockdep.c:4772
> Code: 3d 0e 41 bf 01 00 00 00 0f 86 8c 00 00 00 89 05 06 49 3d 0e e9 81 =
00 00 00 48 b8 00 00 00 00 00 fc ff df 4c 89 f2 48 c1 ea 03 <80> 3c 02 00 =
0f 85 82 2f 00 00 49 81 3e c0 b3 42 8f 0f 84 da f3 ff
> RSP: 0000:ffffc9000d27e9e0 EFLAGS: 00010002
> RAX: dffffc0000000000 RBX: 0000000000000000 RCX: 0000000000000000
> RDX: 000000000000002a RSI: 0000000000000001 RDI: 0000000000000001
> RBP: 0000000000000000 R08: 0000000000000001 R09: 0000000000000000
> R10: fffffbfff1b92d2a R11: 0000000000000000 R12: ffff888017401c40
> R13: 0000000000000000 R14: 0000000000000150 R15: 0000000000000000
> FS:  000000c013afc090(0000) GS:ffff88802cb00000(0000) knlGS:000000000000=
0000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 00000000009a67f0 CR3: 000000001b446000 CR4: 0000000000150ee0
> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
>
>
> ---
> 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.
>