Re: [PATCH] xfs: stabilize fs summary counters for online fsck

[Dave Chinner <david@fromorbit.com>]

On Wed, Aug 02, 2023 at 10:22:18PM -0700, Darrick J. Wong wrote:
> From: Darrick J. Wong <djwong@kernel.org>
> 
> If the fscounters scrubber notices incorrect summary counters, it's
> entirely possible that scrub is simply racing with other threads that
> are updating the incore counters.  There isn't a good way to stabilize
> percpu counters or set ourselves up to observe live updates with hooks
> like we do for the quotacheck or nlinks scanners, so we instead choose
> to freeze the filesystem long enough to walk the incore per-AG
> structures.
> 
> Past me thought that it was going to be commonplace to have to freeze
> the filesystem to perform some kind of repair and set up a whole
> separate infrastructure to freeze the filesystem in such a way that
> userspace could not unfreeze while we were running.  This involved
> adding a mutex and freeze_super/thaw_super functions and dealing with
> the fact that the VFS freeze/thaw functions can free the VFS superblock
> references on return.
> 
> This was all very overwrought, since fscounters turned out to be the
> only user of scrub freezes, and it doesn't require the log to quiesce,
> only the incore superblock counters.  We prevent other threads from
> changing the freeze level by calling freeze_super_excl with a custom
> freeze cookie to keep everyone else out of the filesystem.
> 
> The end result is that fscounters should be much more efficient.  When
> we're checking a busy system and we can't stabilize the counters, the
> custom freeze will do less work, which should result in less downtime.
> Repair should be similarly speedy, but that's in the next patch.
> 
> Signed-off-by: Darrick J. Wong <djwong@kernel.org>
> ---
>  fs/xfs/scrub/fscounters.c |  198 ++++++++++++++++++++++++++++++++++-----------
>  fs/xfs/scrub/fscounters.h |   20 +++++
>  fs/xfs/scrub/scrub.c      |    6 +
>  fs/xfs/scrub/scrub.h      |    1 
>  fs/xfs/scrub/trace.h      |   26 ++++++
>  5 files changed, 203 insertions(+), 48 deletions(-)
>  create mode 100644 fs/xfs/scrub/fscounters.h

Code changes look ok, though I am wondering why struct
xchk_fscounters needs to be moved to it's own header file? AFAICT it
is still only used by fs/xfs/scrub/fscounters.c, so I'm not sure
what purpose the new header file serves....

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