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

[Dave Chinner <david@fromorbit.com>]

On Tue, Mar 24, 2020 at 08:24:36PM -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.

Makes sense. Minor nits:

> 
> Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
> ---
>  fs/xfs/scrub/scrub.c |    4 ++++
>  fs/xfs/xfs_fsmap.c   |    4 ++++
>  fs/xfs/xfs_trans.c   |    5 +++++
>  3 files changed, 13 insertions(+)
> 
> 
> diff --git a/fs/xfs/scrub/scrub.c b/fs/xfs/scrub/scrub.c
> index f1775bb19313..a42bb66b335d 100644
> --- a/fs/xfs/scrub/scrub.c
> +++ b/fs/xfs/scrub/scrub.c
> @@ -168,6 +168,7 @@ xchk_teardown(
>  			xfs_irele(sc->ip);
>  		sc->ip = NULL;
>  	}
> +	sb_end_write(sc->mp->m_super);
>  	if (sc->flags & XCHK_REAPING_DISABLED)
>  		xchk_start_reaping(sc);
>  	if (sc->flags & XCHK_HAS_QUOTAOFFLOCK) {
> @@ -490,6 +491,9 @@ xfs_scrub_metadata(
>  	sc.ops = &meta_scrub_ops[sm->sm_type];
>  	sc.sick_mask = xchk_health_mask_for_scrub_type(sm->sm_type);
>  retry_op:
> +	/* Avoid conflicts with fs freeze. */
> +	sb_start_write(mp->m_super);
> +

Rather than saying something realtively meaningless like "avoid
conflicts with freeze", wouldn't it be better to say something like:

	/*
	 * If freeze runs concurrently with a scrub, the freeze can
	 * be delayed indefinitely as we walk the filesystem and
	 * iterate over metadata buffers. Freeze quiesces the log
	 * which waits for the buffer LRU to be emptied and that
	 * won't happen while checking runs.
	 */

>  	/* Set up for the operation. */
>  	error = sc.ops->setup(&sc, ip);
>  	if (error)
> diff --git a/fs/xfs/xfs_fsmap.c b/fs/xfs/xfs_fsmap.c
> index 918456ca29e1..2bb2cd1e63cf 100644
> --- a/fs/xfs/xfs_fsmap.c
> +++ b/fs/xfs/xfs_fsmap.c
> @@ -896,6 +896,9 @@ xfs_getfsmap(
>  	info.format_arg = arg;
>  	info.head = head;
>  
> +	/* Avoid conflicts with fs freeze. */
> +	sb_start_write(mp->m_super);
> +

And a similar comment here?

-Dave.
-- 
Dave Chinner
david@fromorbit.com