Re: [PATCH v2 1/4] xfs: prohibit fs freezing when using empty transactions

Dave Chinner
2020-03-25T23:26:46Z

On Tue, Mar 24, 2020 at 11:06:27PM -0700, Darrick J. Wong wrote:
> From: Darrick J. Wong <darrick.wong@oracle.com>
>
> I noticed that fsfreeze can take a very long time to freeze an XFS if
> there happens to be a GETFSMAP caller running in the background. I also
> happened to notice the following in dmesg:
>
> ------------[ cut here ]------------
> WARNING: CPU: 2 PID: 43492 at fs/xfs/xfs_super.c:853 xfs_quiesce_attr+0x83/0x90 [xfs]
> Modules linked in: xfs libcrc32c ip6t_REJECT nf_reject_ipv6 ipt_REJECT nf_reject_ipv4 ip_set_hash_ip ip_set_hash_net xt_tcpudp xt_set ip_set_hash_mac ip_set nfnetlink ip6table_filter ip6_tables bfq iptable_filter sch_fq_codel ip_tables x_tables nfsv4 af_packet [last unloaded: xfs]
> CPU: 2 PID: 43492 Comm: xfs_io Not tainted 5.6.0-rc4-djw #rc4
> Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.10.2-1ubuntu1 04/01/2014
> RIP: 0010:xfs_quiesce_attr+0x83/0x90 [xfs]
> Code: 7c 07 00 00 85 c0 75 22 48 89 df 5b e9 96 c1 00 00 48 c7 c6 b0 2d 38 a0 48 89 df e8 57 64 ff ff 8b 83 7c 07 00 00 85 c0 74 de <0f> 0b 48 89 df 5b e9 72 c1 00 00 66 90 0f 1f 44 00 00 41 55 41 54
> RSP: 0018:ffffc900030f3e28 EFLAGS: 00010202
> RAX: 0000000000000001 RBX: ffff88802ac54000 RCX: 0000000000000000
> RDX: 0000000000000000 RSI: ffffffff81e4a6f0 RDI: 00000000ffffffff
> RBP: ffff88807859f070 R08: 0000000000000001 R09: 0000000000000000
> R10: 0000000000000000 R11: 0000000000000010 R12: 0000000000000000
> R13: ffff88807859f388 R14: ffff88807859f4b8 R15: ffff88807859f5e8
> FS: 00007fad1c6c0fc0(0000) GS:ffff88807e000000(0000) knlGS:0000000000000000
> CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 00007f0c7d237000 CR3: 0000000077f01003 CR4: 00000000001606a0
> Call Trace:
> xfs_fs_freeze+0x25/0x40 [xfs]
> freeze_super+0xc8/0x180
> do_vfs_ioctl+0x70b/0x750
> ? __fget_files+0x135/0x210
> ksys_ioctl+0x3a/0xb0
> __x64_sys_ioctl+0x16/0x20
> do_syscall_64+0x50/0x1a0
> entry_SYSCALL_64_after_hwframe+0x49/0xbe
>
> These two things appear to be related. The assertion trips when another
> thread initiates a fsmap request (which uses an empty transaction) after
> the freezer waited for m_active_trans to hit zero but before the the
> freezer executes the WARN_ON just prior to calling xfs_log_quiesce.
>
> The lengthy delays in freezing happen because the freezer calls
> xfs_wait_buftarg to clean out the buffer lru list. Meanwhile, the
> GETFSMAP caller is continuing to grab and release buffers, which means
> that it can take a very long time for the buffer lru list to empty out.
>
> We fix both of these races by calling sb_start_write to obtain freeze
> protection while using empty transactions for GETFSMAP and for metadata
> scrubbing. The other two users occur during mount, during which time we
> cannot fs freeze.
>
> Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
> ---
> v2: improve comments

Looks good now.

Reviewed-by: Dave Chinner <dchinner@redhat.com>
--
Dave Chinner
david@fromorbit.com