Re: [PATCH 5.10.x] btrfs: fix crash after non-aligned direct IO write with O_DSYNC

[Filipe Manana <fdmanana@suse.com>]

On 16/02/21 14:50, Greg KH wrote:
> On Tue, Feb 16, 2021 at 02:40:31PM +0000, fdmanana@kernel.org wrote:
>> From: Filipe Manana <fdmanana@suse.com>
>>
>> Whenever we attempt to do a non-aligned direct IO write with O_DSYNC, we
>> end up triggering an assertion and crashing. Example reproducer:
>>
>>   $ cat test.sh
>>   #!/bin/bash
>>
>>   DEV=/dev/sdj
>>   MNT=/mnt/sdj
>>
>>   mkfs.btrfs -f $DEV > /dev/null
>>   mount $DEV $MNT
>>
>>   # Do a direct IO write with O_DSYNC into a non-aligned range...
>>   xfs_io -f -d -s -c "pwrite -S 0xab -b 64K 1111 64K" $MNT/foobar
>>
>>   umount $MNT
>>
>> When running the reproducer an assertion fails and produces the following
>> trace:
>>
>>   [ 2418.403134] assertion failed: !current->journal_info || flush != BTRFS_RESERVE_FLUSH_DATA, in fs/btrfs/space-info.c:1467
>>   [ 2418.403745] ------------[ cut here ]------------
>>   [ 2418.404306] kernel BUG at fs/btrfs/ctree.h:3286!
>>   [ 2418.404862] invalid opcode: 0000 [#2] PREEMPT SMP DEBUG_PAGEALLOC PTI
>>   [ 2418.405451] CPU: 1 PID: 64705 Comm: xfs_io Tainted: G      D           5.10.15-btrfs-next-87 #1
>>   [ 2418.406026] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.14.0-0-g155821a1990b-prebuilt.qemu.org 04/01/2014
>>   [ 2418.407228] RIP: 0010:assertfail.constprop.0+0x18/0x26 [btrfs]
>>   [ 2418.407835] Code: e6 48 c7 (...)
>>   [ 2418.409078] RSP: 0018:ffffb06080d13c98 EFLAGS: 00010246
>>   [ 2418.409696] RAX: 000000000000006c RBX: ffff994c1debbf08 RCX: 0000000000000000
>>   [ 2418.410302] RDX: 0000000000000000 RSI: 0000000000000027 RDI: 00000000ffffffff
>>   [ 2418.410904] RBP: ffff994c21770000 R08: 0000000000000000 R09: 0000000000000000
>>   [ 2418.411504] R10: 0000000000000000 R11: 0000000000000001 R12: 0000000000010000
>>   [ 2418.412111] R13: ffff994c22198400 R14: ffff994c21770000 R15: 0000000000000000
>>   [ 2418.412713] FS:  00007f54fd7aff00(0000) GS:ffff994d35200000(0000) knlGS:0000000000000000
>>   [ 2418.413326] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>>   [ 2418.413933] CR2: 000056549596d000 CR3: 000000010b928003 CR4: 0000000000370ee0
>>   [ 2418.414528] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
>>   [ 2418.415109] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
>>   [ 2418.415669] Call Trace:
>>   [ 2418.416254]  btrfs_reserve_data_bytes.cold+0x22/0x22 [btrfs]
>>   [ 2418.416812]  btrfs_check_data_free_space+0x4c/0xa0 [btrfs]
>>   [ 2418.417380]  btrfs_buffered_write+0x1b0/0x7f0 [btrfs]
>>   [ 2418.418315]  btrfs_file_write_iter+0x2a9/0x770 [btrfs]
>>   [ 2418.418920]  new_sync_write+0x11f/0x1c0
>>   [ 2418.419430]  vfs_write+0x2bb/0x3b0
>>   [ 2418.419972]  __x64_sys_pwrite64+0x90/0xc0
>>   [ 2418.420486]  do_syscall_64+0x33/0x80
>>   [ 2418.420979]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
>>   [ 2418.421486] RIP: 0033:0x7f54fda0b986
>>   [ 2418.421981] Code: 48 c7 c0 (...)
>>   [ 2418.423019] RSP: 002b:00007ffc40569c38 EFLAGS: 00000246 ORIG_RAX: 0000000000000012
>>   [ 2418.423547] RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00007f54fda0b986
>>   [ 2418.424075] RDX: 0000000000010000 RSI: 000056549595e000 RDI: 0000000000000003
