> On Aug 7, 2020, at 2:02 PM, tytso@xxxxxxx wrote:
>
> Thanks, applied, although I rewrote the commit description to make it
> be a bit more clearer:
>
> fs: prevent BUG_ON in submit_bh_wbc()
>
> If a device is hot-removed --- for example, when a physical device is
> unplugged from pcie slot or a nbd device's network is shutdown ---
> this can result in a BUG_ON() crash in submit_bh_wbc(). This is
> because the when the block device dies, the buffer heads will have
> their Buffer_Mapped flag get cleared, leading to the crash in
> submit_bh_wbc.
>
> We had attempted to work around this problem in commit a17712c8
Should this get a "Fixes:" label with this info, rather than embedding
it in the commit message, so that it could be picked up by stable?
Cheers, Andreas
> ("ext4: check superblock mapped prior to committing"). Unfortunately,
> it's still possible to hit the BUG_ON(!buffer_mapped(bh)) if the
> device dies between when the work-around check in ext4_commit_super()
> and when submit_bh_wbh() is finally called:
>
> Code path:
> ext4_commit_super
> judge if 'buffer_mapped(sbh)' is false, return <== commit a17712c8
> lock_buffer(sbh)
> ...
> unlock_buffer(sbh)
> __sync_dirty_buffer(sbh,...
> lock_buffer(sbh)
> judge if 'buffer_mapped(sbh))' is false, return <== added by this patch
> submit_bh(...,sbh)
> submit_bh_wbc(...,sbh,...)
>
> [100722.966497] kernel BUG at fs/buffer.c:3095! <== BUG_ON(!buffer_mapped(bh))' in submit_bh_wbc()
> [100722.966503] invalid opcode: 0000 [#1] SMP
> [100722.966566] task: ffff8817e15a9e40 task.stack: ffffc90024744000
> [100722.966574] RIP: 0010:submit_bh_wbc+0x180/0x190
> [100722.966575] RSP: 0018:ffffc90024747a90 EFLAGS: 00010246
> [100722.966576] RAX: 0000000000620005 RBX: ffff8818a80603a8 RCX: 0000000000000000
> [100722.966576] RDX: ffff8818a80603a8 RSI: 0000000000020800 RDI: 0000000000000001
> [100722.966577] RBP: ffffc90024747ac0 R08: 0000000000000000 R09: ffff88207f94170d
> [100722.966578] R10: 00000000000437c8 R11: 0000000000000001 R12: 0000000000020800
> [100722.966578] R13: 0000000000000001 R14: 000000000bf9a438 R15: ffff88195f333000
> [100722.966580] FS: 00007fa2eee27700(0000) GS:ffff88203d840000(0000) knlGS:0000000000000000
> [100722.966580] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [100722.966581] CR2: 0000000000f0b008 CR3: 000000201a622003 CR4: 00000000007606e0
> [100722.966582] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> [100722.966583] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> [100722.966583] PKRU: 55555554
> [100722.966583] Call Trace:
> [100722.966588] __sync_dirty_buffer+0x6e/0xd0
> [100722.966614] ext4_commit_super+0x1d8/0x290 [ext4]
> [100722.966626] __ext4_std_error+0x78/0x100 [ext4]
> [100722.966635] ? __ext4_journal_get_write_access+0xca/0x120 [ext4]
> [100722.966646] ext4_reserve_inode_write+0x58/0xb0 [ext4]
> [100722.966655] ? ext4_dirty_inode+0x48/0x70 [ext4]
> [100722.966663] ext4_mark_inode_dirty+0x53/0x1e0 [ext4]
> [100722.966671] ? __ext4_journal_start_sb+0x6d/0xf0 [ext4]
> [100722.966679] ext4_dirty_inode+0x48/0x70 [ext4]
> [100722.966682] __mark_inode_dirty+0x17f/0x350
> [100722.966686] generic_update_time+0x87/0xd0
> [100722.966687] touch_atime+0xa9/0xd0
> [100722.966690] generic_file_read_iter+0xa09/0xcd0
> [100722.966694] ? page_cache_tree_insert+0xb0/0xb0
> [100722.966704] ext4_file_read_iter+0x4a/0x100 [ext4]
> [100722.966707] ? __inode_security_revalidate+0x4f/0x60
> [100722.966709] __vfs_read+0xec/0x160
> [100722.966711] vfs_read+0x8c/0x130
> [100722.966712] SyS_pread64+0x87/0xb0
> [100722.966716] do_syscall_64+0x67/0x1b0
> [100722.966719] entry_SYSCALL64_slow_path+0x25/0x25
>
> To address this, add the check of 'buffer_mapped(bh)' to
> __sync_dirty_buffer(). This also has the benefit of fixing this for
> other file systems.
>
> With this addition, we can drop the workaround in ext4_commit_supper().
>
> [ Commit description rewritten by tytso. ]
>
> Signed-off-by: Xianting Tian <xianting_tian@xxxxxxx>
> Link: https://lore.kernel.org/r/1596211825-8750-1-git-send-email-xianting_tian@xxxxxxx
> Signed-off-by: Theodore Ts'o <tytso@xxxxxxx>
>
> - Ted
Cheers, Andreas
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