[PATCH] ext4: move buffer_mapped() to proper position
From: Xianting Tian
Date: Fri Jul 31 2020 - 12:12:05 EST
As you know, commit a17712c8 has added below code to aviod a
crash( 'BUG_ON(!buffer_mapped(bh))' in submit_bh_wbc) when
device hot-removed(a physical device is unpluged from pcie slot
or a nbd device's network is shutdown).
static int ext4_commit_super():
if (!sbh || block_device_ejected(sb))
return error;
+
+ /*
+ * The superblock bh should be mapped, but it might not be if the
+ * device was hot-removed. Not much we can do but fail the I/O.
+ */
+ if (!buffer_mapped(sbh))
+ return error;
And the call trace, which leads to the crash, as below:
ext4_commit_super()
__sync_dirty_buffer()
submit_bh()
submit_bh_wbc()
BUG_ON(!buffer_mapped(bh));
But recently we met the same crash(with very low probability) when
device hot-removed even though the kernel already contained
above exception protection code. Still, the crash is caused by
'BUG_ON(!buffer_mapped(bh))' in submit_bh_wbc(), and the same
call trace as below.
As my understanding and below codeïthere are still some more
codes needs to run between 'buffer_mapped(sbh)'(which is added
by commit a17712c8) and 'BUG_ON(!buffer_mapped(bh))' in
submit_bh_wbc(), especially lock_buffer is called two times(sometimes,
it may take more times to get the lock). So when do the test of
device hot-remove, there is low probability that the sbh is mapped
when executing 'buffer_mapped(sbh)'(which is added by commit a17712c8)
but sbh is not mapped when executing 'BUG_ON(!buffer_mapped(bh))'
in submit_bh_wbc().
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,...)
This patch is to move the check of 'buffer_mapped(sbh)' to the place just
before calling 'BUG_ON(!buffer_mapped(bh))' in submit_bh_wbc().
[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
Signed-off-by: Xianting Tian <xianting_tian@xxxxxxx>
---
fs/buffer.c | 9 +++++++++
fs/ext4/super.c | 7 -------
2 files changed, 9 insertions(+), 7 deletions(-)
diff --git a/fs/buffer.c b/fs/buffer.c
index 64fe82e..75a8849 100644
--- a/fs/buffer.c
+++ b/fs/buffer.c
@@ -3160,6 +3160,15 @@ int __sync_dirty_buffer(struct buffer_head *bh, int op_flags)
WARN_ON(atomic_read(&bh->b_count) < 1);
lock_buffer(bh);
if (test_clear_buffer_dirty(bh)) {
+ /*
+ * The bh should be mapped, but it might not be if the
+ * device was hot-removed. Not much we can do but fail the I/O.
+ */
+ if (!buffer_mapped(bh)) {
+ unlock_buffer(bh);
+ return -EIO;
+ }
+
get_bh(bh);
bh->b_end_io = end_buffer_write_sync;
ret = submit_bh(REQ_OP_WRITE, op_flags, bh);
diff --git a/fs/ext4/super.c b/fs/ext4/super.c
index 330957e..1c22044 100644
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -5171,13 +5171,6 @@ static int ext4_commit_super(struct super_block *sb, int sync)
return error;
/*
- * The superblock bh should be mapped, but it might not be if the
- * device was hot-removed. Not much we can do but fail the I/O.
- */
- if (!buffer_mapped(sbh))
- return error;
-
- /*
* If the file system is mounted read-only, don't update the
* superblock write time. This avoids updating the superblock
* write time when we are mounting the root file system
--
1.8.3.1