Re: [PATCH v2 02/12] libxfs: track largest metadata LSN in use via verifiers

[Dave Chinner <david@fromorbit.com>]

On Fri, Sep 11, 2015 at 02:55:32PM -0400, Brian Foster wrote:
> The LSN validation helper is called in the I/O verifier codepath for
> metadata that embed a last-modification LSN. While the codepath exists,
> this is not used in userspace as in the kernel because the former
> doesn't have an active log.
> 
> xfs_repair does need to check the validity of the LSN metadata with
> respect to the on-disk log, however. Use the LSN validation mechanism to
> track the largest LSN that has been seen. Export the value so repair can
> use it once it has processed the entire filesystem. Note that the helper
> continues to always return true to preserve existing behavior.
> 
> Signed-off-by: Brian Foster <bfoster@redhat.com>
....
> +xfs_lsn_t		libxfs_max_lsn = 0;
> +pthread_mutex_t		libxfs_max_lsn_lock = PTHREAD_MUTEX_INITIALIZER;
> +
>  void
>  xfs_detect_invalid_lsn(
>  	struct xfs_mount	*mp,
>  	xfs_lsn_t		lsn)
>  {
> +	int			cycle = CYCLE_LSN(lsn);
> +	int			block = BLOCK_LSN(lsn);
> +	int			max_cycle;
> +	int			max_block;
> +
> +	if (lsn == NULLCOMMITLSN)
> +		return;
> +
> +	pthread_mutex_lock(&libxfs_max_lsn_lock);
> +
> +	max_cycle = CYCLE_LSN(libxfs_max_lsn);
> +	max_block = BLOCK_LSN(libxfs_max_lsn);
> +
> +	if ((cycle > max_cycle) ||
> +	    (cycle == max_cycle && block > max_block))
> +		libxfs_max_lsn = lsn;
> +
> +	pthread_mutex_unlock(&libxfs_max_lsn_lock);

This will have the same lock contention problems that the kernel
code would have had - my repair scalablity tests regularly reach
over 1GB/s of metadata being prefetched through tens of threads, so
this is going have a significant impact on performance in those
tests....

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com

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