Re: [RFC v5 PATCH 0/9] xfs: automatic relogging experiment

[Brian Foster <bfoster@redhat.com>]

On Thu, Feb 27, 2020 at 07:09:36AM -0800, Darrick J. Wong wrote:
> On Thu, Feb 27, 2020 at 08:43:12AM -0500, Brian Foster wrote:
> > Hi all,
> > 
> > Here's a v5 RFC of the automatic item relogging experiment. Firstly,
> > note that this is still a POC and experimental code with various quirks.
> 
> Heh, funny, I was going to ask you if you might have time next week to
> review the latest iteration of the btree bulk loading series so that I
> could get closer to merging the rest of online repair and/or refactoring
> offline repair.  I'll take a closer look at this after I read through
> everything else that came in overnight.
> 

Sure.. I can put that next on the list. Is the latest release pending a
post or already posted? Being out for over a month (effectively closer
to two when considering proximity to the holidays) caused me to pretty
much clear everything in my mailbox for obvious reasons. ;) As a result,
anything that might have been on my radar prior to that timeframe has
most likely dropped completely off it. :P

Brian

> --D
> 
> > Some are documented in the code, others might not be (such as abusing
> > the AIL lock, etc.). The primary purpose of this series is still to
> > express and review a fundamental design. Based on discussion on the last
> > version, there is specific focus towards addressing log reservation and
> > pre-item locking deadlock vectors. While the code is still quite hacky,
> > I believe this design addresses both of those fundamental issues.
> > Further details on the design and approach are documented in the
> > individual commit logs.
> > 
> > In addition, the final few patches introduce buffer relogging capability
> > and test infrastructure, which currently has no use case other than to
> > demonstrate development flexibility and the ability to support arbitrary
> > log items in the future, if ever desired. If this approach is taken
> > forward, the current use cases are still centered around intent items
> > such as the quotaoff use case and extent freeing use case defined by
> > online repair of free space trees.
> > 
> > On somewhat of a tangent, another intent oriented use case idea crossed
> > my mind recently related to the long standing writeback stale data
> > exposure problem (i.e. if we crash after a delalloc extent is converted
> > but before writeback fully completes on the extent). The obvious
> > approach of using unwritten extents has been rebuffed due to performance
> > concerns over extent conversion. I wonder if we had the ability to log a
> > "writeback pending" intent on some reasonable level of granularity (i.e.
> > something between a block and extent), whether we could use that to
> > allow log recovery to zero (or convert) such extents in the event of a
> > crash. This is a whole separate design discussion, however, as it
> > involves tracking outstanding writeback, etc. In this context it simply
> > serves as a prospective use case for relogging, as such intents would
> > otherwise risk similar log subsystem deadlocks as the quotaoff use case.
> > 
> > Thoughts, reviews, flames appreciated.
> > 
> > Brian
> > 
> > rfcv5:
> > - More fleshed out design to prevent log reservation deadlock and
> >   locking problems.
> > - Split out core patches between pre-reservation management, relog item
> >   state management and relog mechanism.
> > - Added experimental buffer relogging capability.
> > rfcv4: https://lore.kernel.org/linux-xfs/20191205175037.52529-1-bfoster@redhat.com/
> > - AIL based approach.
> > rfcv3: https://lore.kernel.org/linux-xfs/20191125185523.47556-1-bfoster@redhat.com/
> > - CIL based approach.
> > rfcv2: https://lore.kernel.org/linux-xfs/20191122181927.32870-1-bfoster@redhat.com/
> > - Different approach based on workqueue and transaction rolling.
> > rfc: https://lore.kernel.org/linux-xfs/20191024172850.7698-1-bfoster@redhat.com/
> > 
> > Brian Foster (9):
> >   xfs: set t_task at wait time instead of alloc time
> >   xfs: introduce ->tr_relog transaction
> >   xfs: automatic relogging reservation management
> >   xfs: automatic relogging item management
> >   xfs: automatic log item relog mechanism
> >   xfs: automatically relog the quotaoff start intent
> >   xfs: buffer relogging support prototype
> >   xfs: create an error tag for random relog reservation
> >   xfs: relog random buffers based on errortag
> > 
> >  fs/xfs/libxfs/xfs_errortag.h   |   4 +-
> >  fs/xfs/libxfs/xfs_shared.h     |   1 +
> >  fs/xfs/libxfs/xfs_trans_resv.c |  24 +++-
> >  fs/xfs/libxfs/xfs_trans_resv.h |   1 +
> >  fs/xfs/xfs_buf_item.c          |   5 +
> >  fs/xfs/xfs_dquot_item.c        |   7 ++
> >  fs/xfs/xfs_error.c             |   3 +
> >  fs/xfs/xfs_log.c               |   2 +-
> >  fs/xfs/xfs_qm_syscalls.c       |  12 +-
> >  fs/xfs/xfs_trace.h             |   3 +
> >  fs/xfs/xfs_trans.c             |  79 +++++++++++-
> >  fs/xfs/xfs_trans.h             |  13 +-
> >  fs/xfs/xfs_trans_ail.c         | 216 ++++++++++++++++++++++++++++++++-
> >  fs/xfs/xfs_trans_buf.c         |  35 ++++++
> >  fs/xfs/xfs_trans_priv.h        |   6 +
> >  15 files changed, 399 insertions(+), 12 deletions(-)
> > 
> > -- 
> > 2.21.1
> > 
>