Re: [PATCH 9/9] xfs: log head and tail aren't reliable during shutdown

Darrick J. Wong
2021-07-14T22:12:34Z

On Wed, Jul 14, 2021 at 01:19:58PM +1000, Dave Chinner wrote:
> From: Dave Chinner <dchinner@redhat.com>
>
> I'm seeing assert failures from xlog_space_left() after a shutdown
> has begun that look like:
>
> XFS (dm-0): log I/O error -5
> XFS (dm-0): xfs_do_force_shutdown(0x2) called from line 1338 of file fs/xfs/xfs_log.c. Return address = xlog_ioend_work+0x64/0xc0
> XFS (dm-0): Log I/O Error Detected.
> XFS (dm-0): Shutting down filesystem. Please unmount the filesystem and rectify the problem(s)
> XFS (dm-0): xlog_space_left: head behind tail
> XFS (dm-0): tail_cycle = 6, tail_bytes = 2706944
> XFS (dm-0): GH cycle = 6, GH bytes = 1633867
> XFS: Assertion failed: 0, file: fs/xfs/xfs_log.c, line: 1310
> ------------[ cut here ]------------
> Call Trace:
> xlog_space_left+0xc3/0x110
> xlog_grant_push_threshold+0x3f/0xf0
> xlog_grant_push_ail+0x12/0x40
> xfs_log_reserve+0xd2/0x270
> ? __might_sleep+0x4b/0x80
> xfs_trans_reserve+0x18b/0x260
> .....
>
> There are two things here. Firstly, after a shutdown, the log head
> and tail can be out of whack as things abort and release (or don't
> release) resources, so checking them for sanity doesn't make much
> sense. Secondly, xfs_log_reserve() can race with shutdown and so it
> can still fail like this even though it has already checked for a
> log shutdown before calling xlog_grant_push_ail().
>
> So, before ASSERT failing in xlog_space_left(), make sure we haven't
> already shut down....
>
> Signed-off-by: Dave Chinner <dchinner@redhat.com>
> Reviewed-by: Christoph Hellwig <hch@lst.de>

Makes sense,
Reviewed-by: Darrick J. Wong <djwong@kernel.org>

--D

> ---
> fs/xfs/xfs_log.c | 51 +++++++++++++++++++++++++-----------------------
> 1 file changed, 27 insertions(+), 24 deletions(-)
>
> diff --git a/fs/xfs/xfs_log.c b/fs/xfs/xfs_log.c
> index 01c20b42b2fc..6617cdccaf00 100644
> --- a/fs/xfs/xfs_log.c
> +++ b/fs/xfs/xfs_log.c
> @@ -1272,16 +1272,18 @@ xlog_assign_tail_lsn(
> * wrap the tail, we should blow up. Rather than catch this case here,
> * we depend on other ASSERTions in other parts of the code. XXXmiken
> *
> - * This code also handles the case where the reservation head is behind
> - * the tail. The details of this case are described below, but the end
> - * result is that we return the size of the log as the amount of space left.
> + * If reservation head is behind the tail, we have a problem. Warn about it,
> + * but then treat it as if the log is empty.
> + *
> + * If the log is shut down, the head and tail may be invalid or out of whack, so
> + * shortcut invalidity asserts in this case so that we don't trigger them
> + * falsely.
> */
> STATIC int
> xlog_space_left(
> struct xlog *log,
> atomic64_t *head)
> {
> - int free_bytes;
> int tail_bytes;
> int tail_cycle;
> int head_cycle;
> @@ -1291,29 +1293,30 @@ xlog_space_left(
> xlog_crack_atomic_lsn(&log->l_tail_lsn, &tail_cycle, &tail_bytes);
> tail_bytes = BBTOB(tail_bytes);
> if (tail_cycle == head_cycle && head_bytes >= tail_bytes)
> - free_bytes = log->l_logsize - (head_bytes - tail_bytes);
> - else if (tail_cycle + 1 < head_cycle)
> + return log->l_logsize - (head_bytes - tail_bytes);
> + if (tail_cycle + 1 < head_cycle)
> return 0;
> - else if (tail_cycle < head_cycle) {
> +
> + /* Ignore potential inconsistency when shutdown. */
> + if (xlog_is_shutdown(log))
> + return log->l_logsize;
> +
> + if (tail_cycle < head_cycle) {
> ASSERT(tail_cycle == (head_cycle - 1));
> - free_bytes = tail_bytes - head_bytes;
> - } else {
> - /*
> - * The reservation head is behind the tail.
> - * In this case we just want to return the size of the
> - * log as the amount of space left.
> - */
> - xfs_alert(log->l_mp, "xlog_space_left: head behind tail");
> - xfs_alert(log->l_mp,
> - " tail_cycle = %d, tail_bytes = %d",
> - tail_cycle, tail_bytes);
> - xfs_alert(log->l_mp,
> - " GH cycle = %d, GH bytes = %d",
> - head_cycle, head_bytes);
> - ASSERT(0);
> - free_bytes = log->l_logsize;
> + return tail_bytes - head_bytes;
> }
> - return free_bytes;
> +
> + /*
> + * The reservation head is behind the tail. In this case we just want to
> + * return the size of the log as the amount of space left.
> + */
> + xfs_alert(log->l_mp, "xlog_space_left: head behind tail");
> + xfs_alert(log->l_mp, " tail_cycle = %d, tail_bytes = %d",
> + tail_cycle, tail_bytes);
> + xfs_alert(log->l_mp, " GH cycle = %d, GH bytes = %d",
> + head_cycle, head_bytes);
> + ASSERT(0);
> + return log->l_logsize;
> }
>
>
> --
> 2.31.1
>