>>   [ 2418.424596] RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000000000400
>>   [ 2418.425119] R10: 0000000000000400 R11: 0000000000000246 R12: 00000000ffffffff
>>   [ 2418.425644] R13: 0000000000000400 R14: 0000000000010000 R15: 0000000000000000
>>   [ 2418.426148] Modules linked in: btrfs blake2b_generic (...)
>>   [ 2418.429540] ---[ end trace ef2aeb44dc0afa34 ]---
>>
>> 1) At btrfs_file_write_iter() we set current->journal_info to
>>    BTRFS_DIO_SYNC_STUB;
>>
>> 2) We then call __btrfs_direct_write(), which calls btrfs_direct_IO();
>>
>> 3) We can't do the direct IO write because it starts at a non-aligned
>>    offset (1111). So at btrfs_direct_IO() we return -EINVAL (coming from
>>    check_direct_IO() which does the alignment check), but we leave
>>    current->journal_info set to BTRFS_DIO_SYNC_STUB - we only clear it
>>    at btrfs_dio_iomap_begin(), because we assume we always get there;
>>
>> 4) Then at __btrfs_direct_write() we see that the attempt to do the
>>    direct IO write was not successful, 0 bytes written, so we fallback
>>    to a buffered write by calling btrfs_buffered_write();
>>
>> 5) There we call btrfs_check_data_free_space() which in turn calls
>>    btrfs_alloc_data_chunk_ondemand() and that calls
>>    btrfs_reserve_data_bytes() with flush == BTRFS_RESERVE_FLUSH_DATA;
>>
>> 6) Then at btrfs_reserve_data_bytes() we have current->journal_info set to
>>    BTRFS_DIO_SYNC_STUB, therefore not NULL, and flush has the value
>>    BTRFS_RESERVE_FLUSH_DATA, triggering the second assertion:
>>
>>   int btrfs_reserve_data_bytes(struct btrfs_fs_info *fs_info, u64 bytes,
>>                                enum btrfs_reserve_flush_enum flush)
>>   {
>>       struct btrfs_space_info *data_sinfo = fs_info->data_sinfo;
>>       int ret;
>>
>>       ASSERT(flush == BTRFS_RESERVE_FLUSH_DATA ||
>>              flush == BTRFS_RESERVE_FLUSH_FREE_SPACE_INODE);
>>       ASSERT(!current->journal_info || flush != BTRFS_RESERVE_FLUSH_DATA);
>>   (...)
>>
>> So fix that by setting the journal to NULL whenever check_direct_IO()
>> returns a failure.
>>
>> This bug only affects 5.10 kernels, and the regression was introduced in
>> 5.10-rc1 by commit 0eb79294dbe328 ("btrfs: dio iomap DSYNC workaround").
>> The bug does not exist in 5.11 kernels due to commit ecfdc08b8cc65d
>> ("btrfs: remove dio iomap DSYNC workaround"), which depends on a large
>> patchset that went into the merge window for 5.11. So this is a fix only
>> for 5.10.x stable kernels, as there are people hitting this bug.
>>
>> Fixes: 0eb79294dbe328 ("btrfs: dio iomap DSYNC workaround")
>> CC: stable@vger.kernel.org # 5.10 (and only 5.10)
>> CC: David Sterba <dsterba@suse.cz>
>> Bugzilla: https://bugzilla.suse.com/show_bug.cgi?id=1181605
>> Signed-off-by: Filipe Manana <fdmanana@suse.com>
>> ---
>>  fs/btrfs/inode.c | 6 +++++-
>>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> As this is a one-off patch, I need the btrfs maintainers to ack this and
> really justify why we can't take the larger patch or patch series here
> instead, as that is almost always the correct thing to do instead.

David will likely reply soon, but from another thread:

https://lore.kernel.org/linux-btrfs/20210216142324.GO1993@twin.jikos.cz/

Thanks.
> 
> thanks,
> 
> greg k-h
>